| Popis: |
Categorical moderators are often included in mixed-effects meta-analysis to explain heterogeneity in effect sizes. An assumption in tests of moderator effects is that of a constant between-study variance across all levels of the moderator. Although it rarely receives serious thought, there can be drastic ramifications to upholding this assumption. We propose that researchers should instead assume unequal between-study variances by default. To achieve this, we suggest using a mixed-effects location-scale model (MELSM) to allow group-specific estimates for the between-study variances. In two extensive simulation studies, we show that in terms of Type I error and statistical power, nearly nothing is lost by using the MELSM for moderator tests, but there can be serious costs when a mixed-effects model with equal variances is used. Most notably, in scenarios with balanced sample sizes or equal between-study variance, the Type I error and power rates are nearly identical between the mixed-effects model and the MELSM. On the other hand, with imbalanced sample sizes and unequal variances, the Type I error rate under the mixed-effects model can be grossly inflated or overly conservative, whereas the MELSM excellently controlled the Type I error across all scenarios. With respect to power, the MELSM had comparable or higher power than the mixed-effects model in all conditions where the latter produced valid (i.e., not inflated) Type 1 error rates. Altogether, our results strongly support that assuming unequal between-study variances is preferred as a default strategy when testing categorical moderators |
