There was an evident association between socioeconomic status and being sick-listed in the present study. Although this finding is in line with several previous studies [1
] this is the first study observing a social gradient in sickness absence using new cases of sick-listed employees (> 14 days) in a Swedish randomized working population sample. Since there has been a need for more studies conducted on randomized working population samples [2
] the present findings provide an important contribution to previous findings in Norway [2
] and in Denmark [1
]. The association between socioeconomic status and being sick-listed was observed in both women and men. Yet, it seemed to be stronger in men. Physical work ability constituted the strongest explanatory factor explaining the total association between socioeconomic status and being sick-listed in women. However, among men, a substantial part of this association remained unexplained. The explanatory effect of mental work ability was small. Surprisingly, even in the sick-listed sample most respondents had high mental and physical work ability.
The association between socioeconomic status and sickness absence was present in both women and men. However, albeit comparing Odds ratios from stratified analyses should be done with great caution, when taking these comparisons into account in combination with the proportional distribution the association was somewhat stronger in men. In addition, the specific gradients between different socioeconomic groups were also more pronounced in men. This pattern was particularly evident among men in the logistic regression analyses where the size of the point estimates increased for each downward level of socioeconomic status. Yet, the social gradient was somewhat more pronounced for women in the additional analyses on the more homogenous study sample, why the differences in the main analyses should be interpreted with some caution. Still the difference between women and men persisted and even in the additional analyses the gradient was more pronounced among men. That the social gradient was more prominent in men is in line with a range of studies on various health outcomes but also with a few previous studies on sickness absence [2
]. It has been suggested that measures of socioeconomic status may have less precision in women since they fail to capture gender as a significant element of these structures [22
]. This hypothesis also correspond with Koskinen and Martelin (1994) suggesting that the confounding influence of other socio-demographic variables, may either mask inequities in women or accentuate them in men [25
]. Hence, it is possible that the use of occupational class as a measure of socioeconomic status may have accentuated the observed difference between women and men in the present study. Yet, these differences may also mirror the vertical gender segregation [26
] resulting in highly educated women more often working in lower occupational positions than men with the same level of education. Moreover, although being categorized in the same occupational class women and men most often work in different occupational fields [27
] which sequentially end up in divergent patterns of exposures. In sum, it is possible that the measure of occupational class may not represent the same social structure in both women and men. Thus, a relevant question would be whether the usage of another measure of socioeconomic status would result in divergent findings. However, that substantial co-variations between occupational class, educational level, and income, were found in the present data in both women and men (figures not shown) give that a potential bias compared to using educational level or income for measuring socioeconomic status should be small. Still, future studies should analyze the effect of different measures of socioeconomic status on the social gradient of sickness absence for both women and men.
There was a strong explanatory effect of physical work ability on the association between socioeconomic status and sickness absence in the present study. While previous studies have mainly focused on physical and psychosocial work conditions this is the first study pointing towards the importance of work ability in explaining the social gradient in sickness absence. These results then complement previous research by Laaksonen and colleagues (2010) and Hansen and Ingebrigtsen (2008) showing that physical work conditions provide the strongest explanation for the social gradient in sickness absence [2
]. Consequently, the present findings might support a proposed mechanism comprising that a decreased level of work ability among lower socioeconomic groups will eventually result in a higher proportion that become sick-listed than in higher socioeconomic groups where level of physical work ability is usually higher. Since self-reported work ability is the product of both individual resources and work demands it is not possible to conclude whether the social gradient in physical work ability reflects higher physical work demands faced by the manual workers or their potentially lower individual resources. Still, since previous studies have found a social gradient in both physical health [28
] and physical work demands [3
] it is plausible to think that both these variables work in the same direction. Yet, we do not know whether they represent two independent effects or if an interaction effect may arise amid them.
The explanatory effect of mental work ability was almost negligible, in the main analyses. In the additional analyses on the more homogenous sample mental work ability has a somewhat stronger explanatory effect although most associations remained unexplained in both women and men. Yet, the result that mental work ability has lower explanatory effect may match previous findings showing that the explanatory effect of psychosocial work conditions on the social gradient of sickness absence is generally small [2
]. That physical work ability was the most important explanatory factor also parallels the fact that there was a social gradient in physical but not in mental work ability in the present study (figures not shown). A social gradient in work ability has previously been observed using the work ability index [14
]. Yet, we have found no such studies providing separate analyses for physical and mental work ability respectively. One may hypothesize that while it is probable that lower socioeconomic groups are more exposed to detrimental physical work conditions as a group there is little evidence that individual physical resources should differ in their favor. Consequently, we see a social gradient in physical work ability. On the other hand, it is much harder to hypothesize on the mechanism behind the non-existent social gradient in mental work ability. The main reason is that it is more difficult to interpret what the respondents put into their evaluation of mental work ability. For example, although the requirements of high competence and knowledge among employees in higher socioeconomic positions could also end up in high mental work demands, these demands are often accompanied with higher work control in turn balancing the effect of high demands [29
In women, the association between socioeconomic status and sickness absence was fully explained by differences in physical work ability. However, among men, a substantial part of the difference between non-manual and manual positions in relation to being sick-listed remained unexplained. These results were true also in the additional analyses on the more homogenous study sample. That the social gradient in sickness absence is explained in women but not in men is in line with previous findings by Laaksonen et al (2010) and Hansen and Ingebrigtsen (2008). These issues is further discussed above and may mirror that due to horizontal and vertical gender segregation measures of socioeconomic status may not reflect the same living conditions in women as in men. For example, the association between socioeconomic status and being sick-listed was less pronounced in women. That the association between socioeconomic status and sickness absence was explained in women but not in men may also suggest that we failed to cover factors important for this gradient in men. Another explanation may be that the items on work ability are perceived and, answered in heterogeneous ways by women and men. It is important to note that although reduced work ability is one major criterion for attaining a sick-leave certificate in Sweden; neither mental nor physical work ability explained the total association between socioeconomic status and sickness absence in men. So, are men in Sweden sick-listed for other reasons than reduced work ability? If this is not the case, one could question whether the two items from the Work Ability Index may measure different things in a healthy population and in a sick-listed population and that the items works better in women. We have not found any previous studies suggesting that this would be the case. Still, it is plausible to think that since only newly sick-listed cases were included in the present study they may answer the work ability items with their normal work ability in mind, particularly if the reason for them being on sick-leave is a health problem with short duration. On the other hand, the issue of whether the items are as valid in men as in women persists. In preliminary analyses we have also seen that work ability had different explanatory effect, for being sick-listed, in different socioeconomic groups. This will be furthermore examined in an upcoming study.
That the vast majority even in the sick-listed sample had a high level of work ability imply that the relation between being sick-listed and work ability should be further analyzed in future studies. Particularly, since level of work ability is the main constituent when evaluating individual's right to sickness benefit in Sweden.
One of the strengths of this study is that the measure of sickness absence was not based on self-reports but on new cases of sick-listed employees (< 14 days), reported by the employer. On the other hand this strength could also have worked as a limitation since the delay between inclusion and the respondents filling out the questionnaire allow some of them to go back to work. This give that the ones in the sick-listed sample that had already gone back to work had probably had also retained their work ability. There is unfortunately no way of getting around this problem when using new sick-leave cases but in order to address this limitation, additional analyses were conducted where all individuals not currently being sick-listed were deleted from the study sample (presented in the result section and in the discussion above). Although some differences appeared the results of the additional analyses were in line with the main analyses. That the study was conducted on a randomized working population sample also provide an important contribution to the literature about the social gradient in sickness absence. However, since the two measures on work ability were self-reports interpretations of their explanatory effects have to be made with some caution. It is also possible that their single-item design may have contributed to a lower reliability. Still, these two items are retained from the well-used Work ability index and has previously shown high reliability and validity [18
]. Unfortunately, we had no information on the diagnosed cause for being sick-listed. Still, according to a recent publication [12
] this information could have provided important information even in the present study. The overall response rate was rather low (i.e. 52%) and the analysis on non-respondents suggest that in line with most similar studies the proportions of men, younger individuals, and individuals born outside the Nordic countries are lower than in the total population. Finally, although the present study provide important and complementing knowledge we acknowledge that more complicated methodology may be needed to provide more accurate answers in the question of the social gradient in sickness absence (e.g. see [30