Percentage of drinking days (PDD) also was derived from the Form 90 instrument (Miller and Del Boca, 1994). In the current study, PDD was categorized into (1) nondrinking, defined as 0% drinking days in the 30 days before assessment; (2) infrequent drinking, defined as drinking on <50% of the days in the 30 days before assessment; and (3) frequent drinking, defined as drinking on ≥50% of the days in the 30 days before assessment. Alternative definitions of infrequent drinking were considered (e.g., <30% days abstinent = infrequent drinking). An inspection of the distribution of drinkers who drank at least once at each time point, however, demonstrated strong bimodality with a local minimum at 50%. The average (SD) PDD of those included as infrequent drinkers ranged from 17% (12%) to 24% (11%), whereas the average PDD of frequent drinkers ranged from 84% (17%) to 88% (15%).
