Population and sampling
This study presents a cross sectional study of a sample of 683 adolescents aged 15 to 19 attending high school. The study used a database from the 2006 Health Survey of the Republic of Serbia (without data for Kosovo and Metohija), which was carried out by the Ministry of Health of the Republic of Serbia with financial and professional support of the World Bank, the World Health Organization Regional Office for Europe (country office Serbia) and the Institute of Public Health of Serbia ‘Dr Milan Jovanovic Batut’ .
The 2006 Health Survey of the Republic of Serbia provided statistically reliable estimates of the health indicators at the national level and at the levels of six geographic regions: Vojvodina, Belgrade, West, Central, East and South-East Serbia. By their further division into urban and rural areas, twelve areas were identified as the main sampling strata. The sample was selected in two stages. The primary stage units were 675 enumeration areas from the Census of 2002 in Serbia, selected on the basis of probability proportional sampling. Second stage units were households, selected by simple random sampling without replacement. After updating within each selected census enumeration areas, 10 households and 3 replacement households from the household list were chosen. The replacement households were interviewed only if some of the first 10 households were not found. In the case that a household refused to be interviewed, a replacement household was not contacted In this way, 7673 selected households were made sampling frame and observation units were all members of the selected households.
Out of 7673 households randomly selected for the sample, the members of 6156 households were interviewed. The household response rate was 86.5%. In selected households, 683 adolescents aged 15 to 19 attending high school were identified.
Cross-sectional data were weighted to represent the Serbian population in 2002. The weights were adjusted by population projections for 2006 based on the vital statistics (birth and death rate).
Informed consent was obtained from all respondents. The study was approved by the Review Board of the Ministry of Health of Serbia and the Institute of Public Health of Serbia.
Three types of questionnaires were used to collect data: household questionnaire, questionnaire for children and adolescents aged 7–19 years (face to face) and self-administered questionnaire for children and adolescents aged 12–19 years. Five questions that were related to demographic characteristics and socio-economic status of adolescents of the 81 questions from a face-to-face questionnaire and 6 questions that related to various forms of risky behaviour of the 66 questions from a self-questionnaire for children and adolescents aged 12 to 19 were used. Socio-economic status was measured by calculating the demographic and health survey wealth index (wealth index) on the basis of answers to 9 questions from the household questionnaire that included 30 questions.
Data collection process was standardized in order to ensure the quality of data collection and that a consistent methodology would be used. Before the start of interviewing, training for 201 interviewers was conducted in the form of two-day workshops. The obligation of the interviewers was to interview all household members.
As a measure of demographic and socio-economic characteristics: age (categorized into two age groups: one – 15–16 and two – 17–19), type of settlement (one – non-urban and two – urban), family structure (one –complete: with both biological parents, with one biological parent and stepmother/stepfather or with caregivers, two – incomplete: with one biological parent, be alone or with grandparents), having one’s own room (one – no and two – yes), school success (categorized into three groups: one – high (excellent, very good), two – moderate or low (good, sufficient, insufficient), and the household wealth index (one – poorest, two – poorer, three – middle, four – richer and five – richest class) were used. Assets included in computing household wealth index were number of bedrooms per household member, material used for floor, roof and walls of the house type of drinking water source and sanitation facilities, source of energy used for heating, possession of colour TV, mobile phone, refrigerator, personal computer, washing machine, dishwasher, air conditioning, central heating, car and internet access. The distribution of the household population by household wealth index was performed on 5 categories by 20% quintiles .
In secondary schools in Serbia, the evaluation of success in school for every student is performed using a five-point grading scale and it is assessed by averaging his grades in all subjects. At the end of the school year, final grades for each subject are calculated from those given at the end of each semester and they are determined by the following ranges: 5 (excellent) is given for an average of 4.50 to 5.00; 4 (very good) is given for an average of 3.50 to 4.49; 3 (good) is given for an average of 2.50 to 3.49; 2 (sufficient) the lowest passing grade is given for an average of 2.00 to 2.49; 1 (insufficient) the lowest possible grade, and the failing one, is given if the student does not have grade of at least 2 in each topic of the course.
Data on the prevalence of the single health risk among adolescents were assessed by the responses of adolescents about smoking at least one cigarette per day during the previous month, drinking any alcoholic beverage from the list of drinks: beer, wine, spirits, liqueur cocktail at least one day during the previous month, taking non-prescription tablets (anxiolytics, analgesics, amphetamine etc.) during the previous month, having the experience of casual sexual intercourse during the last 12 months, having first sexual intercourse before the age of 16 and bullying others. In order to determinate risk behaviour that related to bullying somebody, young people were asked whether they had taken part in insults, humiliation or physical harassment of another person during their lifetime. For further analysis, the adolescents were divided into three categories: no risk, one risk, and two or more health risk behaviours.
Data were analysed by descriptive and inferential statistics. At the level of inferential statistics, nonparametric chi-square test was used for testing the statistical significance of the difference between the variables and multivariate logistic regression was used for statistical modeling separately for boys and for girls.
Distribution of boys and girls and their differences according to demographic and socio-economic status variables, single or various models of multiple health risk behaviours were examined by chi-square test or Fisher’s exact test. Also, distribution of boys and girls with various models of health risk behaviours (no risk, one risk, and two or more health risk behaviours) and their differences according to demographic and socio-economic status variables were examined by chi-square test. Finally, multivariate logistic regression model was used to determine predictors of concurrent health risk behaviours. The dependent variable was engaging in multiple risk behaviours (two or more risk behaviours vs no risk behaviours). All analyses were done separately for boys and girls. The odds ratios (ORs) with their corresponding 95% confidence intervals (CIs) were adjusted for age, type of settlement, household wealth index, family structure, having one’s own room and school success.
Statistical package statistical software package SPSS 17 was used for data analysis. Differences were considered statistically significant at P < 0.05.