Information on cannabis use was collected via a postal questionnaire that the young people completed at age 16 years [median age 16 years 7 months, interquartile range (IQR) = 16 years 6 months–16 years 10 months]. Respondents completed the six-item Cannabis Abuse Screen Test (CAST 23), along with an initial stem-question asking whether the respondent has used cannabis since the age of 15. Items in the CAST relate to morning cannabis use, use while alone, experience of cannabis-related problems and failed attempts to quit or reduce use. The CAST sum-score was derived as illustrated by Piontek and colleagues 3, where a response to each item of fairly often/very often is coded as one and never/rarely as zero. A three-level ordered categorical variable was derived indicating: 0, no use since age 15; 1, yes to use but a CAST sum-score of zero; and 2, yes to use and a non-zero CAST sum-score. While a cut-point of 4 has been recommended 3, we opted for a cut of 1 or more points in this sample of young adolescents in order to yield
