Approximate Confidence Intervals for Effect Sizes

Abstract

This article defines an approximate confidence interval for effect size in correlated (repeated measures) groups designs. The authors found that their method was much more accurate than the interval presented and acknowledged to be approximate by Bird. That is, the coverage probability over all the conditions investigated was very close to the theoretical .95 value. By contrast, Bird’s interval could have coverage probability that was substantially below .95. In addition, the authors’interval was less likely than Bird’s method to present an overly optimistic portrayal of the effect. They also examined the operating characteristics of the Bird interval for effect size in an independent groups design and found that, although it is fairly accurate in its approximation of coverage probability, the accuracy of the approximation does vary with the magnitude of the population effect size.