The results of our meta-analysis, which included data from 18 published studies, did not support an association between the COMT Val allele and schizophrenia case status. We concluded that if a genuine association exists between the COMT gene and schizophrenia, the risk conferred is likely to be extremely small, with the largest (albeit non-significant) effect size observed in our meta-analysis being an odds ratio of 1·13, accounting for less than 0·2% of the phenotypic variance. Therefore, given a Val allele frequency of 50% and a disease prevalence of 1%, a sample consisting of in excess of 900 cases and an equal number of controls would be required to detect this effect with 80% power at a relatively non-conservative alpha of 0·05 (Table 3).
