BMC Public Health 2008, 8:159 http://www.biomedcentral.com/1471-2458/8/159 for the last year prior to the investigation AND c) being in an unstable labour market position at the time of the investigation at age 30 (n = 73). Unemployed relatives was measured with one question (yes = 1, no = 0) about having an unemployed person in the family during the last 12 months. 2. Obtained permanent employment: a) ≥ one year of an unstable labour market position between age 25 and 29 AND b) having permanent employment one year prior to and c) at the time of the investigation at age 30 (n = 72). 3. Constant permanent employment: a) < one year of an unstable labour market position between age 25 and 29 AND b) having permanent employment one year prior to and, AND c) having permanent employment at the time of the investigation at age 30 (n = 572). Those who did not belong to any of these three groups were excluded from the analyses (n = 322). The variable "labour market position" was defined inde- pendent of having part-time or full-time employment. Other independent variables Gender was defined as 1 = men, 0 = women. The other independent variables were defined as potential confound- ers (i.e. variables associated with both the dependent and the independent variables) or potential mediators (i.e. var- iables that interplayed between the dependent and the independent variables). Those variables that could not be defined as either confounders or mediators were defined as having an unclear role. Indicator of health-related selection at age 16 As indicator of health-related selection we used psycholog- ical symptoms at age 16, which was measured with the same questions as those in the index of psychological symp- toms at age 30 (and dichotomised in the same way). Potential confounders at age 21 Unemployment was measured with the same battery of questions as the variable "labour market position". Those who were without a job for at least 6 months between ages 16 and 21 were defined as unemployed at a young age (= 1). The variable have children was measured with one ques- tion and defined as those having at least one child (= 1). Financial problems (cash margin) was measured with one question about the possibility of obtaining a certain amount of money (corresponding to 500 euros) within a week by one's own means, with alternative responses yes (= 0) or no (= 1). Potential confounders, mediators and factors with an unclear role at age 30 Socio-economic position was measured with one ques- tion about the subject's own occupation and defined according to the Swedish socio-economic (SEI) classifica- tion of occupational categories [45]. Blue-collar worker (or manual worker) was coded as 1 and white-collar worker (or non-manual worker) was coded as 0 (confounder). A negative answer to the question "Do you do what you want to do?" regarding type of occupation (work, studies, unemployment etc.) was considered to reflect not having the chance to do what one would prefer to do (= 1) (possible mediator) [46]. The psychosocial need for a job was measured by the Work Involvement Scale (WIS) which was developed and validated by Warr, Cook and Wall [25]. The WIS-scale consisted of six seven-grade questions about work com- mitment with a range from 6 to 42. High scores indicated high work involvement. The proportion over the cut-off point was defined as high work involvement (unclear role). Karasek-Theorell's two dimensional model was used to construct the indices of demands and control [22] in rela- tion to the current occupational situation (including employment, studies, unemployment, etc.). The index of high demands was measured from six questions with four answer alternatives each (giving a range of the index from 6 to 24) on qualitative and quantitative demands (refer- ring to quantity of work, intellectual requirements, and time constraints of the job). The index of low control (refer- ring to the possibilities of making decisions, being crea- tive and using and developing one's own skills) was measured in the same way as high demands. The propor- tion over the cut-off point was defined as those with high demands and low control correspondingly (possible media- tors). Validated questions on social network and social support indices developed by Henderson, Duncan-Jones, Byrne and Scott were used [47]. Poor social network was measured with four questions (each with six answer alternatives) about quantitative social network (range 4 to 24). Poor social support (material and emotional) was measured with six questions (with four answer alternatives) about quali- tative aspects of social support (range 6 to 24). The pro- portion above the cut-off point was defined as those with poor social support and poor social network correspond- ingly (unclear role). Page 6 of 11 (page number not for citation purposes)