RANDOM VERSUS NONRANDOM ASSIGNMENT IN CONTROLLED EXPERIMENTS - DO YOUGET THE SAME ANSWER

Psychotherapy meta-analyses commonly combine results from controlled e xperiments that use random and nonrandom assignment without examining whether the 2 methods give the same answer. Results from this article call this practice into question. With the use of outcome studies of m arital and family therapy; 64 experiments using random assignment yiel ded consistently higher mean posttest effects and less variable postte st effects than 36 studies using nonrandom assignment. This difference was reduced by about half by taking into account various covariates, especially pretest effect size levels and various characteristics of c ontrol groups. The importance of this finding depends on(a) whether on e is discussing meta-analysis or primary experiments, (b) how precise an answer is desired. and (c) whether some adjustment to the data from studies using nonrandom assignment is possible. It is concluded that studies using nonrandom assignment may produce acceptable approximatio ns to results from randomized experiments under some circumstances but that reliance on results from randomized experiments as the gold stan dard is still well founded.