What assumption regarding data must be met in statistical tests involving multiple groups?

The assumption that must be met in statistical tests involving multiple groups is the homogeneity of variance. This principle asserts that the variances in different groups being compared should be approximately equal. When conducting tests such as ANOVA, the validity of the results can be significantly impacted if this assumption is violated. If the variance in one group is substantially different from that in another, it can lead to unreliable results and interpretations, potentially skewing the findings of the study.

Meeting this assumption helps ensure that statistical tests have the appropriate sensitivity to detect differences or relationships. If the assumption of homogeneity of variance is not satisfied, corrections or alternative statistical methods may need to be employed to account for the disparities in variance among groups, which is an important consideration when designing and analyzing research in clinical psychology or any field using similar statistical methodologies.