APA Dictionary of Psychology

(symbol: MS) an estimator of variance calculated as a sum of squares divided by its degrees of freedom. It is used primarily in the analysis of variance, in which an F ratio is obtained by dividing the mean square between groups by the mean square within groups. The mean square also is used to determine the accuracy of regression analysis models, indicating the amount of variance explained by a model (MS_regression) compared to the amount of error or unexplained variance (MS_residual).