What is a Scientific Hypothesis? | Definition of Hypothesis
For a hypothesis to be termed a scientific hypothesis, it has to be something that can be supported or refuted through carefully crafted experimentation or observation.
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A scientific hypothesis is 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, the definition can be expanded. A hypothesis also includes an explanation of why the guess may be correct, according to National Science Teachers Association.

A hypothesis is a suggested solution for an unexplained occurrence that does not fit into current accepted scientific theory. The basic idea of a hypothesis is that there is no pre-determined outcome. For a hypothesis to be termed a scientific hypothesis, it has to be something that can be supported or refuted through carefully crafted experimentation or observation. This is called falsifiability and testability, according to the Encyclopedia Britannica.

A key function in this step in the scientific method is deriving predictions from the hypotheses about the results of future experiments, and then performing those experiments to see whether they support the predictions.

A hypothesis is usually written in the form of an if/then statement, according to the University of California. This statement gives a possibility (if) and explains what may happen because of the possibility (then). For example, if garlic repels fleas, then a dog is given garlic every day will not get fleas.

The primary trait of a hypothesis is that something can be tested and that those tests can be replicated, according to Midwestern State University. It is often examined by multiple scientists to ensure the integrity and veracity of the experiment. This process can take years, and in many cases hypotheses do not go any further in the scientific method as it is difficult to gather sufficient supporting evidence.

"As a field biologist my favorite part of the scientific method is being in the field collecting the data, but what really makes that fun is knowing that you are trying to answer an interesting question so the first step in identifying questions and generating possible answers (hypotheses) is also very important and is a creative process. Then once you collect the data you analyze it to see if your hypothesis is supported or not," Jaime Tanner, a professor of biology at Marlboro College, told Live Science.

A null hypothesis is the name given to a hypothesis that is possibly false or has no effect. Often, during a test, the scientist will study another branch of the idea that may work, which is called an alternative hypothesis, according to the University of California, Berkeley

During a test, the scientist may try to prove or disprove just the null hypothesis or test both the null and the alternative hypothesis. If a hypothesis specifies a certain direction, it is called one-tailed hypothesis. This means that the scientist believes that the outcome will be either with effect or without effect. When a hypothesis is created with no prediction to the outcome, it is called a two-tailed hypothesis because there are two possible outcomes. The outcome could be with effect or without effect, but until the testing is complete, there is no way of knowing which outcome it will be, according to The Web Center for Social Research Methods.

During testing, a scientist may come upon two types of errors. A Type I error is when the null hypothesis is rejected when it is true. A Type II error occurs when the null hypothesis is not rejected when it is false, according to the University of California, Berkeley. 

Upon analysis of the results, a hypothesis can be rejected or modified, but it can never be proven to be correct 100 percent of the time. For example, relativity has been tested many times, so it is generally accepted as true, but there could be an instance, which has not been encountered, where it is not true. For example, a scientist can form a hypothesis that a certain type of tomato is red. During research, the scientist then finds that each tomato of this type is red. Though his findings confirm his hypothesis, there may be a tomato of that type somewhere in the world that isn't red. Thus, his hypothesis is true, but it may not be true 100 percent of the time.

Most formal hypotheses consist of concepts that can be connected and their relationships tested. A group of hypotheses comes together to form a conceptual framework. As sufficient data and evidence are gathered to support a hypothesis, it becomes a working hypothesis, which is a milestone on the way to becoming a theory. Though hypotheses and theories are often confused, theories are the result of a tested hypothesis. While hypotheses are ideas, theories explain the findings of the testing of those ideas. 

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

Additional reporting by Kim Ann Zimmermann, Live Science contributor