<P> An experiment usually tests a hypothesis, which is an expectation about how a particular process or phenomenon works . However, an experiment may also aim to answer a "what - if" question, without a specific expectation about what the experiment reveals, or to confirm prior results . If an experiment is carefully conducted, the results usually either support or disprove the hypothesis . According to some philosophies of science, an experiment can never "prove" a hypothesis, it can only add support . On the other hand, an experiment that provides a counterexample can disprove a theory or hypothesis . An experiment must also control the possible confounding factors--any factors that would mar the accuracy or repeatability of the experiment or the ability to interpret the results . Confounding is commonly eliminated through scientific controls and / or, in randomized experiments, through random assignment . </P> <P> In engineering and the physical sciences, experiments are a primary component of the scientific method . They are used to test theories and hypotheses about how physical processes work under particular conditions (e.g., whether a particular engineering process can produce a desired chemical compound). Typically, experiments in these fields focus on replication of identical procedures in hopes of producing identical results in each replication . Random assignment is uncommon . </P> <P> In medicine and the social sciences, the prevalence of experimental research varies widely across disciplines . When used, however, experiments typically follow the form of the clinical trial, where experimental units (usually individual human beings) are randomly assigned to a treatment or control condition where one or more outcomes are assessed . In contrast to norms in the physical sciences, the focus is typically on the average treatment effect (the difference in outcomes between the treatment and control groups) or another test statistic produced by the experiment . A single study typically does not involve replications of the experiment, but separate studies may be aggregated through systematic review and meta - analysis . </P> <P> There are various differences in experimental practice in each of the branches of science . For example, agricultural research frequently uses randomized experiments (e.g., to test the comparative effectiveness of different fertilizers), while experimental economics often involves experimental tests of theorized human behaviors without relying on random assignment of individuals to treatment and control conditions . </P>

What are the two types of variables in science