“…Given that the raw primary data are oftentimes not available, meta-analysts may have to trust the estimation and reporting of the effect sizes in the publication and have hardly any chance to perform further adjustments. However, such adjustments are possible for most ILSA data-in fact, if the raw data of primary studies are available, the meta-analysts are in full control of the effect size estimation and can estimate them and the respective sampling (co-)variances from analytic models that incorporate the complex survey design features of ILSAs, such as multilevel models with sampling weights, stratifying variables, plausible values, and multi-group structures (Campos et al, 2021). Overall, meta-analysts have at least two options to address the complex survey design, especially the nested data structure, in primary studies: (a) Adjust the reported effect sizes by the 𝐼𝐶𝐶 !…”