Experimental Design

An experiment is a randomized comparison used to assess the effects of a treatment or intervention. In the simplest experiment, participants are assigned at random to one of two groups: One receives the treatment or intervention of interest and the other receives either an alternative treatment or no intervention at all. Both groups of participants are subsequently assessed on an outcome measure, and differences between the groups on the outcome measure are used to estimate the size of the treatment effects.

The alternative to a randomized experiment is a quasiexperiment. A comparison is quasiexperimental if participants are not assigned to treatment conditions at random. For example, a quasiexperiment results when the participants choose for themselves which treatment to receive or when treatments are assigned by others ...