Once a suitable effect size measure has been chosen, researchers unfamiliar with effect sizes often face practical challenges in obtaining or calculating their values. Effect sizes are often not directly provided by statistical software output of multivariate regression analyses. In some cases, even statistics which could be used to calculate effect sizes (e.g., R2, the proportion of variance accounted for, in a multiple regression) are not readily accessible from the output. Additional caution is needed when calculating effect sizes using hierarchical or repeated-measures data, as researchers must account for variance accounted for by different sources, for example fixed effects vs. random effects in a mixed-effects regression model.
