, 2005). T3 was completed with 81.4% of the original number of participants (N = 1816, mean age = 16.27 years, SD 0.73, 52.3% girls). Before each assessment wave, informed consent was obtained from all adolescents and their guardian(s) after the nature of the study had been fully explained. Furthermore, all of the TRAILS study procedures were approved by the International ethical committee
‘Central Committee on Research Involving Human Subjects (CCMO)’ in the Netherlands. For the Quizartinib manufacturer analyses of the present study, only Dutch subjects with complete data on predictors and outcome were included in the analyses. Of the fourteen pairs of siblings within the TRAILS-sample, one of the siblings was randomly excluded. This resulted in a final sample of n = 1192. Included participants were PCI-32765 purchase equally likely to be male/female (χ2 (1 df, N = 2230) = 3.67, p > .05), more likely to have a higher socioeconomic status (χ2 (2 df, N = 2230) = 107.55, p < .001), had a higher intelligence (t = 10.02, 2169 df, p < .001), and were less likely to have initiated cannabis use at the second assessment of TRAILS (mean age 13.56 years; SD 0.53) (χ2 (1 df, N = 2230) = 6.60, p < .05) when compared
to the excluded participants. Alcohol and cannabis use: Frequency of alcohol and cannabis use was assessed at T3 by self-report questionnaires filled out at school, supervised by TRAILS assistants. Confidentiality of the study was emphasized so that adolescents were reassured that their parents or teachers would TCL not have access to the information they provided. Among other questions, participants were asked to report the frequency of cannabis and alcohol use ever, in the past year, and in the past four weeks. Response options ranged from 0 to
13, with 0–10 corresponding to the equivalent number of times, and 11, 12 and 13 corresponding to, respectively, 11–19, 20–39, and at least 40 times. In order to create comparable measures of regular alcohol and cannabis use, both were defined according to the number of occasions of use. Regular alcohol consumption was defined as drinking on 10 or more occasions in the past four weeks ( Andersson et al., 2007 and Hibell et al., 2009). Regular cannabis use was defined as the use of cannabis on at least four occasions in the past four weeks. When averaged, this reflects weekly or more frequent than weekly use of cannabis. In order to minimize the possibility of including substance-related phenotypes in the comparison groups, regular users were compared to abstainers. For cannabis use, abstainers were those that reported never to have used cannabis. Because hardly any adolescents reported no alcohol consumption ever, alcohol abstainers were those that reported no consumption of alcohol in the past year. In addition, to make sure that the addressed associations were specific for regular use, rather than for substance use in general, regular users were also compared to experimental users.