Few DD GWAS studies have been attempted, and those that have been published are underpowered by modern standards, partly because they used dichotomous traits (i.e., DD diagnoses). Here, we used a relatively large sample and augmented power with an ordinal trait analytic design that allowed us to take into account both the presence or absence of OD and the severity of affection (including the ability to distinguish between subjects with zero and those with one or two symptoms). This increased power by enabling us to use more of the available phenotypic information than standard diagnosis-based analyses. Some of these strategies have been used previously in successful efforts to map ND risk alleles, most notably the use of large clinical samples (3). We further increased our analytic power by including, for some analyses, data from the Study of Addiction: Genetics and Environment (SAGE) sample (5, 6), which includes SD trait information. This dataset is available to the scientific community through an application process and will henceforth be referred to as “public domain.”
