Most of our measures were self-reported by mothers or their children, which could introduce bias 34. Bias is most likely when both exposure and outcome are reported by the same individual, which was not the case in our study. We used a validated questionnaire instrument to distinguish between problem cannabis use and use without problems. The usual scoring system of the CAST questionnaire employs a higher cut-off score than we used to ascribe problem use. Our choice of the lower cut-off reflected both considerations around the age of our sample and around our patterns of response. Had we used a higher cut-off larger effects might have been apparent, but lower power would have led to these being estimated more imprecisely. The most probable outcome of misclassification in our measures arising through the mechanisms discussed above is a dilution of the apparent effects we estimated. We considered early-life exposures measured prior to measurement of conduct problems in an attempt to address issues of reverse causation. It is possible that some of these exposures, for example maternal substance use, persisted beyond the
