BMC Public Health 2007, 7:45 http://www.biomedcentral.com/1471-2458/7/45 Background The increasing cost of public social sickness insurance poses a serious economic threat to the Swedish welfare state. In recent years, expenditures for social insurance in general, as well as social sickness insurance in particular, have risen steeply in Sweden. Total expenditure on social insurance increased from 17.2 billion Euros in 1985 to 45.3 billion Euros – one sixth of the Swedish gross national product – in 2003 [1]. A 2005 report indicated that 25 of every 100 Euros spent on private consumption in Sweden comes from social insurance [2]. The number of persons on sick leave in Sweden climbed from 180,000 in 1999 to 302,000 in 2003 [3], and between 2000 and 2003 alone, spending on social sickness insurance increased by 14% [4]. Sweden's social insurance system was created in 1955 as part of the establishment of the welfare state. The goal was to provide economic protection to families, children, the elderly, and the unemployed, as well as persons with func- tional disabilities, illnesses, or occupational injuries [5]. Social sickness insurance, one component of the system, was intended to prevent marginalization due to ill health, and to make it possible to preserve an almost unchanged economic standard when ill [5]. In the Swedish system, employers provide sick pay for the first 14 days of illness. A doctor's certificate is required for absences longer than seven days. After 14 days, the gov- ernment takes over from the employer, providing finan- cial compensation called sickness benefit to persons whose working capacity is reduced "due to illness or other impairment in . . . physical or mental performance" [6]. Unlike unemployment benefits in Sweden, which are gen- erally limited to 300 working days [7], sickness benefits are not subject to an official time limit [8]. The level of compensation is based on the degree to which working capacity is reduced, i.e., one quarter, one half, three quar- ters, or fully, and on yearly income prior to onset of reduced working capacity [6]. The phenomenon of sickness absence (SA) in Sweden is complex. The number of people on sick leave varies over time based on a number of factors, including overall eco- nomic conditions, labor market conditions, social policy, and unemployment levels [4]. It has been shown that numerous factors may increase the number of people who take sick leave and the duration of sick leave, including generous sick pay, use of absence as strategy to reduce stress and thus prevent illness, and also need to support family members [9-11]. sick leave [12-14]. The most common diagnoses of per- sons on sick leave in Sweden are musculoskeletal diseases and psychiatric problems, which together account for about 60% of all SA diagnoses [15-17]. Conceivable causes of musculoskeletal complaints include not only physical labor, but also psychosocial factors [18]. Evidence suggests that individual factors may play a con- tributory and perhaps even decisive role in determining who takes sick leave, over and above the presence or absence of sickness, and perhaps even the root cause (physical or psychosocial) of the sickness in question. The authors of a recent Dutch study found that whereas it was mainly work-related physical and psychosocial factors that determined the occurrence of musculoskeletal com- plaints among a group of laundry and dry cleaning work- ers in the Netherlands, individual factors predominantly determined whether persons with these complaints took sick leave or not [18]. The current study is unique in that it links two separate, reliable datasets at the individual level for the first time. One dataset, collected as part of Statistics Sweden's annual Living Conditions Survey, consisted of self-reported infor- mation on longstanding illness and other health prob- lems. The other consisted of objective reports from the Swedish National Social Insurance Board of actual SA taken during the same period by the same individuals. Our first objective was to determine whether sick leave of ≥ 29 days in Sweden was associated with self-reported reduced working capacity due to a longstanding illness. A second objective was to learn if any such associations remained after adjustment for individual sociodemo- graphic and socioeconomic factors, level of employment, and working environment. A final objective was to deter- mine if there were any persons on sick leave ≥29 days who did not report reduced working capacity due to a long- standing illness, and if so, to ascertain their characteristics. Methods Data sources Data used in this study were gathered from two independ- ent sources: the annual Swedish Living Conditions Survey (Undersökningarna av levnadsförhållanden or ULF in Swed- ish), and National Social Insurance Board SA records. The two datasets were linked at the individual level using the national civic registration numbers assigned to all persons in Sweden for their lifetime. National civic registration numbers were changed to serial numbers to insure indi- vidual anonymity. Studies from various countries show that a number of psy- chosocial factors like divorce, stress at work, and work- place reorganization increase the general risk of taking The Living Conditions Survey has been conducted yearly since 1975 by professional interviewers from Statistics Sweden, Sweden's governmental statistics agency. Inter- Page 2 of 11 (page number not for citation purposes)