Statistical significance and effect size in education research: Two sides of a coin

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 decisio ns, both sides should be considered. In a simulation study, the sampling va riability of 2 popular effect-size measures (d and R-2) was examined. The v ariability showed that what is statistically significant may not he practic ally meaningful, and what appears to be practically meaningful could have b een the result of sampling error, thus not trustworthy, Some practical guid elines are suggested for combining the 2 sources of information in research practice.