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1.6: Hypothesis, Theories, and Laws

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Learning Objectives

Describe the difference between hypothesis and theory as scientific terms.
Describe the difference between a theory and scientific law.

Although many have taken science classes throughout the course of their studies, people often have incorrect or misleading ideas about some of the most important and basic principles in science. Most students have heard of hypotheses, theories, and laws, but what do these terms really mean? Prior to reading this section, consider what you have learned about these terms before. What do these terms mean to you? What do you read that contradicts or supports what you thought?

What is a Fact?

A fact is a basic statement established by experiment or observation. All facts are true under the specific conditions of the observation.

What is a Hypothesis?

One of the most common terms used in science classes is a "hypothesis". The word can have many different definitions, depending on the context in which it is being used:

An educated guess: a scientific hypothesis provides a suggested solution based on evidence.
Prediction: if you have ever carried out a science experiment, you probably made this type of hypothesis when you predicted the outcome of your experiment.
Tentative or proposed explanation: hypotheses can be suggestions about why something is observed. In order for it to be scientific, however, a scientist must be able to test the explanation to see if it works and if it is able to correctly predict what will happen in a situation. For example, "if my hypothesis is correct, we should see ___ result when we perform ___ test."

A hypothesis is very tentative; it can be easily changed.

What is a Theory?

The United States National Academy of Sciences describes what a theory is as follows:

"Some scientific explanations are so well established that no new evidence is likely to alter them. The explanation becomes a scientific theory. In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena."

"A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experimentation. Such fact-supported theories are not "guesses" but reliable accounts of the real world. The theory of biological evolution is more than "just a theory." It is as factual an explanation of the universe as the atomic theory of matter (stating that everything is made of atoms) or the germ theory of disease (which states that many diseases are caused by germs). Our understanding of gravity is still a work in progress. But the phenomenon of gravity, like evolution, is an accepted fact.

Note some key features of theories that are important to understand from this description:

Theories are explanations of natural phenomena. They aren't predictions (although we may use theories to make predictions). They are explanations as to why we observe something.
Theories aren't likely to change. They have a large amount of support and are able to satisfactorily explain numerous observations. Theories can, indeed, be facts. Theories can change, but it is a long and difficult process. In order for a theory to change, there must be many observations or pieces of evidence that the theory cannot explain.
Theories are not guesses. The phrase "just a theory" has no room in science. To be a scientific theory carries a lot of weight; it is not just one person's idea about something

Theories aren't likely to change.

What is a Law?

Scientific laws are similar to scientific theories in that they are principles that can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. Usually scientific laws refer to rules for how nature will behave under certain conditions, frequently written as an equation. Scientific theories are more overarching explanations of how nature works and why it exhibits certain characteristics. As a comparison, theories explain why we observe what we do and laws describe what happens.

For example, around the year 1800, Jacques Charles and other scientists were working with gases to, among other reasons, improve the design of the hot air balloon. These scientists found, after many, many tests, that certain patterns existed in the observations on gas behavior. If the temperature of the gas is increased, the volume of the gas increased. This is known as a natural law. A law is a relationship that exists between variables in a group of data. Laws describe the patterns we see in large amounts of data, but do not describe why the patterns exist.

What is a Belief?

A belief is a statement that is not scientifically provable. Beliefs may or may not be incorrect; they just are outside the realm of science to explore.

Laws vs. Theories

A common misconception is that scientific theories are rudimentary ideas that will eventually graduate into scientific laws when enough data and evidence has accumulated. A theory does not change into a scientific law with the accumulation of new or better evidence. Remember, theories are explanations and laws are patterns we see in large amounts of data, frequently written as an equation. A theory will always remain a theory; a law will always remain a law.

Video \(\PageIndex{1}\): What’s the difference between a scientific law and theory?

A hypothesis is a tentative explanation that can be tested by further investigation.
A theory is a well-supported explanation of observations.
A scientific law is a statement that summarizes the relationship between variables.
An experiment is a controlled method of testing a hypothesis.

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Marisa Alviar-Agnew ( Sacramento City College )

Henry Agnew (UC Davis)

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Scientific Law Definition and Examples

A scientific law is a statement or mathematical equation that describes or predicts a natural phenomenon. It does not explain why or how a phenomenon occurs. Another name for a scientific law is a law of nature or law of science . All scientific laws are based on empirical evidence and the scientific method. In science, an assertion can be disproven, but never proven, so it’s possible for a scientific law to be revised or disproven by future experiments. In contrast, a mathematical theorem or identity is proven to be true.

Examples of Scientific Laws

There are laws in all scientific disciplines, although primarily they are physical laws. Here are some examples:

Beer’s law
Dalton’s law of partial pressures
Ideal gas law
Kepler’s laws of planetary motion
Law of conservation of mass
Law of conservation of energy
Law of conservation of momentum
Law of reflection
Laws of thermodynamics
Newton’s law of universal gravitation
Newton’s laws of motion

Difference Between a Scientific Law and Scientific Theory

Both scientific laws and scientific theories are based in the scientific method and are falsifiable. However, the two terms have very different meanings. A law describes what happens, but does not explain it. A theory explains how or why something works.

For example, Newton’s law of universal gravitation describes what happens when two masses are a given distance apart. The law can be written as a mathematical equation [F = G(m 1 m 2 /r 2 )] and used to make predictions and calculations. However, the law does not explain how gravity works or why two masses are attracted to one another. Scientists didn’t really have an explanation for gravity until Einstein’s theory of general relativity, which continues to be revised as we understand more about the nature of spacetime.

As another example, Hubble’s law of Cosmic Expansion (velocity = Hubble constant x distance) describes the movement of galaxies away from each other. It does explain why this occurs. The Big Bang Theory is one of the theories that explains why galaxies move apart, but the theory does not offer a formula for calculating this motion.

Can a Hypothesis or Theory Become a Law?

A hypothesis , theory, and law are all parts of scientific inquiry, but one never becomes another . They are different things. A hypothesis never becomes a theory, no matter how many experiments support it, because a hypothesis is simply a prediction about how one variable responds when another is changed. A theory takes into account the results of many experiments, testing different hypotheses. A theory explains how something works. Like a theory, a law draws on the results of repeated observations and experiments. But, a law states in words or mathematical equations what happens. Laws don’t explain why.

Barrow, John (1991). Theories of Everything: The Quest for Ultimate Explanations . ISBN 0-449-90738-4.
Feynman, Richard (1994). The Character of Physical Law (Modern Library ed.). New York: Modern Library. ISBN 978-0-679-60127-2.
Gould, Stephen Jay (1981). “ Evolution as Fact and Theory “. Discover . 2 (5): 34–37.
McComas, William F. (2013). The Language of Science Education: An Expanded Glossary of Key Terms and Concepts in Science Teaching and Learning. Springer Science & Business Media. ISBN 978-94-6209-497-0.

Theories, Hypotheses, and Laws: Definitions, examples, and their roles in science

by Anthony Carpi, Ph.D., Anne E. Egger, Ph.D.

Listen to this reading

Did you know that the idea of evolution had been part of Western thought for more than 2,000 years before Charles Darwin was born? Like many theories, the theory of evolution was the result of the work of many different scientists working in different disciplines over a period of time.

A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and is logical, testable, and predictive.

As new evidence comes to light, or new interpretations of existing data are proposed, theories may be revised and even change; however, they are not tenuous or speculative.

A scientific hypothesis is an inferred explanation of an observation or research finding; while more exploratory in nature than a theory, it is based on existing scientific knowledge.

A scientific law is an expression of a mathematical or descriptive relationship observed in nature.

Imagine yourself shopping in a grocery store with a good friend who happens to be a chemist. Struggling to choose between the many different types of tomatoes in front of you, you pick one up, turn to your friend, and ask her if she thinks the tomato is organic . Your friend simply chuckles and replies, "Of course it's organic!" without even looking at how the fruit was grown. Why the amused reaction? Your friend is highlighting a simple difference in vocabulary. To a chemist, the term organic refers to any compound in which hydrogen is bonded to carbon. Tomatoes (like all plants) are abundant in organic compounds – thus your friend's laughter. In modern agriculture, however, organic has come to mean food items grown or raised without the use of chemical fertilizers, pesticides, or other additives.

So who is correct? You both are. Both uses of the word are correct, though they mean different things in different contexts. There are, of course, lots of words that have more than one meaning (like bat , for example), but multiple meanings can be especially confusing when two meanings convey very different ideas and are specific to one field of study.

Scientific theories

The term theory also has two meanings, and this double meaning often leads to confusion. In common language, the term theory generally refers to speculation or a hunch or guess. You might have a theory about why your favorite sports team isn't playing well, or who ate the last cookie from the cookie jar. But these theories do not fit the scientific use of the term. In science, a theory is a well-substantiated and comprehensive set of ideas that explains a phenomenon in nature. A scientific theory is based on large amounts of data and observations that have been collected over time. Scientific theories can be tested and refined by additional research , and they allow scientists to make predictions. Though you may be correct in your hunch, your cookie jar conjecture doesn't fit this more rigorous definition.

All scientific disciplines have well-established, fundamental theories . For example, atomic theory describes the nature of matter and is supported by multiple lines of evidence from the way substances behave and react in the world around us (see our series on Atomic Theory ). Plate tectonic theory describes the large scale movement of the outer layer of the Earth and is supported by evidence from studies about earthquakes , magnetic properties of the rocks that make up the seafloor , and the distribution of volcanoes on Earth (see our series on Plate Tectonic Theory ). The theory of evolution by natural selection , which describes the mechanism by which inherited traits that affect survivability or reproductive success can cause changes in living organisms over generations , is supported by extensive studies of DNA , fossils , and other types of scientific evidence (see our Charles Darwin series for more information). Each of these major theories guides and informs modern research in those fields, integrating a broad, comprehensive set of ideas.

So how are these fundamental theories developed, and why are they considered so well supported? Let's take a closer look at some of the data and research supporting the theory of natural selection to better see how a theory develops.

Comprehension Checkpoint

The development of a scientific theory: Evolution and natural selection

The theory of evolution by natural selection is sometimes maligned as Charles Darwin 's speculation on the origin of modern life forms. However, evolutionary theory is not speculation. While Darwin is rightly credited with first articulating the theory of natural selection, his ideas built on more than a century of scientific research that came before him, and are supported by over a century and a half of research since.

The Fixity Notion: Linnaeus

Figure 1: Cover of the 1760 edition of Systema Naturae .

Research about the origins and diversity of life proliferated in the 18th and 19th centuries. Carolus Linnaeus , a Swedish botanist and the father of modern taxonomy (see our module Taxonomy I for more information), was a devout Christian who believed in the concept of Fixity of Species , an idea based on the biblical story of creation. The Fixity of Species concept said that each species is based on an ideal form that has not changed over time. In the early stages of his career, Linnaeus traveled extensively and collected data on the structural similarities and differences between different species of plants. Noting that some very different plants had similar structures, he began to piece together his landmark work, Systema Naturae, in 1735 (Figure 1). In Systema , Linnaeus classified organisms into related groups based on similarities in their physical features. He developed a hierarchical classification system , even drawing relationships between seemingly disparate species (for example, humans, orangutans, and chimpanzees) based on the physical similarities that he observed between these organisms. Linnaeus did not explicitly discuss change in organisms or propose a reason for his hierarchy, but by grouping organisms based on physical characteristics, he suggested that species are related, unintentionally challenging the Fixity notion that each species is created in a unique, ideal form.

The age of Earth: Leclerc and Hutton

Also in the early 1700s, Georges-Louis Leclerc, a French naturalist, and James Hutton , a Scottish geologist, began to develop new ideas about the age of the Earth. At the time, many people thought of the Earth as 6,000 years old, based on a strict interpretation of the events detailed in the Christian Old Testament by the influential Scottish Archbishop Ussher. By observing other planets and comets in the solar system , Leclerc hypothesized that Earth began as a hot, fiery ball of molten rock, mostly consisting of iron. Using the cooling rate of iron, Leclerc calculated that Earth must therefore be at least 70,000 years old in order to have reached its present temperature.

Hutton approached the same topic from a different perspective, gathering observations of the relationships between different rock formations and the rates of modern geological processes near his home in Scotland. He recognized that the relatively slow processes of erosion and sedimentation could not create all of the exposed rock layers in only a few thousand years (see our module The Rock Cycle ). Based on his extensive collection of data (just one of his many publications ran to 2,138 pages), Hutton suggested that the Earth was far older than human history – hundreds of millions of years old.

While we now know that both Leclerc and Hutton significantly underestimated the age of the Earth (by about 4 billion years), their work shattered long-held beliefs and opened a window into research on how life can change over these very long timescales.

Fossil studies lead to the development of a theory of evolution: Cuvier

Figure 2: Illustration of an Indian elephant jaw and a mammoth jaw from Cuvier's 1796 paper.

With the age of Earth now extended by Leclerc and Hutton, more researchers began to turn their attention to studying past life. Fossils are the main way to study past life forms, and several key studies on fossils helped in the development of a theory of evolution . In 1795, Georges Cuvier began to work at the National Museum in Paris as a naturalist and anatomist. Through his work, Cuvier became interested in fossils found near Paris, which some claimed were the remains of the elephants that Hannibal rode over the Alps when he invaded Rome in 218 BCE . In studying both the fossils and living species , Cuvier documented different patterns in the dental structure and number of teeth between the fossils and modern elephants (Figure 2) (Horner, 1843). Based on these data , Cuvier hypothesized that the fossil remains were not left by Hannibal, but were from a distinct species of animal that once roamed through Europe and had gone extinct thousands of years earlier: the mammoth. The concept of species extinction had been discussed by a few individuals before Cuvier, but it was in direct opposition to the Fixity of Species concept – if every organism were based on a perfectly adapted, ideal form, how could any cease to exist? That would suggest it was no longer ideal.

While his work provided critical evidence of extinction , a key component of evolution , Cuvier was highly critical of the idea that species could change over time. As a result of his extensive studies of animal anatomy, Cuvier had developed a holistic view of organisms , stating that the

number, direction, and shape of the bones that compose each part of an animal's body are always in a necessary relation to all the other parts, in such a way that ... one can infer the whole from any one of them ...

In other words, Cuvier viewed each part of an organism as a unique, essential component of the whole organism. If one part were to change, he believed, the organism could not survive. His skepticism about the ability of organisms to change led him to criticize the whole idea of evolution , and his prominence in France as a scientist played a large role in discouraging the acceptance of the idea in the scientific community.

Studies of invertebrates support a theory of change in species: Lamarck

Jean Baptiste Lamarck, a contemporary of Cuvier's at the National Museum in Paris, studied invertebrates like insects and worms. As Lamarck worked through the museum's large collection of invertebrates, he was impressed by the number and variety of organisms . He became convinced that organisms could, in fact, change through time, stating that

... time and favorable conditions are the two principal means which nature has employed in giving existence to all her productions. We know that for her time has no limit, and that consequently she always has it at her disposal.

This was a radical departure from both the fixity concept and Cuvier's ideas, and it built on the long timescale that geologists had recently established. Lamarck proposed that changes that occurred during an organism 's lifetime could be passed on to their offspring, suggesting, for example, that a body builder's muscles would be inherited by their children.

As it turned out, the mechanism by which Lamarck proposed that organisms change over time was wrong, and he is now often referred to disparagingly for his "inheritance of acquired characteristics" idea. Yet despite the fact that some of his ideas were discredited, Lamarck established a support for evolutionary theory that others would build on and improve.

Rock layers as evidence for evolution: Smith

In the early 1800s, a British geologist and canal surveyor named William Smith added another component to the accumulating evidence for evolution . Smith observed that rock layers exposed in different parts of England bore similarities to one another: These layers (or strata) were arranged in a predictable order, and each layer contained distinct groups of fossils . From this series of observations , he developed a hypothesis that specific groups of animals followed one another in a definite sequence through Earth's history, and this sequence could be seen in the rock layers. Smith's hypothesis was based on his knowledge of geological principles , including the Law of Superposition.

The Law of Superposition states that sediments are deposited in a time sequence, with the oldest sediments deposited first, or at the bottom, and newer layers deposited on top. The concept was first expressed by the Persian scientist Avicenna in the 11th century, but was popularized by the Danish scientist Nicolas Steno in the 17th century. Note that the law does not state how sediments are deposited; it simply describes the relationship between the ages of deposited sediments.

Figure 3: Engraving from William Smith's 1815 monograph on identifying strata by fossils.

Smith backed up his hypothesis with extensive drawings of fossils uncovered during his research (Figure 3), thus allowing other scientists to confirm or dispute his findings. His hypothesis has, in fact, been confirmed by many other scientists and has come to be referred to as the Law of Faunal Succession. His work was critical to the formation of evolutionary theory as it not only confirmed Cuvier's work that organisms have gone extinct , but it also showed that the appearance of life does not date to the birth of the planet. Instead, the fossil record preserves a timeline of the appearance and disappearance of different organisms in the past, and in doing so offers evidence for change in organisms over time.

The theory of evolution by natural selection: Darwin and Wallace

It was into this world that Charles Darwin entered: Linnaeus had developed a taxonomy of organisms based on their physical relationships, Leclerc and Hutton demonstrated that there was sufficient time in Earth's history for organisms to change, Cuvier showed that species of organisms have gone extinct , Lamarck proposed that organisms change over time, and Smith established a timeline of the appearance and disappearance of different organisms in the geological record .

Figure 4: Title page of the 1859 Murray edition of the Origin of Species by Charles Darwin.

Charles Darwin collected data during his work as a naturalist on the HMS Beagle starting in 1831. He took extensive notes on the geology of the places he visited; he made a major find of fossils of extinct animals in Patagonia and identified an extinct giant ground sloth named Megatherium . He experienced an earthquake in Chile that stranded beds of living mussels above water, where they would be preserved for years to come.

Perhaps most famously, he conducted extensive studies of animals on the Galápagos Islands, noting subtle differences in species of mockingbird, tortoise, and finch that were isolated on different islands with different environmental conditions. These subtle differences made the animals highly adapted to their environments .

This broad spectrum of data led Darwin to propose an idea about how organisms change "by means of natural selection" (Figure 4). But this idea was not based only on his work, it was also based on the accumulation of evidence and ideas of many others before him. Because his proposal encompassed and explained many different lines of evidence and previous work, they formed the basis of a new and robust scientific theory regarding change in organisms – the theory of evolution by natural selection .

Darwin's ideas were grounded in evidence and data so compelling that if he had not conceived them, someone else would have. In fact, someone else did. Between 1858 and 1859, Alfred Russel Wallace , a British naturalist, wrote a series of letters to Darwin that independently proposed natural selection as the means for evolutionary change. The letters were presented to the Linnean Society of London, a prominent scientific society at the time (see our module on Scientific Institutions and Societies ). This long chain of research highlights that theories are not just the work of one individual. At the same time, however, it often takes the insight and creativity of individuals to put together all of the pieces and propose a new theory . Both Darwin and Wallace were experienced naturalists who were familiar with the work of others. While all of the work leading up to 1830 contributed to the theory of evolution , Darwin's and Wallace's theory changed the way that future research was focused by presenting a comprehensive, well-substantiated set of ideas, thus becoming a fundamental theory of biological research.

Expanding, testing, and refining scientific theories
Genetics and evolution: Mendel and Dobzhansky

Since Darwin and Wallace first published their ideas, extensive research has tested and expanded the theory of evolution by natural selection . Darwin had no concept of genes or DNA or the mechanism by which characteristics were inherited within a species . A contemporary of Darwin's, the Austrian monk Gregor Mendel , first presented his own landmark study, Experiments in Plant Hybridization, in 1865 in which he provided the basic patterns of genetic inheritance , describing which characteristics (and evolutionary changes) can be passed on in organisms (see our Genetics I module for more information). Still, it wasn't until much later that a "gene" was defined as the heritable unit.

In 1937, the Ukrainian born geneticist Theodosius Dobzhansky published Genetics and the Origin of Species , a seminal work in which he described genes themselves and demonstrated that it is through mutations in genes that change occurs. The work defined evolution as "a change in the frequency of an allele within a gene pool" ( Dobzhansky, 1982 ). These studies and others in the field of genetics have added to Darwin's work, expanding the scope of the theory .

Evolution under a microscope: Lenski

More recently, Dr. Richard Lenski, a scientist at Michigan State University, isolated a single Escherichia coli bacterium in 1989 as the first step of the longest running experimental test of evolutionary theory to date – a true test meant to replicate evolution and natural selection in the lab.

After the single microbe had multiplied, Lenski isolated the offspring into 12 different strains , each in their own glucose-supplied culture, predicting that the genetic make-up of each strain would change over time to become more adapted to their specific culture as predicted by evolutionary theory . These 12 lines have been nurtured for over 40,000 bacterial generations (luckily bacterial generations are much shorter than human generations) and exposed to different selective pressures such as heat , cold, antibiotics, and infection with other microorganisms. Lenski and colleagues have studied dozens of aspects of evolutionary theory with these genetically isolated populations . In 1999, they published a paper that demonstrated that random genetic mutations were common within the populations and highly diverse across different individual bacteria . However, "pivotal" mutations that are associated with beneficial changes in the group are shared by all descendants in a population and are much rarer than random mutations, as predicted by the theory of evolution by natural selection (Papadopoulos et al., 1999).

Punctuated equilibrium: Gould and Eldredge

While established scientific theories like evolution have a wealth of research and evidence supporting them, this does not mean that they cannot be refined as new information or new perspectives on existing data become available. For example, in 1972, biologist Stephen Jay Gould and paleontologist Niles Eldredge took a fresh look at the existing data regarding the timing by which evolutionary change takes place. Gould and Eldredge did not set out to challenge the theory of evolution; rather they used it as a guiding principle and asked more specific questions to add detail and nuance to the theory. This is true of all theories in science: they provide a framework for additional research. At the time, many biologists viewed evolution as occurring gradually, causing small incremental changes in organisms at a relatively steady rate. The idea is referred to as phyletic gradualism , and is rooted in the geological concept of uniformitarianism . After reexamining the available data, Gould and Eldredge came to a different explanation, suggesting that evolution consists of long periods of stability that are punctuated by occasional instances of dramatic change – a process they called punctuated equilibrium .

Like Darwin before them, their proposal is rooted in evidence and research on evolutionary change, and has been supported by multiple lines of evidence. In fact, punctuated equilibrium is now considered its own theory in evolutionary biology. Punctuated equilibrium is not as broad of a theory as natural selection . In science, some theories are broad and overarching of many concepts, such as the theory of evolution by natural selection; others focus on concepts at a smaller, or more targeted, scale such as punctuated equilibrium. And punctuated equilibrium does not challenge or weaken the concept of natural selection; rather, it represents a change in our understanding of the timing by which change occurs in organisms , and a theory within a theory. The theory of evolution by natural selection now includes both gradualism and punctuated equilibrium to describe the rate at which change proceeds.

Hypotheses and laws: Other scientific concepts

One of the challenges in understanding scientific terms like theory is that there is not a precise definition even within the scientific community. Some scientists debate over whether certain proposals merit designation as a hypothesis or theory , and others mistakenly use the terms interchangeably. But there are differences in these terms. A hypothesis is a proposed explanation for an observable phenomenon. Hypotheses , just like theories , are based on observations from research . For example, LeClerc did not hypothesize that Earth had cooled from a molten ball of iron as a random guess; rather, he developed this hypothesis based on his observations of information from meteorites.

A scientist often proposes a hypothesis before research confirms it as a way of predicting the outcome of study to help better define the parameters of the research. LeClerc's hypothesis allowed him to use known parameters (the cooling rate of iron) to do additional work. A key component of a formal scientific hypothesis is that it is testable and falsifiable. For example, when Richard Lenski first isolated his 12 strains of bacteria , he likely hypothesized that random mutations would cause differences to appear within a period of time in the different strains of bacteria. But when a hypothesis is generated in science, a scientist will also make an alternative hypothesis , an explanation that explains a study if the data do not support the original hypothesis. If the different strains of bacteria in Lenski's work did not diverge over the indicated period of time, perhaps the rate of mutation was slower than first thought.

So you might ask, if theories are so well supported, do they eventually become laws? The answer is no – not because they aren't well-supported, but because theories and laws are two very different things. Laws describe phenomena, often mathematically. Theories, however, explain phenomena. For example, in 1687 Isaac Newton proposed a Theory of Gravitation, describing gravity as a force of attraction between two objects. As part of this theory, Newton developed a Law of Universal Gravitation that explains how this force operates. This law states that the force of gravity between two objects is inversely proportional to the square of the distance between those objects. Newton 's Law does not explain why this is true, but it describes how gravity functions (see our Gravity: Newtonian Relationships module for more detail). In 1916, Albert Einstein developed his theory of general relativity to explain the mechanism by which gravity has its effect. Einstein's work challenges Newton's theory, and has been found after extensive testing and research to more accurately describe the phenomenon of gravity. While Einstein's work has replaced Newton's as the dominant explanation of gravity in modern science, Newton's Law of Universal Gravitation is still used as it reasonably (and more simply) describes the force of gravity under many conditions. Similarly, the Law of Faunal Succession developed by William Smith does not explain why organisms follow each other in distinct, predictable ways in the rock layers, but it accurately describes the phenomenon.

Theories, hypotheses , and laws drive scientific progress

Theories, hypotheses , and laws are not simply important components of science, they drive scientific progress. For example, evolutionary biology now stands as a distinct field of science that focuses on the origins and descent of species . Geologists now rely on plate tectonics as a conceptual model and guiding theory when they are studying processes at work in Earth's crust . And physicists refer to atomic theory when they are predicting the existence of subatomic particles yet to be discovered. This does not mean that science is "finished," or that all of the important theories have been discovered already. Like evolution , progress in science happens both gradually and in short, dramatic bursts. Both types of progress are critical for creating a robust knowledge base with data as the foundation and scientific theories giving structure to that knowledge.

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Theories, hypotheses, and laws drive scientific progress

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What Is a Hypothesis? (Science)

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Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
B.A., Physics and Mathematics, Hastings College

A hypothesis (plural hypotheses) is a proposed explanation for an observation. The definition depends on the subject.

In science, a hypothesis is part of the scientific method. It is a prediction or explanation that is tested by an experiment. Observations and experiments may disprove a scientific hypothesis, but can never entirely prove one.

In the study of logic, a hypothesis is an if-then proposition, typically written in the form, "If X , then Y ."

In common usage, a hypothesis is simply a proposed explanation or prediction, which may or may not be tested.

Writing a Hypothesis

Most scientific hypotheses are proposed in the if-then format because it's easy to design an experiment to see whether or not a cause and effect relationship exists between the independent variable and the dependent variable . The hypothesis is written as a prediction of the outcome of the experiment.

Null Hypothesis and Alternative Hypothesis

Statistically, it's easier to show there is no relationship between two variables than to support their connection. So, scientists often propose the null hypothesis . The null hypothesis assumes changing the independent variable will have no effect on the dependent variable.

In contrast, the alternative hypothesis suggests changing the independent variable will have an effect on the dependent variable. Designing an experiment to test this hypothesis can be trickier because there are many ways to state an alternative hypothesis.

For example, consider a possible relationship between getting a good night's sleep and getting good grades. The null hypothesis might be stated: "The number of hours of sleep students get is unrelated to their grades" or "There is no correlation between hours of sleep and grades."

An experiment to test this hypothesis might involve collecting data, recording average hours of sleep for each student and grades. If a student who gets eight hours of sleep generally does better than students who get four hours of sleep or 10 hours of sleep, the hypothesis might be rejected.

But the alternative hypothesis is harder to propose and test. The most general statement would be: "The amount of sleep students get affects their grades." The hypothesis might also be stated as "If you get more sleep, your grades will improve" or "Students who get nine hours of sleep have better grades than those who get more or less sleep."

In an experiment, you can collect the same data, but the statistical analysis is less likely to give you a high confidence limit.

Usually, a scientist starts out with the null hypothesis. From there, it may be possible to propose and test an alternative hypothesis, to narrow down the relationship between the variables.

Example of a Hypothesis

Examples of a hypothesis include:

If you drop a rock and a feather, (then) they will fall at the same rate.
Plants need sunlight in order to live. (if sunlight, then life)
Eating sugar gives you energy. (if sugar, then energy)
White, Jay D. Research in Public Administration . Conn., 1998.
Schick, Theodore, and Lewis Vaughn. How to Think about Weird Things: Critical Thinking for a New Age . McGraw-Hill Higher Education, 2002.
Null Hypothesis Definition and Examples
Definition of a Hypothesis
What Are the Elements of a Good Hypothesis?
Six Steps of the Scientific Method
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What Are Examples of a Hypothesis?
Understanding Simple vs Controlled Experiments
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Scientific Method Vocabulary Terms
What Is a Testable Hypothesis?
Null Hypothesis Examples
What 'Fail to Reject' Means in a Hypothesis Test
How To Design a Science Fair Experiment
What Is an Experiment? Definition and Design
Hypothesis Test for the Difference of Two Population Proportions

The Scientific Hypothesis

The Key to Understanding How Science Works

Hypotheses, Theories, Laws (and Models)… What’s the difference?

Untold hours have been spent trying to sort out the differences between these ideas. should we bother.

Ask what the differences between these concepts are and you’re likely to encounter a raft of distinctions; typically with charts and ladders of generality leading from hypotheses to theories and, ultimately, to laws. Countless students have been exposed to and forced to learn how the schemes are set up. Theories are said to be well-tested hypotheses, or maybe whole collections of linked hypotheses, and laws, well, laws are at the top of the heap, the apex of science having enormous reach, quantitative predictive power, and validity. It all seems so clear.

Yet there are many problems with the general scheme. For one thing, it is never quite explained how a hypothesis turns into a theory or law and, consequently, the boundaries are blurry, and definitions tend vary with the speaker. And there is no consistency in usage across fields, I’ll give some examples in a minute. There are branches of science that have few if any theories and no laws – neuroscience comes to mind – though no one doubts that neuroscience is a bona fide science that has discovered great quantities of reliable and useful information and wide-ranging generalizations. At the other extreme, there are sciences that spin out theories at a dizzying pace – psychology, for instance – although the permanence and indeed the veracity of psychological theories are rarely on par with those of physics or chemistry.

Some people will tell you that theories and laws are “more quantitative” than hypotheses, but the most famous theory in biology, the Theory of Evolution, which is based on concepts such as heritability, genetic variability, natural selection, etc. is not as neatly expressible in quantitative terms as is Newton’s Theory of Gravity, for example. And what do we make of the fact that Newton’s “Law of Gravity” was superceded by Einstein’s “General Theory (not Law) of Relativity?”

What about the idea that a hypothesis is a low-level explanation that somehow transmogrifies into a theory when conditions are right? Even this simple rule is not adhered to. Take geology (or “geoscience” nowadays): We have the Alvarez Hypothesis about how an asteroid slamming into the earth caused the extinction of dinosaurs and other life-forms ~66 million years ago. The Alvarez Hypothesis explains, often in quantitative detail, many important phenomena and makes far-reaching predictions, most remarkably of a crater, which was eventually found in the Yucatan peninsula, that has the right age and size to be the site of an extinction-causing asteroid impact. The Alvarez Hypothesis has been rigorously tested many times since it was proposed, without having been promoted to a theory.

But perhaps the Alvarez Hypothesis is still thought to be a tentative explanation, not yet worthy of a more exalted status? It seems that the same can’t be said about the idea that the earth’s crust consists of 12 or so rigid “plates” of solid material that drift around very slowly and create geological phenomena, such as mountain ranges and earth-quakes, when they crash into each other. This is called either the “Plate Tectonics Hypothesis” or “Plate Tectonics Theory” by different authors. Same data, same interpretations, same significance, different names.

And for anyone trying to make sense of the hypothesis-theory-law progression, it must be highly confusing to learn that the crowning achievement of modern physics – itself the “queen of the sciences” – is a complex, extraordinarily precise, quantitative structure is known as the Standard Model of Particle Physics, not the Standard Theory, or the Standard Law! The Standard Model incorporates three of the four major forces of nature, describes many subatomic particles, and has successfully predicted numerous subtle properties of subatomic particles. Does this mean that “model” now implies a large, well-worked out and self-consistent body of scientific knowledge? Not at all; in fact, “model” and “hypothesis” are used interchangeably at the simplest levels of experimental investigation in biology, neuroscience, etc., so definition-wise, we’re back to the beginning.

The reason that the Standard Model is a model and not a theory seems basically to be the same as the reason that the Alvarez Hypothesis is a hypothesis and not a theory or that Evolution is a theory and not a law: essentially it is a matter of convention, tradition, or convenience. The designations, we can infer, are primarily names that lack exact substantive, generally agreed-on definitions.

So, rather than worrying about any profound distinctions between hypotheses, theories, laws (and models) it might be more helpful to look at the properties that they have in common:

1. They are all “conjectural” which, for the moment, means that they are inventions of the human mind.

2. They make specific predictions that are empirically testable, in principle.

3. They are falsifiable – if their predictions are false, they are false – though not provable, by experiment or observation.

4. As a consequence of point 3., hypotheses, theories, and laws are all provisional; they may be replaced as further information becomes available.

“Hypothesis,” it seems to me, is the fundamental unit, the building block, of scientific thinking. It is the term that is most consistently used by all sciences; it is more basic than any theory; it carries the least baggage, is the least susceptible to multiple interpretations and, accordingly, is the most likely to communicate effectively. These advantages are relative of course; as I’ll get into elsewhere, even “hypothesis” is the subject of misinterpretation. In any case, its simplicity and clarity are why this website is devoted to the Scientific Hypothesis and not the others.

Theory vs. Hypothesis vs. Law… Explained!

Some people try to attack things like evolution by natural selection and man-made climate change by saying “Oh, that’s just a THEORY!”

Yes, they are both theories. Stop saying it like it’s a bad thing! It’s time to learn the difference between a fact, a theory, a hypothesis, and a scientific law.

Special thanks to Joe Hanson, Ph.D., for allowing us to publish his terrific videos.

It’s Okay To Be Smart is written and hosted by Joe Hanson, Ph.D. @jtotheizzoe Facebook: http://www.facebook.com/itsokaytobesmart For more awesome science, check out: http://www.itsokaytobesmart.com Produced by PBS Digital Studios: http://www.youtube.com/user/pbsdigita…

Joe Hanson – Creator/Host/Writer Joe Nicolosi – Director Amanda Fox – Producer, Spotzen Inc. Kate Eads – Producer Andrew Matthews – Editing/Motion Graphics/Animation Katie Graham – Camera John Knudsen – Gaffer

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What is a scientific hypothesis?

It's the initial building block in the scientific method.

A girl looks at plants in a test tube for a science experiment. What's her scientific hypothesis?

Hypothesis basics

What makes a hypothesis testable.

Types of hypotheses
Hypothesis versus theory

Additional resources

Bibliography.

A scientific hypothesis is a tentative, testable explanation for a phenomenon in the natural world. It's the initial building block in the scientific method . Many describe it as an "educated guess" based on prior knowledge and observation. While this is true, a hypothesis is more informed than a guess. While an "educated guess" suggests a random prediction based on a person's expertise, developing a hypothesis requires active observation and background research.

The basic idea of a hypothesis is that there is no predetermined outcome. For a solution to be termed a scientific hypothesis, it has to be an idea that can be supported or refuted through carefully crafted experimentation or observation. This concept, called falsifiability and testability, was advanced in the mid-20th century by Austrian-British philosopher Karl Popper in his famous book "The Logic of Scientific Discovery" (Routledge, 1959).

A key function of a hypothesis is to derive predictions about the results of future experiments and then perform those experiments to see whether they support the predictions.

A hypothesis is usually written in the form of an if-then statement, which gives a possibility (if) and explains what may happen because of the possibility (then). The statement could also include "may," according to California State University, Bakersfield .

Here are some examples of hypothesis statements:

If garlic repels fleas, then a dog that is given garlic every day will not get fleas.
If sugar causes cavities, then people who eat a lot of candy may be more prone to cavities.
If ultraviolet light can damage the eyes, then maybe this light can cause blindness.

A useful hypothesis should be testable and falsifiable. That means that it should be possible to prove it wrong. A theory that can't be proved wrong is nonscientific, according to Karl Popper's 1963 book " Conjectures and Refutations ."

An example of an untestable statement is, "Dogs are better than cats." That's because the definition of "better" is vague and subjective. However, an untestable statement can be reworded to make it testable. For example, the previous statement could be changed to this: "Owning a dog is associated with higher levels of physical fitness than owning a cat." With this statement, the researcher can take measures of physical fitness from dog and cat owners and compare the two.

Types of scientific hypotheses

Elementary-age students study alternative energy using homemade windmills during public school science class.

In an experiment, researchers generally state their hypotheses in two ways. The null hypothesis predicts that there will be no relationship between the variables tested, or no difference between the experimental groups. The alternative hypothesis predicts the opposite: that there will be a difference between the experimental groups. This is usually the hypothesis scientists are most interested in, according to the University of Miami .

For example, a null hypothesis might state, "There will be no difference in the rate of muscle growth between people who take a protein supplement and people who don't." The alternative hypothesis would state, "There will be a difference in the rate of muscle growth between people who take a protein supplement and people who don't."

If the results of the experiment show a relationship between the variables, then the null hypothesis has been rejected in favor of the alternative hypothesis, according to the book " Research Methods in Psychology " (BCcampus, 2015).

There are other ways to describe an alternative hypothesis. The alternative hypothesis above does not specify a direction of the effect, only that there will be a difference between the two groups. That type of prediction is called a two-tailed hypothesis. If a hypothesis specifies a certain direction — for example, that people who take a protein supplement will gain more muscle than people who don't — it is called a one-tailed hypothesis, according to William M. K. Trochim , a professor of Policy Analysis and Management at Cornell University.

Sometimes, errors take place during an experiment. These errors can happen in one of two ways. A type I error is when the null hypothesis is rejected when it is true. This is also known as a false positive. A type II error occurs when the null hypothesis is not rejected when it is false. This is also known as a false negative, according to the University of California, Berkeley .

A hypothesis can be rejected or modified, but it can never be proved correct 100% of the time. For example, a scientist can form a hypothesis stating that if a certain type of tomato has a gene for red pigment, that type of tomato will be red. During research, the scientist then finds that each tomato of this type is red. Though the findings confirm the hypothesis, there may be a tomato of that type somewhere in the world that isn't red. Thus, the hypothesis is true, but it may not be true 100% of the time.

Scientific theory vs. scientific hypothesis

The best hypotheses are simple. They deal with a relatively narrow set of phenomena. But theories are broader; they generally combine multiple hypotheses into a general explanation for a wide range of phenomena, according to the University of California, Berkeley . For example, a hypothesis might state, "If animals adapt to suit their environments, then birds that live on islands with lots of seeds to eat will have differently shaped beaks than birds that live on islands with lots of insects to eat." After testing many hypotheses like these, Charles Darwin formulated an overarching theory: the theory of evolution by natural selection.

"Theories are the ways that we make sense of what we observe in the natural world," Tanner said. "Theories are structures of ideas that explain and interpret facts."

Read more about writing a hypothesis, from the American Medical Writers Association.
Find out why a hypothesis isn't always necessary in science, from The American Biology Teacher.
Learn about null and alternative hypotheses, from Prof. Essa on YouTube .

Encyclopedia Britannica. Scientific Hypothesis. Jan. 13, 2022. https://www.britannica.com/science/scientific-hypothesis

Karl Popper, "The Logic of Scientific Discovery," Routledge, 1959.

California State University, Bakersfield, "Formatting a testable hypothesis." https://www.csub.edu/~ddodenhoff/Bio100/Bio100sp04/formattingahypothesis.htm

Karl Popper, "Conjectures and Refutations," Routledge, 1963.

Price, P., Jhangiani, R., & Chiang, I., "Research Methods of Psychology — 2nd Canadian Edition," BCcampus, 2015.‌

University of Miami, "The Scientific Method" http://www.bio.miami.edu/dana/161/evolution/161app1_scimethod.pdf

William M.K. Trochim, "Research Methods Knowledge Base," https://conjointly.com/kb/hypotheses-explained/

University of California, Berkeley, "Multiple Hypothesis Testing and False Discovery Rate" https://www.stat.berkeley.edu/~hhuang/STAT141/Lecture-FDR.pdf

University of California, Berkeley, "Science at multiple levels" https://undsci.berkeley.edu/article/0_0_0/howscienceworks_19

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Formative Assessment Probe

What Is a Hypothesis?

By Page Keeley

Uncovering Student Ideas in Science, Volume 3: Another 25 Formative Assessment Probes

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This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

Access this probe as a Google form: English

Download this probe as an editable PDF: English

The purpose of this assessment probe is to elicit students’ ideas about hypotheses. The probe is designed to find out if students understand what a hypothesis is, when it is used, and how it is developed.

Type of Probe

Justified List

Related Concepts

hypothesis, nature of science, scientific inquiry, scientific method

Explanation

The best choices are A, B, G, K, L, and M. However, other possible answers open up discussions to contrast with the provided definition. A hypothesis is a tentative explanation that can be tested and is based on observation and/or scientific knowledge such as that that has been gained from doing background research. Hypotheses are used to investigate a scientific question. Hypotheses can be tested through experimentation or further observation, but contrary to how some students are taught to use the “scientific method,” hypotheses are not proved true or correct. Students will often state their conclusions as “My hypothesis is correct because my data prove…,” thereby equating positive results with proof (McLaughlin 2006, p. 61). In essence, experimentation as well as other means of scientific investigation never prove a hypothesis—the hypothesis gains credibility from the evidence obtained from data that support it. Data either support or negate a hypothesis but never prove something to be 100% true or correct.

Hypotheses are often confused with questions. A hypothesis is not framed as a question but rather provides a tentative explanation in response to the scientific question that leads the investigation. Sometimes the word hypothesis is oversimplified by being defined as “an educated guess.” This terminology fails to convey the explanatory or predictive nature of scientific hypotheses and omits what is most important about hypotheses: their purpose. Hypotheses are developed to explain observations, such as notable patterns in nature; predict the outcome of an experiment based on observations or prior scientific knowledge; and guide the investigator in seeking and paying attention to the right data. Calling a hypothesis a “guess” undermines the explanation that underscores a hypothesis.

Predictions and hypotheses are not the same. A hypothesis, which is a tentative explanation, can lead to a prediction. Predictions forecast the outcome of an experiment but do not include an explanation. Predictions often use if-then statements, just as hypotheses do, but this does not make a prediction a hypothesis. For example, a prediction might take the form of, “If I do [X], then [Y] will happen.” The prediction describes the outcome but it does not provide an explanation of why that outcome might result or describe any relationship between variables.

Sometimes the words hypothesis , theory , and law are inaccurately portrayed in science textbooks as a hierarchy of scientific knowledge, with the hypothesis being the first step on the way to becoming a theory and then a law. These concepts do not form a sequence for the development of scientific knowledge because each represents a different type of knowledge.

Not every investigation requires a hypothesis. Some types of investigations do not lend themselves to hypothesis testing through experimentation. A good deal of science is observational and descriptive—the study of biodiversity, for example, usually involves looking at a wide variety of specimens and maybe sketching and recording their unique characteristics. A biologist studying biodiversity might wonder, “What types of birds are found on island X?” The biologist would observe sightings of birds and perhaps sketch them and record their bird calls but would not be guided by a specific hypothesis. Many of the great discoveries in science did not begin with a hypothesis in mind. For example, Charles Darwin did not begin his observations of species in the Galapagos with a hypothesis in mind.

Contrary to the way hypotheses are often stated by students as an unimaginative response to a question posed at the beginning of an experiment, particularly those of the “cookbook” type, the generation of hypotheses by scientists is actually a creative and imaginative process, combined with the logic of scientific thought. “The process of formulating and testing hypotheses is one of the core activities of scientists. To be useful, a hypothesis should suggest what evidence would support it and what evidence would refute it. A hypothesis that cannot in principle be put to the test of evidence may be interesting, but it is not likely to be scientifically useful” (AAAS 1988, p. 5).

Curricular and Instructional Considerations

Elementary Students

In the elementary school grades, students typically engage in inquiry to begin to construct an understanding of the natural world. Their inquiries are initiated by a question. If students have a great deal of knowledge or have made prior observations, they might propose a hypothesis; in most cases, however, their knowledge and observations are too incomplete for them to hypothesize. If elementary school students are required to develop a hypothesis, it is often just a guess, which does little to contribute to an understanding of the purpose of a hypothesis. At this grade level, it is usually sufficient for students to focus on their questions, instead of hypotheses (Pine 1999).

Middle School Students

At the middle school level, students develop an understanding of what a hypothesis is and when one is used. The notion of a testable hypothesis through experimentation that involves variables is introduced and practiced at this grade level. However, there is a danger that students will think every investigation must include a hypothesis. Hypothesizing as a skill is important to develop at this grade level but it is also important to develop the understandings of what a hypothesis is and why and how it is developed.

High School Students

At this level, students have acquired more scientific knowledge and experiences and so are able to propose tentative explanations. They can formulate a testable hypothesis and demonstrate the logical connections between the scientific concepts guiding a hypothesis and the design of an experiment (NRC 1996).

Administering the Probe

This probe is best used as is at the middle school and high school levels, particularly if students have been previously exposed to the word hypothesis or its use. Remove any answer choices students might not be familiar with. For example, if they have not encountered if-then reasoning, eliminate this distracter. The probe can also be modified as a simpler version for students in grades 3–5 by leaving out some of the choices and simplifying the descriptions.

K–4 Understandings About Scientific Inquiry

Scientific investigations involve asking and answering a question and comparing the answer with what scientists already know about the world.
Scientists develop explanations using observations (evidence) and what they already know about the world (scientific knowledge).

5–8 Understandings About Scientific Inquiry

Different kinds of questions suggest different kinds of investigations. Some investigations involve observing and describing objects, organisms, or events; some involve collecting specimens; some involve experiments; some involve seeking more information; some involve discovery of new objects and phenomena; and some involve making models.
Current scientific knowledge and understanding guide scientific investigations. Different scientific domains employ different methods, core theories, and standards to advance scientific knowledge and understanding.

5–8 Science as a Human Endeavor

Science is very much a human endeavor, and the work of science relies on basic human qualities such as reasoning, insight, energy, skill, and creativity.

9–12 Abilities Necessary to Do Scientific Inquiry

Identify questions and concepts that guide scientific investigations.*

9–12 Understandings About Scientific Inquiry

Scientists usually inquire about how physical, living, or designed systems function. Conceptual principles and knowledge guide scientific inquiries. Historical and current scientific knowledge influence the design and interpretation of investigations and the evaluation of proposed explanations made by other scientists.

*Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal.

K–2 Scientific Inquiry

People can often learn about things around them by just observing those things carefully, but sometimes they can learn more by doing something to the things and noting what happens.

3–5 Scientific Inquiry

Scientists’ explanations about what happens in the world come partly from what they observe and partly from what they think. Sometimes scientists have different explanations for the same set of observations. That usually leads to their making more observations to resolve the differences.

6–8 Scientific Inquiry

Scientists differ greatly in what phenomena they study and how they go about their work. Although there is no fixed set of steps that all scientists follow, scientific investigations usually involve the collection of relevant evidence, the use of logical reasoning, and the application of imagination in devising hypotheses and explanations to make sense of the collected evidence.*

6–8 Values and Attitudes

Even if they turn out not to be true, hypotheses are valuable if they lead to fruitful investigations.*

9–12 Scientific Inquiry

Hypotheses are widely used in science for choosing what data to pay attention to and what additional data to seek and for guiding the interpretation of the data (both new and previously available).*

Related Research

Students generally have difficulty with explaining how science is conducted because they have had little contact with real scientists. Their familiarity with doing science, even at older ages, is “school science,” which is often not how science is generally conducted in the scientific community (Driver et al. 1996).
Despite over 10 years of reform efforts in science education, research still shows that students typically have inadequate conceptions of what science is and what scientists do (Schwartz 2007).
Upper elementary school and middle school students may not understand experimentation as a method of testing ideas, but rather as a method of trying things out or producing a desired outcome (AAAS 1993).
Middle school students tend to invoke personal experiences as evidence to justify their hypothesis. They seem to think of evidence as selected from what is already known or from personal experience or secondhand sources, not as information produced through experiment (AAAS 1993).

Related NSTA Resources

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

McLaughlin, J. 2006. A gentle reminder that a hypothesis is never proven correct, nor is a theory ever proven true. Journal of College Science Teaching 36 (1): 60–62.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

Schwartz, R. 2007. What’s in a word? How word choice can develop (mis)conceptions about the nature of science. Science Scope 31 (2): 42–47.

VanDorn, K., M. Mavita, L. Montes, B. Ackerson, and M. Rockley. 2004. Hypothesis-based learning. Science Scope 27: 24–25.

Suggestions for Instruction and Assessment

The “scientific method” is often the first topic students encounter when using textbooks and this can erroneously imply that there is a rigid set of steps that all scientists follow, including the development of a hypothesis. Often the scientific method described in textbooks applies to experimentation, which is only one of many ways scientists conduct their work. Embedding explicit instruction of the various ways to do science in the actual investigations students do throughout the year as well as in their studies of investigations done by scientists is a better approach to understanding how science is done than starting off the year with the scientific method in a way that is devoid of a context through which students can learn the content and process of science.
Students often participate in science fairs that may follow a textbook scientific method of posing a question, developing a hypothesis, and so on, that incorrectly results in students “proving” their hypothesis. Make sure students understand that a hypothesis can be disproven, but it is never proven, which implies 100% certainty.
Help students understand that science begins with a question. The structure of some school lab reports may lead students to believe that all investigations begin with a hypothesis. While some investigations do begin with a hypothesis, in most cases, they begin with a question. Sometimes it is just a general question.
A technique to help students maintain a consistent image of science as inquiry throughout the year by paying more careful attention to the words they use is to create a “caution words” poster or bulletin board (Schwartz 2007). Important words that have specific meanings in science but are often used inappropriately in the science classroom and through everyday language can be posted in the room as a reminder to pay careful attention to how students are using these words. For example, words like hypothesis and scientific method can be posted here. Words that are banned when referring to hypotheses include prove, correct, and true.
Use caution when asking students to write lab reports that use the same format regardless of the type of investigation conducted. The format used in writing about an investigation may imply a rigid, fixed process or erroneously misrepresent aspects of science, such as that hypotheses are developed for every scientific investigation.
Avoid using hypotheses with younger children when they result in guesses. It is better to start with a question and have students make a prediction about what they think will happen and why. As they acquire more conceptual understanding and experience a variety of observations, they will be better prepared to develop hypotheses that reflect the way science is done.
Avoid using “educated guess” as a description for hypothesis. The common meaning of the word guess implies no prior knowledge, experience, or observations.
Scaffold hypothesis writing for students by initially having them use words like may in their statements and then formalizing them with if-then statements. For example, students may start with the statement, “The growth of algae may be affected by temperature.” The next step would be to extend this statement to include a testable relationship, such as, “If the temperature of the water increases, then the algae population will increase.” Encourage students to propose a tentative explanation and then consider how they would go about testing the statement.

American Association for the Advancement of Science (AAAS). 1988. Science for all Americans. New York: Oxford University Press.

Driver, R., J. Leach, R. Millar, and P. Scott. 1996. Young people’s images of science. Buckingham, UK: Open University Press.

Pine, J. 1999. To hypothesize or not to hypothesize. In Foundations: A monograph for professionals in science, mathematics, and technology education. Vol. 2. Inquiry: Thoughts, views, and strategies for the K–5 classroom. Arlington, VA: National Science Foundation.

Definition of hypothesis

Did you know.

The Difference Between Hypothesis and Theory

A hypothesis is an assumption, an idea that is proposed for the sake of argument so that it can be tested to see if it might be true.

In the scientific method, the hypothesis is constructed before any applicable research has been done, apart from a basic background review. You ask a question, read up on what has been studied before, and then form a hypothesis.

A hypothesis is usually tentative; it's an assumption or suggestion made strictly for the objective of being tested.

A theory , in contrast, is a principle that has been formed as an attempt to explain things that have already been substantiated by data. It is used in the names of a number of principles accepted in the scientific community, such as the Big Bang Theory . Because of the rigors of experimentation and control, it is understood to be more likely to be true than a hypothesis is.

In non-scientific use, however, hypothesis and theory are often used interchangeably to mean simply an idea, speculation, or hunch, with theory being the more common choice.

Since this casual use does away with the distinctions upheld by the scientific community, hypothesis and theory are prone to being wrongly interpreted even when they are encountered in scientific contexts—or at least, contexts that allude to scientific study without making the critical distinction that scientists employ when weighing hypotheses and theories.

The most common occurrence is when theory is interpreted—and sometimes even gleefully seized upon—to mean something having less truth value than other scientific principles. (The word law applies to principles so firmly established that they are almost never questioned, such as the law of gravity.)

This mistake is one of projection: since we use theory in general to mean something lightly speculated, then it's implied that scientists must be talking about the same level of uncertainty when they use theory to refer to their well-tested and reasoned principles.

The distinction has come to the forefront particularly on occasions when the content of science curricula in schools has been challenged—notably, when a school board in Georgia put stickers on textbooks stating that evolution was "a theory, not a fact, regarding the origin of living things." As Kenneth R. Miller, a cell biologist at Brown University, has said , a theory "doesn’t mean a hunch or a guess. A theory is a system of explanations that ties together a whole bunch of facts. It not only explains those facts, but predicts what you ought to find from other observations and experiments.”

While theories are never completely infallible, they form the basis of scientific reasoning because, as Miller said "to the best of our ability, we’ve tested them, and they’ve held up."

proposition
supposition

hypothesis , theory , law mean a formula derived by inference from scientific data that explains a principle operating in nature.

hypothesis implies insufficient evidence to provide more than a tentative explanation.

theory implies a greater range of evidence and greater likelihood of truth.

law implies a statement of order and relation in nature that has been found to be invariable under the same conditions.

Examples of hypothesis in a Sentence

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'hypothesis.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Greek, from hypotithenai to put under, suppose, from hypo- + tithenai to put — more at do

1641, in the meaning defined at sense 1a

Phrases Containing hypothesis

counter - hypothesis
nebular hypothesis
null hypothesis
planetesimal hypothesis
Whorfian hypothesis

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Cite this Entry

“Hypothesis.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/hypothesis. Accessed 2 May. 2024.

Kids Definition

Kids definition of hypothesis, medical definition, medical definition of hypothesis, more from merriam-webster on hypothesis.

Nglish: Translation of hypothesis for Spanish Speakers

Britannica English: Translation of hypothesis for Arabic Speakers

Britannica.com: Encyclopedia article about hypothesis

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Avogadro's Law

What is avogadro’s law.

Avogadro’s law, also known as Avogadro’s principle or Avogadro’s hypothesis, is a gas law which states that the total number of atoms/molecules of a gas (i.e. the amount of gaseous substance) is directly proportional to the volume occupied by the gas at constant temperature and pressure.

Avogadro’s law is closely related to the ideal gas equation since it links temperature, pressure, volume, and amount of substance for a given gas.

Table of Content

Formula and graphical representation, molar volume of a gas, examples of avogadros law, what are the limitations of avogadro’s law, solved exercises on avogadro’s law, recommended videos.

Frequently Asked Questions – FAQs

Avogadro’s law is named after the Italian scientist Amedeo Carlo Avogadro, who suggested that two dissimilar ideal gases occupying the same volume at a given (constant) temperature and pressure must contain an equal number of molecules.

At constant pressure and temperature, Avogadro’s law can be expressed via the following formula:

Where V is the volume of the gas, n denotes the amount of gaseous substance (often expressed in moles), and k is a constant. When the amount of gaseous substance is increased, the corresponding increase in the volume occupied by the gas can be calculated with the help of the following formula:

V 1 /n 1 = V 2 /n 2 ( = k, as per Avogadro’s law).

The graphical representation of Avogadro’s law (with the amount of substance on the X-axis and volume on the Y-axis) is illustrated below.

Here, the straight line (which indicates that the two quantities are directly proportional) passes through the origin, implying that zero moles of gas will occupy zero volume.

Avogadro’s law can be derived from the ideal gas equation, which can be expressed as follows:

‘P’ is the pressure exerted by the gas on the walls of its container
‘V’ is the volume occupied by the gas
‘n’ is the amount of gaseous substance (number of moles of gas)
‘R’ is the universal gas constant
‘T’ is the absolute temperature of the gas

Rearranging the ideal gas equation, the following equation can be obtained.

V/n = (RT)/P

Here, the value of (RT)/P is a constant (since the temperature and pressure kept constant and the product/quotient of two or more constants is always a constant). Therefore:

Thus, the proportionality between the volume occupied by a gas and the number of gaseous molecules is verified.

As per Avogadro’s law, the ratio of volume and amount of gaseous substance is a constant (at constant pressure and temperature). The value of this constant (k) can be determined with the help of the following equation:

Under standard conditions for temperature and pressure, the value of T corresponds to 273.15 Kelvin and the value of P corresponds to 101.325 kilo Pascals. Therefore, the volume occupied by one mole of a gas at STP is:

Volume occupied by 1 mole of gas = (8.314 J.mol -1 .K -1 )*(273.15 K)/(101.325 kPa) = 22.4 litres

Therefore, one mole of any gaseous substance occupies 22.4 litres of volume at STP .

The process of respiration is a great example of Avogadro’s law. When humans inhale, the increase in the molar quantity of air in the lungs is accompanied by an increase in the volume of the lungs (expansion of the lungs). An image detailing the change in volume brought on by an increase in the number of gaseous molecules is provided below.

Another common example of Avogadro’s law is the deflation of automobile tyres. When the air trapped inside the tyre escapes, the number of moles of air present in the tyre decreases. This results in a decrease in the volume occupied by the gas, causing the tyre to lose its shape and deflate.

Despite being perfectly applicable to ideal gases, Avogadro’s law provides only approximate relationships for real gases. The deviation of real gases from ideal behaviour increases at low pressure and high temperature.

It is important to note that gases molecules having relatively low molecular masses (such as helium and hydrogen) obey Avogadro’s law to a greater extent than heavier molecules.

One mole of helium gas fills up an empty balloon to a volume of 1.5 litres. What would be the volume of the balloon if an additional 2.5 moles of helium gas is added? (Assume that the temperature and the pressure are kept constant)

The initial amount of helium (n 1 ) = 1 mol

The initial volume of the balloon (V 1 ) = 1.5 L

The final amount of helium (n 2 ) = 1 mol + 2.5 mol = 3.5 mol

As per Avogadro’s law, V 1 /n 1 = V 2 /n 2

Therefore, the final volume of the balloon (V 2 ) = (V 1 n 2 )/n 1 = (1.5L*3.5mol)/1mol = 5.25 L

The balloon would occupy a volume of 5.25 litres when it contains 3.5 moles of helium gas.

A tyre containing 10 moles of air and occupying a volume of 40L loses half its volume due to a puncture. Considering that the pressure and temperature remain constant, what would be the amount of air in the deflated tyre?

The initial amount of air (n 1 ) = 10 mol

The initial volume of the tyre (V 1 ) = 40 L

The final volume of the tyre (V 2 ) = 20 L

According to Avogadro’s law, the final amount of air in the tyre (n 2 ) = (V 2 n 1 )/V 1 = 5 moles.

The deflated tyre would contain 5 moles of air.

Frequently Asked Questions – FAQs

What does avogadro’s law state.

Avogadro’s law states that equal volumes of different gases contain an equal number of molecules under the same conditions of temperature and pressure.

Why is Avogadro’s law important?

The link between the amount of gas (n) and the volume (V) is investigated via Avogadro’s law (v). It’s a direct relationship, which means the volume of a gas is proportional to the number of moles contained in the gas sample. The law is significant because it allows us to save time and money over time.

What does Charles law state?

The physics theory known as Charles’ law asserts that the volume of a gas equals a constant value multiplied by its Kelvin temperature.

What is Avogadro’s Law in simple terms?

Why is avogadro’s law only for gases.

This is because there is so much space between each molecule that the size of the molecule has no bearing on the volume of the material. This is why the volume of a gas is governed by the pressure applied to it, and why under the same pressure, all gases have the same volume.

What are the limitations of Avogadro law?

What are the applications of avogadro law, can we apply the ideal gas law to liquids, when was avogadro’s law discovered, why was avogadro’s law rejected.

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Universities Face an Urgent Question: What Makes a Protest Antisemitic?

Pro-Palestinian student activists say their movement is anti-Zionist but not antisemitic. It is not a distinction that everyone accepts.

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An overhead view of Columbia University’s campus at night, with multicolored tents and tarps partly filling one section of lawn and a star of David on another stretch of grass.

By Katherine Rosman

Katherine Rosman reported from the Columbia University campus.

In a video shared widely online, a leader of the pro-Palestinian student movement at Columbia University stands near the center of a lawn on the campus and calls out, “We have Zionists who have entered the camp.”

Dozens of protesters, who have created a tent village called the “Gaza Solidarity Encampment,” repeat his words back to him: “We have Zionists who have entered the camp.”

“Walk and take a step forward,” the leader says, as the students continue to repeat his every utterance, “so that we can start to push them out of the camp. ”

The protesters link arms and march in formation toward three Jewish students who have come inside the encampment.

“It was really scary because we had like 75 people quickly gathered around, encircling us, doing exactly what he said to do,” Avi Weinberg, one of the Jewish students, said in an interview. He and his friends had gone to see the encampment, not intending to provoke, he said. When it began to feel tense, one of the students started to record the encounter. They are not sure precisely how the protest leader determined they were supportive of Israel.

“Suddenly we are being called ‘the Zionists’ in their encampment,” Mr. Weinberg said. “He put a target on our back.”

On Thursday, the incident took on new significance when a video from January resurfaced on social media showing the same protest leader, Khymani James, saying “Zionists don’t deserve to live” and “Be grateful that I’m not just going out and murdering Zionists.”

The next day, Columbia officials announced they had barred Mr. James from campus.

Columbia has been ground zero in a national student movement against Israel’s treatment of Palestinians, with protesters setting up encampments on campuses across the country. Hundreds of demonstrators — at Columbia, Yale, Emerson College, the University of Southern California and beyond — have been arrested.

Protesters occupied

Hamilton Hall early

Tuesday morning

West 114th St.

Tent encampment at

Columbia University

Faculty and staff

members guarding

access to the tents

Amsterdam Ave.

Source: Google Earth

Note: Photograph taken Monday, April 29

By Leanne Abraham; Photograph by Bing Guan

Pro-Palestinian demonstrators across the country say Israel is committing what they see as genocide against the Palestinian people, and they aim to keep a spotlight on the suffering. But some Jewish students who support Israel and what they see as its right to defend itself against Hamas say the protests have made them afraid to walk freely on campus. They hear denunciations of Zionism and calls for a Palestinian uprising as an attack on Jews themselves.

The tension goes to the heart of a question that has touched off debate among observers and critics of the protests: At what point does pro-Palestinian political speech in a time of war cross the line into the type of antisemitism colleges have vowed to combat?

If this is a matter that has vexed political leaders, university administrators and some Jewish college students, inside the encampments the very notion of antisemitism is barely discussed, in part because the demonstrators do not believe the label applies to their activism. Protest leaders point to the involvement of Jewish student activists and challenge the idea that the comfort of Israel’s supporters should be a concern.

And they draw a distinction between anti-Zionism, which describes opposition to the Jewish state of Israel, and hatred toward Jewish people in general. It is an argument many Jews see as a fig leaf for bigotry.

In a letter to Columbia students last week, university officials made clear the challenge they are facing. “We know that many of you feel threatened by the atmosphere and the language being used and have had to leave campus,” they wrote. “That is unacceptable.”

They continued, “Chants, signs, taunts and social media posts from our own students that mock and threaten to ‘kill’ Jewish people are totally unacceptable, and Columbia students who are involved in such incidents will be held accountable.”

A call for divestment

The protests beyond New York City have been inspired by the Columbia students, but they are largely diffuse, spreading via social media much like other recent movements, including Black Lives Matter and the Arab Spring.

At Columbia, the demonstration is led by a group known as CUAD — Columbia University Apartheid Divest — a coalition representing more than 100 Columbia student organizations including Students for Justice in Palestine and Jewish Voice for Peace. Leadership is amorphous. The organizers communicate on the Telegram messaging app and provide media training to the activists they make available to speak to the press.

It is unclear what financial support the group receives, and from whom. When asked, one student leader declined to comment.

But supporters from across New York have responded to the group’s Instagram pleas for water, blankets, gloves and cigarettes. Last week, Palestine Legal, an advocacy group, filed a federal civil rights complaint on behalf of the protesters, arguing that they have been subjected to anti-Palestinian and anti-Islamic harassment on campus.

Student demonstrators are specifically calling for their universities to make transparent all financial holdings and divest from companies and funds they say are profiting from or supporting Israel and its government’s policies. They also want “amnesty” for students and faculty who have been disciplined by the university as a result of their protest.

At Columbia, students are also calling on the university to end its five-year-old dual-degree program with Tel Aviv University. Some also object to the presence on the university board of Jeh Johnson, who served as homeland security secretary during the Obama administration and sits on the board of Lockheed Martin , a supplier of fighter jets to the Israel Defense Force.

Mr. Johnson declined to comment.

At encampments around the country, signs also point to the broader politics of many of the protesters. They support the anti-Israel Boycott, Divestment and Sanctions movement, which predates the war in Gaza. The students invoke historical issues of colonialism and apartheid.

Student activists who are not themselves Palestinian say that they have joined the movement for a wide variety of reasons: anguish over a humanitarian crisis in Gaza ; a rebuke of university and police response to protests; a commitment to intersectional justice where any group’s fight should be everyone’s fight; the idealistic desire to be a part of a community effort; and a sense that the fight for Palestinians is a continuation of the work started on behalf of oppressed people during the Black Lives Matter movement.

Many Jewish students taking part in the current protests say they are doing so as an expression of their Jewish values that emphasize social justice and equality. Encampments have hosted Shabbat dinners and Passover seders. At Columbia, one student said that donors have supplied kosher meals.

Samuel Law, a graduate student at the University of Texas at Austin who is Jewish and involved in the protests, was inspired by the encampments popping up around the country. “I strongly believe that the university should be there for us to care about what we care about,” he said.

‘They don’t feel safe’

Outside the pro-Palestinian encampments, the movement has drawn accusations of anti-Jewish bigotry and harassment — from political leaders as well as from some students, Jewish and not.

Jimmy Hayward, a Columbia freshman who is not Jewish, said that he has many friends studying at the Columbia-affiliated Jewish Theological Seminary who are unnerved. “I have friends in JTS that need to be walked to campus,” he said. “They want me to walk them because they don’t feel safe walking alone.”

Signs in and around the Columbia encampment include inspirational quotes, including “The world belongs to the people, and the future belongs to us,” attributed to Jiang Qing, a Chinese communist revolutionary. But there are also celebrations of violence, like “Whoever is in solidarity with our corpses but not our rockets is a hypocrite and not one of us.”

At the University of Michigan, some Jewish students said they felt rattled as they walked to class passing by protesters chanting, “Long live the intifada,” using the word for “uprising” in Arabic, which has been used to describe periods of violent protests by Palestinians against Israelis.

Tessa Veksler, a Jewish student at the University of California Santa Barbara was alarmed to see, at the school’s multicultural center, a sign on the door to a student lounge that said, “Zionist Not Allowed.”

Campus protesters dispute the notion that their movement has made pro-Israel students unsafe.

Nas Issa, a Columbia graduate who is supporting and advising protest organizers, sees a difference between feeling uncomfortable and feeling that you are in danger — “especially if you feel that your identity is tied to the practices of a particular state or to a political ideology.”

“That can be personally affecting and I think that’s understandable,” said Ms. Issa, who is Palestinian. “But I think the conflation between that and safety — it can be a bit misleading.”

When pressed, the protesters say they are anti-Zionist but not antisemitic.

It is not a distinction everyone buys.

“Let’s take any other ethnic or religious minority,” said Eden Yadegar, a junior at Columbia. “Would you only accept them if they were willing to denounce an integral part of their religious or ethnic identity? The answer is absolutely not. So how come it’s OK to say, you know, we accept Jews, but only if you denounce your religious and social and ethnic connection to your homeland? It’s ridiculous.”

Last Tuesday afternoon, Isidore Karten, an Israeli Jew, hopped a fence and entered the pro-Palestinian encampment at Columbia.

“I think it’s super important to go and show our side also,” said Mr. Karten, a 2022 Columbia graduate. “We should be allowed to be there as much as anyone else .”

Once inside, he unfurled an Israeli flag. A friend who had come with him toted a poster showing the faces and names of Israelis who were kidnapped into Gaza by Hamas on Oct. 7.

As they did, they were trailed by pro-Palestinian protesters holding a large black sheet to keep journalists from seeing them and the flag.

A few students, Mr. Karten said, chanted, “Burn Tel Aviv to the ground.”

And as he tried to talk with the demonstrators, he said, his efforts were blocked by protest leaders.

One of them was Khymani James, the student who was later barred from campus for his incendiary video. “We don’t engage with Zionists,” he said, according to Mr. Karten.

‘A wake-up call’

Mr. James’s video , which was publicized by a right-wing outlet on Thursday and then reported on by The New York Times and others, drew wide attention, including from President Biden, whose spokesman issued a statement saying, “These dangerous, appalling statements turn the stomach and should serve as a wake-up call.”

Others cautioned not to use the words of one activist to define a much larger group.

The Rev. Michael McBride , a founder of Black Church PAC, who has pressed for a cease-fire in Gaza, said Mr. James’s comments were not representative of the antiwar movement.

“You can go to a protest and find anything you’re looking for,” said the Rev. McBride, who leads a church in Berkeley, Calif. “If you’re looking for that, then you’ll find it.”

At Columbia, the CUAD student protest organization on Friday posted a statement on Instagram that said, “Khymani’s words in January do not reflect his view, our values, nor the encampment’s community agreements.” The statement added, “In the same way some of us were once Zionists and are now anti-Zionists, we believe unlearning is always possible.”

But for university administrators, Mr. James’s case has presented a serious challenge.

He made some of his comments about killing Zionists — including that “taking someone’s life in certain case scenarios is necessary and better for the overall world” — during a college disciplinary hearing in January.

But he was not barred from campus until the January video began to spread last week. A notification sent to Mr. James by the university and shared with The New York Times by one of his friends described it as an “interim suspension.” Mr. James, who said in a statement last week that his words were “wrong,” could not be reached for comment.

“When leadership learned of the video, it took immediate steps to ban James from campus,” a Columbia spokesman said this weekend. “We initiated disciplinary proceedings which encompass this and additional potential violations of university policies.”

It is not clear whether the Columbia administrator conducting the disciplinary hearing alerted a superior or public safety official to Mr. James’s remarks at the time — or whether Columbia policy dictated that the administrator should have.

A spokesman for the university declined to comment further.

The episode left Avi Weinberg, the pro-Israel student who was surrounded by Mr. James and other protesters at the encampment, distressed. “The university was aware that this is his mind-set, and the university put their students in danger,” he said. “That is very present on my mind.”

Eryn Davis, Neelam Bohra, Katie Glueck, Stephanie Saul, Olivia Bensimon and Karla Marie Sanford contributed reporting.

Katherine Rosman covers newsmakers, power players and individuals making an imprint on New York City. More about Katherine Rosman

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