When calculating a polygenic score from markers in GABA system genes, each SNP was permitted to contribute to the score only if its individual effect was such that the p-value associated with the marker was below a particular cutoff. Scores were calculated at ten incrementally increasing p-value cutoffs ranging from p < 0.1 to p <= 1.0 (that is, at the final threshold, all SNPs were permitted to contribute to the polygenic score). At each p-value cutoff threshold, allowing more markers of smaller individual effect to contribute to the score potentially resulted in an increase in the number of markers reflecting minute but genuine genotypic influences, but also permitted the inclusion of more markers that had p-values below the cutoff merely due to chance.
