The statistical analysis of mediation effects has become an indispensable tool for helping scientists
investigate processes thought to be causal. Yet, in spite of many recent advances in the estimation and
testing of mediation effects, little attention has been given to methods for communicating effect size and
the practical importance of those effect sizes. Our goals in this article are to (a) outline some general
desiderata for effect size measures, (b) describe current methods of expressing effect size and practical
importance for mediation, (c) use the desiderata to evaluate these methods, and (d) develop new methods
to communicate effect size in the context of mediation analysis. The first new effect size index we
describe is a residual-based index that quantifies the amount of variance explained in both the mediator
and the outcome. The second new effect size index quantifies the indirect effect as the proportion of the
maximum possible indirect effect that could have been obtained, given the scales of the variables
involved.