Dependent variable reliability and determination of sample size

Abstract

Arguments have recently been put forth that standard textbook procedures for determining the sample size necessary to achieve a certain level of power in a completely randomized design are incorrect when the dependent variable is fallible. In fact, however, there are several correct procedures-one of which is the standard textbook approach-because there are several ways of defining the magnitude of group differences. The standard formula is appropriate when group differences are defined relative to the within-group standard deviation of observed scores. Advantages and disadvantages of the various approaches are discussed.