A Simple Method for Removing Bias From a Popular Measure of Standardized Effect Size: Adjusted Partial Eta Squared

Abstract

Accurate estimates of population effect size are critical to empirical science, for both reporting experimental results and conducting a priori power analyses. Unfortunately, the current most-popular measure of standardized effect size, partial eta squared ( (Formula presented.) ), is known to have positive bias. Two less-biased alternatives, partial epsilon squared ( (Formula presented.) ) and partial omega squared ( (Formula presented.) ), have both existed for decades, but neither is often employed. Given that researchers appear reluctant to abandon (Formula presented.), this article provides a simple method for removing bias from this measure, to produce a value referred to as adjusted partial eta squared (adj (Formula presented.) ). Some of the many benefits of adopting this measure are briefly discussed.