The Health Behaviour in School-aged Children (HBSC) survey is a cross-national research study of the health and health-related behaviour of school-going children carried out every four years. The research is conducted by an international network of research teams in collaboration with the World Health Organization (WHO) Regional Office for Europe. The most recent survey was conducted successfully in 35 countries and regions, including Ireland. Findings from the Irish survey, including data on drug use, have been previously published.1

 On 4 June 2004 the WHO published an international report which used data from the last HBSC survey.2 The report, Young people’s health in context, provides information on the prevalence of priority health-related behaviour and key health indicators, and places these findings in their social and developmental contexts. Previously unpublished data on cannabis use by young people in Ireland are presented in part of the report. The section on cannabis use is authored by Tom ter Bogt, Anastasios Fotiou and Irish researcher Saoirse Nic Gabhainn.

 The target population in the HBSC survey is school-going children who are aged 11, 13 and 15 years. The HBSC protocol recommends a minimum survey sample size of 1,536 children at each of these ages. In Ireland the numbers of children surveyed at these ages were 1,012, 944 and 919 respectively, which were lower than the desired targets. It should be kept in mind that, in general, the smaller the survey sample the wider are the confidence intervals around the estimates obtained from the sample.3 The protocol also required that children be selected using a cluster sampling design, where the initial sampling unit was the class (or school in the absence of a sampling frame of classes). Each child of the appropriate age in the classes selected and present in class on the day of the survey was asked to participate. In Ireland, individual schools within regions were first randomly selected and classes within schools were subsequently randomly selected for participation. The objective was to achieve a nationally representative sample of school-going children. The Irish HBSC survey was carried out between April and June 2002.

 The findings on cannabis use presented in the report relate only to 15-year-olds. The numbers of children who responded to the cannabis questions are not given in the report. Results for boys and girls in Ireland compared with the average for all 35 HBSC countries and regions are shown in the table below.  

Source: Young people’s health in context (p. 86)

 In Ireland one in every four boys reported using cannabis in the year prior to the survey. The rate was lower for girls: one in every seven had used cannabis in the previous year.  Cannabis use for boys in Ireland was higher than the HBSC average, while that for girls was lower. However, the differences in the proportions are small and may be due to sampling variation.4

 Four groups were identified among those who reported having used cannabis at least once in their lives, based on the number of times they had used it in the previous 12 months. The four groups were ‘former users’ who had not used cannabis in the last year, ‘experimental users’ who had used it once or twice, ‘recreational users’ who had used it between 3 and 39 times, and ‘heavy users’ who had used it 40 or more times. The percentage of young people in Ireland in each of these four groups compared with the HBSC average is shown in the table below.

Source: Young people’s health in context (p. 88)

 Compared to the HBSC average, Ireland had a much lower percentage of ‘former users’, suggesting that young children in Ireland are more likely to continue their use after starting. Indeed, of the 35 HBSC countries and regions, Ireland ranked 30th on this indicator. The percentage of experimental cannabis users in Ireland was slightly higher than the HBSC average, while the percentage of recreational users was lower. Of most concern was the level of heavy cannabis use: a higher percentage of children in the sample in Ireland had used cannabis 40 or more times in the last year (3.6%) than the average for all samples in the 35 HBSC countries and regions (2.8%). The report notes that, while most users in all HBSC countries and regions belonged to the experimental and recreational use groups, the ‘small numbers reporting heavy use at this stage of their lives may well be at risk of adverse health and social consequences and should be the focus of targeted interventions’.  

1. Kelleher C et al. (2003) The National Health & Lifestyle Surveys. Survey of Lifestyle, Attitudes and Nutrition (SLÁN) & the Irish Health Behaviour in School-aged Children Survey (HBSC). Galway: Centre for Health Promotion Studies, National University of Ireland. 

2. Currie C et al. (eds) (2004) Young people’s health in context. Health Behaviour in School-aged Children (HBSC) study: international report from 2001/2002 survey. Copenhagen: World Health Organization. 

3. A confidence interval is a range of values which we can be confident includes the true value. By convention, a 95% confidence interval is reported. For example, if the proportion of cannabis users in a survey sample drawn from the population is found to be 17% and the 95% confidence interval is reported as ranging from 14% to 20%, then we can say that we are 95% confident that this interval includes the true proportion of cannabis users in the population. It is good practice to provide confidence intervals to indicate the level of precision associated with survey estimates. 

4. Because researchers must normally take a sample rather than study an entire population, inaccuracies can occur in making inferences about a population value based on a sample value. Sampling variation or sampling error is an estimate of how much a sample value can be expected to differ from the true value in the population. The difference expected is accounted for by the level of precision chosen when calculating the sample size. Thus, differences observed between two estimated proportions may simply be due to a lack of precision. 

The report Young people’s health in context can be obtained from the WHO website at http://www.euro.who.int