BMC Public Health 2008, 8:159 http://www.biomedcentral.com/1471-2458/8/159 having lower scores of psychological symptoms at age 30 and having permanent employment was 0.19 (95% CI 0.08–0.47) respectively 0.22 (95% CI 0.09–0.55). Simi- larly to age 16, there was a strong association between the presence of psychological symptoms at age 30 and the indicators of health-related selection at age 18 (OR = 1.35, 95% CI 1.21–1.51) and age 21 (OR = 1.36, 95% CI 1.24– 1.50). Multiplicative interaction analysis was run in order to test for interactions between gender and labour market posi- tion. No significant interaction effects were found, even when controlled for all other independent variables in the model. Other possible confounders (education, alcohol con- sumption, smoking, use of narcotics) were also tested to be included in the multivariate regression analyses. None of them changed the significant association between psy- chological symptoms and labour market status. 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. Discussion The results showed that even after controlling for all the independent variables, including the indicator of health- related selection, a positive association was found between obtaining permanent employment after an unstable labour market position and a lower probability of having psychological symptoms. On the method The attrition rate in this study was as low as 3.6 per cent, which may be due to both the Swedish system of personal identification numbers, which makes it possible to find the address of almost anyone who lives in the country, and the fact that much effort had been expended to find even those who are usually left out in other studies [40]. Structural changes in the labour market in Sweden, including increasing unemployment and increasing work demands in all sectors, took place in the mid 1990s. Thus, we could assume that those who gained permanent employment during these conditions might be positively health-selected into employment. Those who remained unemployed could be selected out of the labour market due to poor health. However, the longitudinal design of the study made it possible to control for the health-related selection, i.e. the health situation at age 16 before entry into the labour market. Several possible limitations of our study need to be dis- cussed. A problem when analysing indicators of health- related selection is which time is best for measuring it. At age 16 the cohort participants were still in compulsory school and thus had not yet been exposed to the labour market. But the late teenage years and early adulthood is the period of greatest risk for the onset of affective disor- ders and therefore it might have been more appropriate to use health at the age of 18 or 21. However, at those ages the cohort had already been exposed to the labour market which could have affected their health status. For exam- ple, half of the cohort had experienced unemployment at age 21 and earlier research has shown the negative health consequences of unemployment already at an early age such as 18 [48] and 21 [7,8,27]. We tested health-related selection also at the ages of 18 and 21, but the lower prob- ability of psychological symptoms among those who had obtained permanent employment and among those hav- ing permanent employment remained at both ages 18 and 21. Thus, we chose to consider health at age 16 as the main measure of health-related selection. As young people are generally healthy, the probability is low that medical diagnoses will appear as a consequence of an unstable labour market position. Therefore, it seemed accurate to use self-reported symptoms. Moreo- ver, it was shown that self-reported symptoms are predica- tive of future medical diagnosis [49]. Due to the young age of the cohort, the number of persons with high scores in the measure of psychological symptoms is small, and the cut-off point for psychological symptoms was set at the 75th percentile in order to detect a group at risk. Another possible limitation of our study is the dichotomi- sation of the variable psychological symptoms at age 16. The reason for dichotomisation of the variable was that it should be measured in the same way as the identically measured outcome variable (i.e., psychological symptoms at age 30) which had to be dichotomised in the logistic regression analyses. Moreover, separate analyses showed that the association between lower scores of psychological symptoms at age 30 and obtaining permanent employ- ment as well as having permanent employment remained significant even if the indicator of health-related selection variable was used as a continuous variable. A further limitation is that the labour market position var- iable was measured from age 25, but we have no measure of the health situation or life circumstances at this age. The health of the cohort was measured with quite long intervals (age 16, age 18, age 21 and age 30). Therefore, the discussion of possible confounders in our study has to be hypothetical. The problem of attributing psychological health at age 30 to changes in the labour market position during a relatively long time period has been addressed by limiting the time period of the labour market position to Page 8 of 11 (page number not for citation purposes)