Longitudinal-Experimental Combinations

Abstract

An experiment is a systematic attempt to investigate the effect of variations in one factor, the independent variable, on another, the dependent variable. The independent variable is under the control of the experimenter; in other words, the experimenter decides which people receive which treatment, using the word treatment very widely to include all kinds of interventions. The focus here is on randomized experiments in which people are assigned at random to different treatments. Given a large number of people, randomization ensures that subjects in experimental and control groups on average are equivalent on all possible extraneous variables, within the limits of small statistical fluctuations. This feature of randomization makes it possible to isolate and disentangle the effect of the independent variable from the effect of all other extraneous variables (see e.g., Farrington 1983).