Methods
Setting and Study Population
The Public Health Service of Amsterdam, the Netherlands, organizes general health surveys in a region around Amsterdam (the Amstelland region) among high school students (secondary education). Two studies, in 2005 and 2009, provided the possibility to monitor substance use among adolescents in this region. The Amstelland region consists of four municipalities (Aalsmeer, Uithoorn, Ouder-Amstel and Amstelveen) with about 145,000 inhabitants. Out of a total of nine high schools in the Amstelland region eight were included in the surveys. One school was unable to participate in the study due to lack of time. Second and fourth year students, aged 13, 14, 15 or 16, living in the Amstelland region were asked to complete an online questionnaire in a classroom setting. Participation was anonymous and voluntary. Students were divided into two schooling levels; VMBO students (lower educational level) and HAVO/VWO students (higher educational level). Seven participating schools included the entire second and fourth year, one school only the fourth year. The study population in 2005 consisted of 1,854 students and in 2009 of 2,088 students. For 66 classes in 2005 (1,564 students) and 56 classes in 2009 (1,324 students) participation was followed. The response was 91.4 % in 2005 and 92.5 % in 2009; the main reason for non-response was illness of the student.
Questionnaires
The questionnaires in 2005 and 2009 included questions about demographic and baseline characteristics (such as age, gender, educational level and ethnicity) and several health-related subjects including questions about alcohol consumption, smoking behaviour and cannabis use. In both surveys, of 2005 and 2009, the same questions were used to investigate drug using habits. The eight questions to assess alcohol-drinking habits were also similar in both surveys except for the possible answers to the first question "What kind of alcohol do you drink?". The answer possibilities in 2005 and 2009 differed in order to assess whether a student had ever drunk alcohol. In 2005 the possible answer was "I don't drink alcohol at all" and in 2009 "I have never drunk alcohol". Both answers excluded the students from further questions about alcohol use. In the survey of 2005 smoking habits were investigated using one question. In 2009 this question was split up into two questions with the same answer options.
Statistical Analyses
The outcome variables were last month alcohol-drinking, last month binge drinking, last month being drunk or tipsy, weekly and daily smoking and lifetime cannabis use and last month cannabis use. Explanatory variables were gender, age and educational level.
Differences in baseline characteristics (age, gender, educational level and ethnicity) between the study population in 2005 and 2009 were examined using Chi-squared tests.
For comparing the surveys (2005 and 2009) logistic regression analyses were applied to correct for differences between the surveys by gender, age and educational level. For the subgroup analyses by gender, by age and by educational level respectively the analyses were corrected by including the two remaining variables to the model. The regression coefficients for the years of investigation were used to calculate new 'adjusted' prevalence rates in percentages for 2009 using the SISA's logit module. To study differences between the two surveys (2005 and 2009) for the subgroups by educational level, by age and by gender, 2 order interaction terms of these variables with survey year were added to the regression models. Analyses were performed with SPSS version 19.
Ethical Approval and Consent
This study has been reported to the Dutch data protection authority and meets national ethics and privacy requirements. Parents were informed of the data collection by mail and they could refuse entry of their child into the data collection. This method of passive agreement is in accordance with Dutch legal standards.