BMC Public Health 2007, 7:45 http://www.biomedcentral.com/1471-2458/7/45 SES was operationalized as paid employment status, occu- pation, and housing tenure. Employment status included two groups: (1) full-time and (2) part-time. Occupational status was divided into four groups: (1) unskilled manual workers, (2) skilled manual workers, (3) lower-level employees, and (4) middle-level employees and profes- sionals. Housing tenure was broken down into two groups: (1) those who owned their home and (2) those who rented their home. Quality of physical working environment was categorized as good or bad based on the sum of the answers to five yes (1) or no (0) questions: "Does your work involve repetitive and monotonous movements?" "Are you forced to work in bent and twisted postures?" "Do you become sweaty every day from effort in your work?" "Are you exposed to heavy shaking or vibrations in your work?" "Does your work require heavy lifting?" If the sum of the answers was between 0 and 2, the phys- ical working environment was categorized as good, and if it was between 3 and 5, the physical working environment was categorized as poor. Presence of stress at work was determined by asking two questions: "Is your job hectic?" and "Is your job psycho- logically demanding?" Response alternatives were yes (1) and no (0), and an index was constructed by summing up the two answers. Those who scored 0 to 1 were catego- rized as experiencing low stress at work (0), and those who scored 2 were categorized as experiencing high stress at work (1). A work-related social support index was constructed by summing up the responses to two statements, resulting in a scale from 0 to 4. The first statement was "can interrupt work to talk with coworkers," and choices included "no" (0), "sometimes" (1), and "often" (2). The second was "meets coworkers outside the workplace," and choices included "no" (0), "meets one workmate" (1), and "meets two workmates" (2). Those whose answers summed to above an approximate median were classified as having high work-related social support (0) and those whose answers summed to below the median were classified as having low work-related social support (1). Ethical considerations This study was approved by the Ethics Committee at Karo- linska Institute, Stockholm. Statistical analysis The SAS software package was used in the statistical anal- yses [20]. Sex- and age-adjusted prevalence rates (percent- age) of the different outcomes were calculated as the mean prevalence during the study period, 1995 to 2002. A logis- tic regression model was used to estimate the odds ratio (OR) of the outcomes, taking each of the different explan- atory variables into account. The results are shown as ORs with 95% confidence intervals. There were no interactions (p < 0.05) of interest between length of SA and the other explanatory variables. The fit of the models was judged by the Hosmer-Lemeshow goodness-of-fit test. The models were considered acceptable if p > 0.05, and all models met this demand. Results On average, 10.9% of the persons in the sample were on sick leave for ≥29 days during the study period, and 5.7% for >90 days. The SA pattern shows that women took longer sick leave than men (Table 1). For a few of the explanatory variables there was a gradient of increasing percentage of SA with increasing length of SA: people aged 55–64, unskilled manual workers, people with poor qual- ity of physical working environment, people with pres- ence of stress at work, and people who had poor work- related social support. Sex- and age-standardized prevalence of self-reported reduced working capacity due to longstanding illness of >90 days among people with paid employment is shown in Table 2. Not surprisingly, the prevalence of self- reported reduced working capacity increased in tandem with number of sick days taken. The age-adjusted preva- lence of reduced working capacity was higher among women and those living alone than among men or those who were married or cohabiting. More manual workers than middle-level employees and professionals reported reduced working capacity. Those working part-time had more than twice the prevalence of reduced working capac- ity than those working full-time, and those with a poor working environment, stress at work, and low work- related support reported reduced working capacity more often than those with a good working environment, no stress at work, and high work-related social support. A total of 10.5% of all persons who took 0–28 days of SA during the year reported reduced working capacity due to a longstanding illness. Table 3 shows the odds ratios (ORs) of self-reported reduced working capacity due to longstanding illness by number of SA days taken per year. Those who had more Page 4 of 11 (page number not for citation purposes)