A Pearson Chi-square test was used to test departures from Hardy–Weinberg equilibrium (HWE) expectations in controls defined by the absence of a DSM-IV diagnosis of ND and an FTND score ≤1 and in cases of ND or FTND. Only the SNPs in HWE in controls (P > 0.01, Bonferroni corrected) were included in subsequent analyses. The CPD phenotype was log transformed to achieve approximate normality and the association between log (CPD) and each SNP was tested using linear regression. The association between each SNP and the two binary traits was estimated in a logistic regression framework. We tested the genetic effects under a log-additive model with age, sex and race as covariates in the combined sample, and age and sex were included as covariates in each population-specific subgroup analysis. All of above analysis was performed in the R package “SNPassoc” (González et al., 2007).
