BMC Public Health 2008, 8:159 http://www.biomedcentral.com/1471-2458/8/159 the regression analyses. In all cases the results were the same in the sensitivity analyses and then the guiding prin- ciple was to dichotomise at the 75th percentile. In order to analyse whether the transition from an unsta- ble labour market position to permanent employment was associated with the lower probability of having psy- chological symptoms, multivariate logistic regression analysis was used to estimate the odds ratio (OR) with 95% confidence intervals (CI) for the health outcome in relation to the variable "labour market position" after control for gender, possible confounders and mediators as well as for the indicator of health-related selection (Table 2). Bivariate regression analyses (not shown in the Table) were performed for the variable "labour market position" and the outcome variable in order to analyse the change in OR between the bivariate and the multivariate regres- sion analyses. Possible interaction between gender and labour market position was assessed with multiplicative interaction. Written consent to participate was given by the partici- pants in the study. The Ethics Committees of Umeå and Uppsala Universities approved the study. Outcome variable The outcome variable psychological symptoms at age 30 was measured with a well-known and validated scale from the Swedish Survey of living conditions [41,42]. The scale consisted of questions about symptoms during the last year with the answer alternatives "yes" (coded as 1) or "no" (coded as 0). The index of psychological symptoms included six items on restlessness, concentration prob- lems, nervousness, palpitations, anxiety and other nerv- ous symptoms. The range of the index was from 0 to 6, with higher values corresponding to more psychological problems. The proportion over the cut-off point (≥ 1) was defined as those with psychological symptoms. Those below the cut- off point (<1) were defined as not having psychological symptoms. Independent variables Labour market position The variable "labour market position" was measured with a specially constructed battery of questions where the par- ticipants were asked to report how long they had been unemployed, employed, studying, and participating in labour market programmes, etc., since the last follow-up. The reports were done for each school-year term (autumn and spring) during the period between 1986 and 1995. If the information about the labour market programmes and unemployment was missing or incomplete, it was Table 2: Multivariate logistic regression for psychological symptoms (75th percentile) in relation to labour market position after controlling for the indicator of health-related selection as well as possible confounders and mediators [odds ratios (OR), 95% confidence intervals (95%CI)]. Labour market position between age 25 and age 30 Without employment (Reference) Obtained employment Permanent employment Gender Psychological symptoms (16) Unemployed at young age (16–21) Have children (21) Financial problems (21) Unemployed relatives (21) Blue-collar worker (30) Do not have a chance to do what I would prefer to do (30) High WIS (30) High demands (30) Low control (30) Poor social network (30) Poor social support (30) Risk of unemployment (30) People look down on me (30) OR 1 0.35 0.22 0.93 2.18 1.26 0.92 1.47 1.19 0.67 1.76 1.01 1.57 0.79 0.71 1.45 1.04 2.01 95% CI 0.19–0.63 0.10–0.51 0.63–1.36 1.48–3.23 0.74–2.15 0.46–1.83 0.99–2.17 0.62–2.25 0.44–1.02 1.19–2.61 0.66–1.55 1.07–2.31 0.50–1.24 0.46–1.10 0.97–2.18 0.69–1.59 1.33–3.04 The model Chi-square was significant at p = <0.001 (x2 = 112.02, DF = 17). The explained variance in the regression models calculated by Cox & Snell R Square was 0.14). supplemented with data from the Swedish Labour Market Board's register. An unstable labour market position was defined as being in unemployment, occasional jobs or labour market pro- grammes. The aim of the labour market programmes in Sweden during the 1990s was to provide the unemployed with work experience and training in the regular labour market. A "vocational training" aimed at enhancing mobility. Matching on the labour market was also a part of the labour market programmes, and provided qualified theoretical or practical education, which was either pur- chased from educational enterprises or the regular educa- tion system [43,44]. Since we were interested in the transition from an unsta- ble labour market position to employment, the popula- tion was classified into three types of labour market groups. Through fulfilling the three criteria – a), b) and c), groups with the highest exposure to different kinds of labour market positions were defined. 1. Reference group (without permanent employment): a) ≥ one year of an unstable labour market position between age 25 AND 29 and b) unstable labour market position Page 5 of 11 (page number not for citation purposes)