A hypothesis is a provisional idea or explanation requiring evaluation. It is a key component of the scientific method. Every scientific study, whether experimental or descriptive, begins with a hypothesis that the study is designed to test -- that is, depending on the results of the study, the hypothesis will be either confirmed or disconfirmed.
Every well-designed study is designed to test something that we do not already know or which is reasonably subject to investigation. Though hypotheses are often “best guesses” about the outcome of a study, the outcome itself should not be something already known by the researcher. Outcomes that the researcher already knows are called “consequences” and should be taken into consideration when forming a study’s hypothesis. A consequence cannot in any real sense be confirmed, since it’s something that is already known.
Reducing the Question to Variables
As a first step in creating a hypothesis, researchers reduce the question that they're investigating to variables -- that is, measurable values. If a given question cannot be reduced to variables, it most likely is not a question answerable by scientific study. In an experimental study, which is one that attempts to show that one thing causes or affects another, these variables are directional -- the hypothesis will claim that if there is a certain change in one variable, there will be a corresponding change in another, but not necessarily the other way around. In a descriptive study, which attempts to show correlation but not necessarily causation between two or more things, there is no directionality to the variables.
Falsifiability refers to the idea that there must be some set of conditions that could occur that would show that a given hypothesis is false. For example, if a hypothesis states, “If mice eat twice as many calories, they will show weight gain,” there is a set of conditions under which the hypothesis would be false -- the mice eat twice as many calories but do not show weight gain. If there is no such set of conditions, then the hypothesis has been poorly designed -- because it cannot be disconfirmed, it cannot in any real sense be confirmed.
If a well-designed study delivers the results predicted by the hypothesis, then that hypothesis is confirmed. Note, however, that there is a difference between a confirmed hypothesis and a “proven” hypothesis. Scientific studies can support a given hypothesis, but they do not claim to absolutely prove hypotheses -- there could always be some other explanation for why a given study obtained the results it did. Generally speaking, however, the more often a study or experiment obtains the same results, the more heavily supported -- and thus, more likely to be correct -- a given hypothesis is.