The Analysis of Covariance

The analysis of covariance serves the functions of a.) adjusting group means on a dependent variable to account for mean differences among the groups on a concomitant variable, and b.) increasing power by “explaining away” part of the within-group variability. Although most educational researchers are familiar with the first function of covariance, many are unaware of the second. The author shows, through a simple numerical example, how covariance can be useful even when treatment groups have identical covariate means. Data from an actual experiment is used to demonstrate the same point. Finally, the assumption of random assignment of Ss is discussed to support the author’s contention that the main value of covariance is increased power through reduced within-group variability.