For Cohort I, lifetime history of AD was coded from DSM-IV AD symptoms based on a DSM-III-R AD assessment. The SSAGA administered to Cohort I predates DSM-IV; however, an approximate algorithm for DSM-IV AD without clustering criteria was subsequently applied by Heath and colleagues (see Heath, Whitfield, Madden, Bucholz, Dinwiddie et al., 2001). Consistent with other reports (Grant, Heath, Bucholz, Madden, Agrawal et al., 2007; Knopik, Heath, Jacob, Slutske, Bucholz et al., 2006), respondents in Cohort I were diagnosed with AD if they endorsed three or more DSM-IV AD symptoms. For Cohort II, DSM-IV AD was directly assessed, with age of onset defined as age at first 12-month clustering of three or more AD symptoms. An equivalent measure of AD age of onset was not available for Cohort I.
