The majority (i.e., 55) of the 69 key variables were categorical, with four possible response categories: “yes,” “no,” “obligate no,” and “missing.” For the 5,390 participants, complete data were available for 98.93% of the entries for the 69 key variables. Fifteen of the key variables asked about the signs and symptoms that a patient experienced when he/she stopped, reduced, or went without opioids. Using “yes-or-no” questions, respondents were asked whether they had ever experienced the 15 withdrawal symptoms (“ever occur” symptoms) and whether two or more symptoms occurred together (“occur together” symptoms). If participants never tried to stop or reduce their opioid use, these variables were scored as “obligate no.” Our previous subtyping efforts (Chan et al., 2011; Gelernter et al., 2005; Gelernter et al., 2006; Kranzler, et al., 2008) used the set of “ever occur” withdrawal variables. In the present study, based on the analysis described below, we used the “occur together” withdrawal variables.
