Experimental Designs

The Role of Randomization in Experimental Designs

Establishing cause and effect requires that several conditions be met. For X to cause Y, X must occur before Y; changes in X must be associated with changes in Y; and all other plausible explanations for the observed association between X and Y must be controlled. The condition that all other plausible explanations are controlled is one of the defining characteristics of experimental research. [Page 643]When researchers conduct an experiment, they apply these three conditions by (1) manipulating the hypothesized cause and observing the outcome afterward, (2) testing whether variation in the hypothesized cause is associated with variation in the outcome, and (3) using randomization to reduce the plausibility of other explanations for the results observed. In technical terms, the final condition stipulates that plausible threats to internal validity are controlled. These include subject characteristics threats, testing threats, instrumentation threats, history threats, attrition threats, and regression to the mean.

Arguably, the subject characteristics threat is the most important threat minimized by the process of randomization. A subject characteristics threat occurs when individuals in the groups being compared are not equivalent to each other prior to the implementation of the treatment. In this case, equivalence implies that on average, the two groups are approximately the same on all measured and unmeasured characteristics. Without group equivalence, the researcher cannot be confident that any observed posttreatment differences were caused by the treatment. It is important to note that randomization does not eliminate all threats to internal validity; instead by reducing subject characteristics threats, randomization aims to ensure that threats are distributed evenly across conditions and are not conflated with participants’ condition membership. In experimental designs, randomization is the primary mechanism for minimizing plausible internal validity threats, distinguishing them from quasi-experimental designs, which without randomization cannot fully minimize all plausible threats.

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