BMC Public Health 2007, 7:45 http://www.biomedcentral.com/1471-2458/7/45 The authors of the Whitehall II study suggest that SA is particularly suited for use as an overall global health indi- cator, because longer periods of SA are based on a physi- cian's examination rather than self-evaluation [28]. The results of the current study provide some support for this interpretation, as we demonstrated not only that more persons on sick leave in Sweden reported longstanding ill- ness and reduced working capacity than did those not on sick leave, but also that increasing length of SA is associ- ated with increasing rates of self-reported longstanding ill- ness and reduced working capacity. Controlling for SES (operationalized as occupational sta- tus, form of tenure, and employment), as well as physical and psychosocial working conditions (significant risk fac- tors for longstanding illness that results in reduced work- ing capacity) only reduced the odds of SA by about 10%. However, we did find an association between socioeco- nomic variables and longstanding illness – especially between employment, occupational status, quality of physical working environment, and longstanding illness – which is in agreement with the findings of at least one other study [29]. Limitations This study has several limitations. For example, existing survey data were not collected specifically to test the pro- posed study aims. Respondent bias is a possible limita- tion, particularly if nonrespondents (20%) differ from respondents with respect to the measurements being made. We found that those who refused to participate in the Swedish Living Conditions survey during the study period (2/3 of nonrespondents) had the same mortality rate as respondents, whereas the other two groups of non- respondents (those who were not found and those who were ill) had a significantly higher mortality rate than respondents. There is potential for self-report bias in some of the data from the Swedish Living Conditions Survey. There is, however, no potential for self-report bias for the SA varia- ble in this study because they were collected from other sources after the Living Conditions Survey was complete. Another limitation of this study is that conclusions about causal relationships or direction of causality cannot be drawn because of the cross-sectional design. This limita- tion is further exacerbated by the possibility that some of the Living Conditions Survey interviews may have taken place before the SA; in other words, the illness might have occurred later during the year than the interview. How- ever, this weakness is somewhat mitigated by the fact that our outcome measure required a longstanding illness of at least 3 months duration. In addition, the range of specific health indicators in this study is limited. The main question asked ("Do you suffer from any long-standing illness, after-effects from an acci- dent, disability, or other ailment?") was quite general. Thus, it is possible that some health problems that led to ≥29 days of SA may have been missed in the study. Many problems not identified via the main question would have been found via the questions on additional health complaints, but it is possible that some were not. For example, some instances of musculoskeletal disor- ders, a common cause of SA in Sweden that can be related to psychosocial stress and repetitive work tasks, may have been missed. This would be the case if respondents had a such a disorder but did not consider it to be a longstand- ing illness or the after-effect of an accident, disability, or other ailment, and furthermore reported good health, no psychosomatic complaints (fatigue, sleeping difficulties, migraine headaches), and no anxiety. Moreover, those who did not report any "long-standing illness, after-effects from an accident, disability, or other ailment" were not asked whether their working capacity was reduced. To sum up, it is clear that we cannot exclude the possibil- ity that some persons with reduced working capacity were not identified. Finally, we had data on continuous absences long enough to be paid for by the National Social Insurance Board, but no data on possible multiple shorter absences paid for by employers. Thus, any instances of repeated shorter absences from work would have been missed and the peo- ple who took such absences placed in the "0–28 days of SA" category even if their accumulated days of absence amounted to ≥29. Strengths These limitations are balanced by the strengths of the study. For example, the Living Conditions Survey data were collected in face-to-face interviews undertaken by trained interviewers. The quality of the variables has been studied in re-interviews and is mostly high [30]. Another advantage is the large sample size. The sample was drawn from the entire population of employed (paid) men and women interviewed as part of the Living Conditions Sur- vey. The response rate was high (80%). A strong advan- tage is that the National Swedish Social Insurance Board has complete data on all instances of SA longer than 14 days for all employees in Sweden. The insurance board obtains this information from employers, who are obli- gated to report all instances of SA of more than 14 days. By linking the data from the Living Conditions Survey with the sick-leave data from the National Swedish Social Insurance Board, we avoided the problems associated with recall bias with regard to SA. Page 9 of 11 (page number not for citation purposes)