The technique used to statistically equate groups that differ on a pretest or some other variable or variables is called "matching." In research studies, matching is a technique used to create two or more groups that are similar in terms of one or more variables that may affect the outcome of the study. Matching can be done on a variety of variables, such as age, gender, socioeconomic status, or baseline scores on a pretest. The goal of matching is to reduce the potential confounding effects of the variables being matched, so that any observed differences between groups can be more confidently attributed to the treatment or intervention being studied.