BMC Public Health 2008, 8:159 http://www.biomedcentral.com/1471-2458/8/159 reemployment because reemployment may have more financial importance among blue-collar workers, while reemployment may mean the status of having a job among white-collar workers [33,34]. Second, most of the studies have a design that makes it impossible to control for health-related selection due to lack of information about the health status before enter- ing the labour market. Health selection means that early health status could have an impact on adult health status [35]. Poor health before entering the labour market could lead both to difficulties in obtaining a job and to contin- ued poor health if reemployed [3,36]. Earlier research has also shown that a history of poor health is associated with a higher risk of mobility out of employment and a lower chance of mobility into employment for both men and women [37]. Third, the labour market during the last two decades has become increasingly polarised between two extremes – "the secure centre", characterised by permanent employ- ees, and the "insecure periphery", characterised by differ- ent kinds of temporary employment [38] as well as by unemployment and labour market programmes [17]. Thus, the frontier between employment and unemploy- ment is becoming blurred and it is methodologically dif- ficult to obtain accurate data about exposure [39]. It has therefore been suggested that studies on the labour mar- ket position should move away from the comparison between employed and unemployed towards a more dynamic comparison of workers with stable versus unsta- ble jobs [10,39]. As young people often move between different temporary situations such as unemployment, labour market programmes, studies and precarious employments [7] the analysis of an unstable labour mar- ket position rather than unemployment alone would give a more comprehensive picture of the transition to perma- nent employment. Fourth, the non-response rate is often high in longitudinal studies which are necessary for analysing health changes related to labour market trajectories. A high response rate is crucial for including marginalised people who are often missing in other studies, and earlier research has shown that even a low non-response rate can introduce serious bias and uncertain results [40]. Thus, there is a strong need for longitudinal studies with high response rates, starting at a young age before the cohort enters the labour market, so that health-related selection can be controlled for. The aim of this study was to analyse whether transition from an unstable labour market position to permanent employment could protect mental health. The hypothesis was that obtaining permanent employment after being in an unstable labour market position is associated with a lower risk of psychological symptoms after controlling for gender, possible confounders and mediators as well as an indicator of health-related selection. Methods The study was carried out in a medium-sized industrial town in the north of Sweden. The cohort, consisting of all 1083 pupils (506 girls and 577 boys) aged 16 who attended or should have attended the last year of compul- sory school in 1981, was followed up at the ages of 16, 18, 21 and 30. Extensive work was carried out to reach every participant, including those who had moved, in order to minimize the non-response rate to a minimum [8]. Com- plete data for the whole 14-year follow-up period were collected for 1044 individuals (547 men and 497 women), i.e. 96.4% of the original sample. Data were mainly collected with comprehensive question- naires during school hours (at the ages of 16 and 18) and during class reunions (at the ages of 21 and 30). The ques- tionnaires covered the following main themes: psycholog- ical and physical symptoms, health behaviour, experiences of work and unemployment, as well as socio- economic data. The questionnaires, including a reminder, were sent to those who could not attend these reunions. Participants who failed to reply were contacted by tele- phone and, if they agreed to participate, a telephone inter- view was performed. A within-subject design was used, based on three repeated measurements. The surveys from 1981 (when the respondents were 16 years old), 1986 (21 years) and 1995 (30 years) were used in this article. The method is exten- sively described elsewhere [35,40]. Statistics Windows version 15.0 of SPSS was used for data analysis. We assessed the distribution of the dependent variable (health outcome) and independent variables between the three groups of labour market position for men and women separately (Tables 1a and 1b). A chi-square test was used to analyse differences for the dichotomous vari- ables between the studied groups in Tables 1a and 1b. A p-value <0.05 was chosen as statistically significant. The correlations between all independent variables included in the analyses were calculated [see Additional file 1]. The internal consistency for all indices included in the model was measured by Cronbach's alpha [see Additional file 2]. Sensitivity analyses were performed for both the depend- ent and the independent variables with ordinal scales in order to define how to dichotomise variables. Thus, differ- ent cut-off points of each specific variable were tested in Page 3 of 11 (page number not for citation purposes)