The Hordaland Health Study
The data collection was conducted as part of the Hordaland Health Study (HUSK) in collaboration with the Norwegian National Health Screening Service and the local health services. HUSK is one of several Norwegian population based health surveys, and took place between 1997 and 1999 in Hordaland county, a county in western Norway with both urban and rural areas.
All inhabitants in Hordaland County born between 1953 and 1957 were invited to participate (Figure 1), giving a target population of 29,400 subjects (15,051 men and 14,349 women), aged 39-46 years at the time of the study. The participants responded to questionnaires on various health related matters and sociodemographic factors, including the highest completed level of education. Participants also went through a clinical examination, with measures on height, weight, blood pressure and blood level cholesterol. 18,565 persons (8585 men and 9980 women) met to the clinical examination for inclusion in the study, giving a response rate of 63% (57% for men, 70% for women).
Exposure: Socioeconomic status
Several measures of SES have been used in previous research, including education, income (personal or household) or classification of occupation. Education has been shown to be a particularly good measure of SES in health related issues [16, 17]. In addition, level of income is generally reduced when on disability pension, and this would diminish the relevance of income as a measure of SES with our approach.
As a measurement of SES we used self-reported information on achieved level of education, stratified into three groups; compulsory school only (9 years of schooling), high school (a total of 12 years of schooling), and higher education (more than 12 years of schooling, corresponding to a college or university degree).
Outcome: Disability pension
According to the law, a permanent disability pension is awarded for life for chronic conditions after proper treatment and rehabilitation is performed. Consequently, we wanted to compare current recipients of permanent disability pensions to those who do not receive such a pension (whereof the majority are in labour work).
Using personal identification numbers, which are issued to all Norwegians by the state at time of birth (for immigrants at time of immigration) the health survey was linked to the National Insurance Administration's (NIA) records on disability pensioning covering the period January 1992 to December 2004. These registries are updated annually, and their accuracy is well documented [13]. We excluded, however, all subjects who were granted a disability pension after the collection of health data in HUSK (N = 708), as these disability pensions could be based on health problems originating after the assessment of health data in HUSK.
NIA has recorded exact dates for initiation of all disability pensions granted after January 1992, but for disability pensions granted before this, we did not have precise data on time of initiation. A disability pension granted early in life may influence on the education level reached. Thus all subjects who had been granted a disability pension before January 1992 (N = 314, aged 32-40 years at the time) were also excluded from the analyses (Figure 1).
For participants included in the study, the time range assessing disability pension was therefore from 1992 until their time of participation in HUSK (which for each participant was some time in the period of 1997-1999).
Health variables
The Norwegian National Insurance Act states, in line with most OECD countries, that to be considered for a disability pension your earning capacity must be reduced due to permanent illness, injury or impairment. Thus we adjusted for as many such health factors as possible (both somatic and mental) to estimate how much of the relationship between SES and disability pension was left unexplained.
Mental health problems have been shown to be considerable risk factors for disability pension, including disability pensions awarded for somatic disorders [18, 19]. Symptoms of anxiety and depression were assessed in HUSK with the Hospital Anxiety and Depression Scale (HADS) [20]. The scale consists of 14 items, 7 related to anxiety (HADS-A) and 7 related to depression (HADS-D). The two HADS sub-scales are often used with a cut-off value for probable positive cases [21], but have been shown to be better predictors of impairment when used dimensionally rather than categorically [20]. Thus, in this study, the HADS sub-scales were used as two separate ordinal variables.
As a measure of somatic conditions the subjects were asked whether they "have or have had" coronary infarction, angina, stroke, diabetes, asthma or multiple sclerosis. The number of conditions checked was summed up in one ordinal variable; number of self-reported somatic conditions. Scores above 3 was collapsed at 3. The variable thus included 4 categories (0-3). The participants were asked if they had taken any medications the previous day, and, if so, for which medical condition and the name of the medication. On the basis of this information, a team of physicians appointed appropriate diagnoses based on the International Classification of Primary Care system (ICPC). An ordinal variable describing the number of different diagnoses for which the respondent received medications was established; number of medicated diagnoses with values ranging from 0 to 3 (scores above 3 collapsed).
As a substantial proportion of disability pensions are likely to be granted on basis of more diffuse pain disorders [22], we also adjusted for somatic symptoms. Participants were asked about 17 somatic symptoms from the ICD-10 research criteria for Somatisation disorder (F45.0). For each symptom, all participants checked off how often they experienced the symptom: "almost never", "rarely", "sometimes", "often" or "almost always". Responses were coded from 0 to 4 and summed up in one continuous variable [23].
Musculoskeletal pain and disorders rank among the most frequent causes of disability pensioning [24], and fibromyalgia has been one of the most frequent single diagnosis causing disability pensions in Norway [25]. In our study the participants were asked if they "suffered from Fibromyalgia/Fibrositis/Chronic pain syndrome", and the answers were entered into a dichotomous variable (yes/no). The participants were also asked if they during the previous year had been suffering from pain and/or stiffness that lasted for at least 3 consecutive months, and if so, they were asked to specify the localization of this pain to any of the following 10 anatomical locations: neck, shoulders, elbows, hands/wrists, chest/abdomen, upper back, lower back, hips, knees and ankles/feet. One dichotomous variable was established for each pain site.
Obesity is a known risk factor for sickness absence and disability pension [26, 27], and weight and height were measured at the clinical examinations in HUSK. A Body Mass Index (BMI) of 30 or more was classified as "obesity" in the analyses and treated as a dichotomous variable.
Missing values
A total of 2476 participants were excluded from the study for having missing information on symptoms of anxiety, depression or somatisation (Figure 1) [28]. Missing data on self reported somatic diagnoses (n = 44), diagnoses based on medication (n = 267) and on fibromyalgia (n = 56) were substituted by the mode [29]. Missing data for measured BMI (n = 27) was substituted by mean value of 25.4 [29].
Analyses
Univariate logistic regression analysis was applied to estimate the unadjusted Odds Ratios (ORs) for being granted a disability pension with level of education as independent variable and higher education (>12 years) as the referent group. Multivariate analyses were then performed controlling for all relevant health variables measured in the survey, modelled as continuous or dichotomous variables as described above. All analyses were performed in SPSS version 14.0 for Windows.
The proportion of the association between educational level and disability pension that was accounted for by health was calculated as
where β is the regression coefficient (whereof OR is calculated as eβ).
Stratified analyses for men and women were performed to explore eventual sex differences.
To analyse associations between health and socioeconomic status and between health and disability pension, we used one-way ANOVA tests for the continuous variables and chi-square tests for the dichotomous variables. For these particular analyses of association, the ten dichotomous variables on localized pain/stiffness were summed up in one variable indicating number of pain sites, and analysed as one continuous variable.
Ethics
The study protocol was approved by the Regional Committee for Medical Research Ethics, Western Norway and by the Norwegian Data Inspectorate. All HUSK participants gave their written informed consent when they met at the examination premises.