Understanding Effect Sizes

Converting Among Effect Sizes

Earlier in this Part we discussed the case where different study designs were used to compute the same effect size. For example, studies that used independent groups and studies that used matched groups were both used to yield estimates of the standardized mean difference, g. There is no problem in combining these estimates in a meta-analysis since the effect size has the same meaning in all st...

Effect Sizes and the Integrative Understanding of Urban Bird Song (A Reply to Slabbekoorn et al.)

Effect sizes for experimenting psychologists.

This article describes three families of effect size estimators and their use in situations of general and specific interest to experimenting psychologists. The situations discussed include both between- and within-group (repeated measures) designs. Also described is the counternull statistic, which is useful in preventing common errors of interpretation in null hypothesis significance testing....

Effect Sizes Based on Means

When the outcome is reported on a meaningful scale and all studies in the analysis use the same scale, the meta-analysis can be performed directly on the raw difference in means (henceforth, we will use the more common term, raw mean difference). The primary advantage of the raw mean difference is that it is intuitively meaningful, either inherently (for example, blood pressure, which is measur...

Planning sample sizes when effect sizes are uncertain: The power-calibrated effect size approach.

Statistical power and thus the sample size required to achieve some desired level of power depend on the size of the effect of interest. However, effect sizes are seldom known exactly in psychological research. Instead, researchers often possess an estimate of an effect size as well as a measure of its uncertainty (e.g., a standard error or confidence interval). Previous proposals for planning ...