A scientific experiment is a controlled set of observations aimed at testing whether two or more variables are causally related. William Shadish, Thomas Cook, and Donald Campbell describe two broad types of experiments: (a) randomized experiments, in which study units are randomly assigned to observational conditions; and (b) quasi-experiments, in which study units are not randomly assigned to observational conditions because of ethical or practical constraints. Although it is more difficult to draw causal inferences from quasi-experiments than from randomized experiments, careful planning of quasi-experiments can lead to designs that allow for strong causal inferences.

In order to infer a relationship between cause and effect, three requirements must be met: Cause must precede effect; cause must be related to effect; and, aside from the cause, no ...