Furthermore, previous studies have focused on the diagnosis of alcohol dependence as a categorical phenotype (Dick et al. 2006; Edenberg et al. 2004; Covault et al. 2004; Lappalainen et al. 2005; Fehr et al. 2006; Drgon et al. 2006; Soyka et al. 2008; Covault et al. 2008; Enoch et al. 2009; Bierut et al. 2010; Olfson and Bierut 2012; Ittiwut et al. 2012; Li et al. 2013; Sakai et al. 2010). In this study we examined the sum of AAD symptoms as a quantitative trait. There are three reasons for this approach: 1) item-response theory work that has shown that abuse and dependence symptoms provide overlapping information on severity (Gelhorn et al. 2008; Martin et al. 2006; Saha et al. 2006; Langenbucher et al. 2004), 2) this analysis is clinically relevant given the field has moved toward new DSM 5 criteria that merged abuse and dependence symptoms, and 3) continuous variables can provide a more accurate estimate of the phenotype.
