Study design and study population: Health Behaviour in School-aged Children (HBSC) is an international study which aims to examine adolescents’ health and health behaviours in their social setting . The HBSC study includes a series of cross-sectional school-surveys of nationally representative samples of three age groups, 11-, 13- and 15-year-olds, carried out every fourth year in currently 50 countries in Europe and North America. The studies are comparable since they adhere to a standard protocol for sampling, measurement, and data collection .
Our study used data from eight comparable, cross-sectional, periodical surveys in Denmark in 1991, 1994, 1998, 2002, 2006, 2010, 2014 and 2018. We collected data from random samples of schools, a new sample in each survey, drawn from complete lists of public and private schools. In each school we invited all students in the fifth, seventh and ninth grade (corresponding to the age groups 11, 13 and 15) to participate and complete the internationally standardized HBSC questionnaire in the classroom . The participation rate across all eight surveys was 88.0%, n=35,320. This study included students with complete information about sex, age, prevalence of backpain and the family’s SES, n=29,952 (Table 1).
Measurements: Backpain was measured by one item in the HBSC Symptom Check List (HBSC-SCL) [42-44]: “In the last 6 months, how often have you had backpain?” We dichotomized the responses into chronic ("about every day" and “more than once a week”) vs. episodic ("about every week”, “about every month”, and “rarely or never”). The HBSC-SCL is reliable assessed by consistent response patterns and valid assessed by qualitative interviews [42-44]. This measurement was similar in all eight surveys.
SES was measured by family occupational social class (OSC). The students answered six questions about their parents’ occupation: “Does your father/mother have a job?”, “If no, why does he/she not have a job?”, “If yes, please say in what place he/she works (for example: hospital, bank, restaurant)” and “Please write down exactly what job he/she does there (for example: teacher, bus driver)”. The research group coded the answers into OSC from I (high) to V (low). We added OSC VI for economically inactive parents who receive unemployment benefits, disability pension or other kinds of transfer income, similarly based on students’ responses. The questions about occupation were identical across surveys and so was the coding procedure . Most students (87.6%) provided enough information for the coding of OSC. Several studies showed that 11-15-year-old schoolchildren can report their parents' occupation with a high agreement with parents’ own information [46-49] and Pförtner et al.  showed that OSC is an appropriate variable for studies of social inequality in adolescents’ health. Each student was categorized by the highest-ranking parent into three levels of OSC: High (I-II, e.g. professionals and managerial positions), middle (III-IV, e.g. technical and administrative staff, skilled workers), and low (V, unskilled workers and VI, economically inactive).
Statistical procedures: We calculated age-standardized prevalence proportions of chronic backpain with 95% confidence intervals. The analyses included chi2-test for homogeneity and Cochran-Armitage test for trends over time. Initial analyses found similar patterns among boys and girls, so the final analyses combined boys and girls and standardized for sex. The analyses of social inequality of chronic backpain included two approaches: 1) Prevalence difference between low and high OSC as an indicator of absolute social inequality and 2) logistic regression analyses to examine the relative social inequality. The logistic regression analyses included OSC, sex, age group and survey year in mutually adjusted models and a final model with inclusion of an interaction term (survey year * OSC) to assess potential interaction between survey year and OSC. The analyses accounted for the applied cluster sampling by means of multilevel modelling (PROC GLIMMIX in SAS). Sensitivity analyses included analyses with two other cut-points of backpain frequency, 1) daily backpain ("about every day") vs. less often and 2) weekly backpain ("about every day", “more than once a week”, "about every week”) vs. less often.