For our ANCOVA analyses, the desirable sample size was calculated as 158 for the effect size 0.25 to achieve 80% power. This might suggest that a sample size of 129 subjects in our study was somewhat insufficient to reliably detect moderate differences between the SNP genotypes, and a far greater sample may be required to detect smaller differences. Considering this caveat and relatively small effect size observed in the haplotype analysis (e.g., R2 for haplotype effect on the frequency of analgesic administration was 0.120), future studies with larger sample sizes may reveal additional associations between polymorphisms and opioid sensitivity.
