is relatively poor at recall, it can be improved significantly when probed with careful questioning involving specific time periods and events. For cannabis, subjects were asked, “When you were [AGE] how easy would it have been to get CANNABIS if you wanted to use it?” Responses were scored on a four point ordinal scale [0 = “very difficult or don't know”, 1 = “somewhat difficult”, 2 = “somewhat easy”, and 3 = “very easy”]. We combined “very difficult” and “don't know” because during the pilot phase, interviewers consistently noted that “don't know” responses typically meant not knowing how to obtain a drug rather than not knowing anything about the drug. True “don't know” responses were very rare. Complete cannabis availability data were available from 1788 males. Cronbach's alpha for the five measures of cannabis availability was 0.80. Retest correlations, based on a sample of 141 subjects for whom the mean interval was 29 days averaged 0.68 across five age periods (28).
