What is an experiment?

An experiment is defined as a research method in which one or more independent variables are systematically manipulated to observe their effects on a dependent variable. This approach allows researchers to establish cause-and-effect relationships by controlling the conditions under which the observations are made. By manipulating the independent variable(s), researchers can rigorously test hypotheses and draw conclusions about how changes in one variable may influence another.

The key element that distinguishes an experiment from other research methods is this manipulation aspect, which enables the researcher to control for extraneous variables that could affect the outcome. This control strengthens the internal validity of the study, making it possible to infer causation rather than merely correlation.

In contrast, the other options describe research approaches that do not involve manipulation of variables. Qualitative studies (like the one mentioned in the first option) focus on exploring and understanding participant behaviors without quantifiable measures. Longitudinal studies track the same subjects over time to observe changes, but they do not manipulate variables. Correlational studies measure the relationship between variables without any manipulation, thus limiting the ability to draw causal conclusions. This makes the definition provided in the second option the correct characterization of what constitutes an experiment.