females if sex differences in self reported drug availability exist. Second, our data were collected retrospectively. Twins were asked to recollect drug availability, which introduces the potential for recall bias and telescoping effects (54). In order to improve the quality of retrospective recall of the cannabis availability, the interview utilized a Life History Calendar (LHC) format as developed by Thornton (31). By applying the principles of cognitive psychology, the LHC provides multiple cues to improve the subject's chances of accurate recall by making the task more akin to the accurate and well-retained process of recognition than to the less reliable task of free recall. In addition, LHCs have been shown empirically to improve the accuracy of retrospective reporting (31, 55). As shown elsewhere (28), retest correlations were highest for substances more easily obtained and lower for the more illicit substances. Moreover, the correlations declined with greater intervening time intervals. All of our analyses were therefore corrected for the linear effects of age at interview to remove, in part, possible cohort effects. Finally, our model fitting strategy was not exhaustive. The CCC model is an improvement over previous separate modelling of drug use and abuse (1) because it models for the
