BMC Public Health 2007, 7:45 http://www.biomedcentral.com/1471-2458/7/45 viewees are a simple random sample from the Swedish population register and interviewed in person over a two- year period about a wide range of topics, including their socioeconomic and demographic characteristics and working conditions. have poor self-reported health. There were five response alternatives between 1996 and 2002: very good, good, moderate, poor, and very poor. Those who rated their health moderate, poor, or very poor were considered to have poor self-reported health. We retrieved survey data on a total of 22,281 persons for the eight-year period between 1995 and 2002. We included only those persons interviewed who were between 25 and 65 years of age and had paid employ- ment. Some Living Conditions Survey participants are re- interviewed after eight years. We purposely chose to use an eight-year study period to avoid including the same persons twice. The response rate during the study period averaged approximately 80%. More detailed information on the Living Conditions Survey has been published else- where [19]. Main outcome measure The main outcome measure, self-reported reduced work- ing capacity due to a longstanding illness, was based on the answers to two questions in the Living Conditions Survey. The first was: "Do you suffer from any long-stand- ing illness, after-effects from an accident, disability, or other ailment?" Those who reported a longstanding ill- ness of at least 3 months duration were asked an attendant question: "Is your working capacity reduced due to any of the reported illnesses?" Those who reported working capacity reduced "to a great extent" or "to a certain extent" were considered to have reduced working capacity due to a longstanding illness; all others were considered to have normal working capacity. Additional outcomes There were three additional outcomes: psychosomatic complaints, anxiety, and poor self-reported health. Pres- ence of psychosomatic complaints was determined by the answers to three yes or no questions in the Living Condi- tions Survey, which asked if a respondent had experienced fatigue, sleeping difficulties, or migraine headaches in the last 14 days. Those who reported two of three complaints were considered to have psychosomatic complaints. Presence of anxiety was based on the answer to the ques- tion, "Do you suffer from anxiety?" There were three response alternatives: severe, moderate, or none. Those who reported severe or moderate anxiety were considered to have anxiety. Presence of poor self-reported health was based on the answer to the question, "How would you describe your general health?" There were three response alternatives in 1995: good, poor, or somewhere between good and poor. Those who answered that their self-rated health was some- where between good and poor or poor were considered to Sickness absence (SA) among people with paid employment The National Social Insurance Board's records available to us included the length of all SA instances per year paid for by the board. During the study period, the length SA paid for by employers varied. Thus, for most of the study period, National Social Insurance Board records included length of all SA >14 days, but for 1997 and the first quarter of 1998, data were available only for SA ≥29 days. We have taken these differences into consideration in the analyses. SA data from the National Social Insurance Board include only continuous spells of absence >7 days in Sweden, and thus all corresponding data in this study, are medically certified. All SA data of ≤7 days in Sweden are self-certi- fied. Employers keep the medical certification for all SA between 8 and 14 days in length (7–28 days in 1997 and the first quarter of 1998). Information about repeated shorter absences (paid for by employers) was not availa- ble to us. We used individual serial numbers to retrieve the board's SA record for each of the 22,281 participants from the Liv- ing Conditions Survey. SA data were obtained only for the year of the Living Conditions Survey interview and are summarized in number of SA days. Because the exact dates of SA were not available, we took steps to correlate SA and interview data as closely as pos- sible. First, we chose a duration of illness >90 days to increase the probability that the SA occurred because of the illness reported in the survey. Second, we looked at SA ≥ 29 days, which also increases the probability that this SA was caused by the reported illness. Length of SA comprised three levels: 0–28, 29–90, and >90 days per year. Explanatory variables Eight potential confounders were taken into account: sex, age, de facto marital status, socioeconomic status (SES), quality of physical working environment, presence of stress at work, and work-related social support. Age was categorized into the following groups: 25–34, 35–44, 45– 54, and 55–64 years. De facto marital status included two groups: married/cohabiting or living alone. Page 3 of 11 (page number not for citation purposes)