Unbalanced ascertainment in case/control designs can introduce bias in studies using meta-analytic data. Effective sample sizes (Neffs) can help to reduce potential bias in this situation. In this study, we used Neff=41/ncase+1/ncontrol to calculate the Neff for each phenotype, and used the per-SNP Neffs when available in the summary statistics (e.g. PTSD). This approach takes in account the impact of potential bias and reduced power introduced by unbalanced ascertainment of the number of cases and controls across cohorts analyzed under a liability scale (e.g. Walters et al., 2018; Zhou et al., 2020a).
