Statistical Significance and Effect Size in Education Research: Two Sides of a Coin

Abstract In education research, statistical significance and effect size are 2 sides of 1 coin; they complement each other but they do not substitute for each other. Good research practice requires that, to make sound research decisions, both sides should be considered. In a simulation study, the sampling variability of 2 popular effect-size measures (d and R 2) was examined. The variability showed that what is statistically significant may not be practically meaningful, and what appears to be practically meaningful could have been the result of sampling error, thus not trustworthy. Some practical guidelines are suggested for combining the 2 sources of information in research practice.