Dependent Variable Reliability and Determination of Sample Size

Abstract

Arguments have recently been put forth that stan dard textbook procedures for determining the sam ple 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—be cause there are several ways of defining the magni tude of group differences. The standard formula is appropriate when group differences are defined rel ative to the within-group standard deviation of ob served scores. Advantages and disadvantages of the various approaches are discussed.