Group Matching: Is This a Research Technique to Be Avoided?

Abstract

Systematic assignment procedures are sometimes advocated as contributing to experimental designs that will result in more nearly precise estimates of treatment effects than those with completely random assignment. Several maintain that this systematic assignment is counter-productive because it reduces the power of statistical tests relative to random assignment. To help resolve these positions, the variance of the sample difference and the power of the F test for mean differences were studied under group matching on covariates and also under random assignment. This procedure was followed for all combinations of (1) one or three covariates; (2) small, medium, or large error variance; and (3) simple analysis of variance (ANOVA) or analysis of covariance (ANOCOVA). ANOCOVA with group matching always resulted in the correct Type-I error, yielded the greatest power in 5 of 6 situations, and gave the smallest variance of sample differences in 5 of 6 situations. However, the superiority of ANOCOVA with group matching over ANOCOVA with random assignment was small. It is concluded that there is little to choose between these two methods when a model of linear regression obtains. If it does not, matched assignment is a judicious technique.