Department of Community Medicine and General Practice, Norwegian University of Science and Technology, Trondheim, Norway.
Correspondence: Steinar Krokstad, Department of Community Medicine and General Practice, Norwegian University of Science and Technology, HUNT Research Centre, Neptunveien 1, 7650 Verdal, Norway. E-mail: steinar.krokstad{at}medisin.ntnu.no
Abstract
Background Non-medical factors may be important determinants for granting disability pension (DP) even though disability is medically defined, as in Norway. The aim of this analysis was to identify determinants of DP in a total county population in a 10-year follow-up study.
Methods Participants were people without DP, 20- to 66-years-old in 19841986. The baseline data were obtained in the Nord-Trøndelag Health Study (HUNT): 90 000 people were invited to answer questionnaires on health, disease, social, psychological, occupational, and lifestyle factors. Information on those who later received DP was obtained from the National Insurance Administration database in 1995. Data analyses were performed using Cox regression analyses.
Results The incidence of DP showed great variation with regards to age and gender, accounting for an overall increase in the follow-up period. Low level of education, low self-perceived health, occupation-related factors and any long-standing health problem were found to be the strongest independent determinants of DP. Low level of education and socioeconomic factors contributed more to younger peoples risk compared to those over 50 years. For people under 50 years of age with a low level of education compared to those with a high level of education, the age-adjusted relative risk for DP was 6.35 for men and 6.95 for women. The multivariate-adjusted relative risk was 2.91 and 4.77, respectively.
Conclusions Even for a medically based DP, low socioeconomic status, low level of education and occupational factors might be strong determinants when compared to medical factors alone. These non-medical determinants are usually not addressed by individual based health or rehabilitation programmes.
Keywords Insurance, disability, pensions, socioeconomic factors, risk factors, social medicine, epidemiology
Accepted 23 July 2002
Most industrialized countries have public income-maintenance programmes to protect workers in case of disability.1 For long-term illness or injury, disability pension (DP) is typically comprised of both universal and earnings-related programmes.2 Many countries in Europe and North America have experienced a dramatic increase in rates of such government paid benefits.1,35 In Norway, incidence of the medically based DP started to increase in 19821983, after a stable period throughout the 1970s. In 1999 there were 33 551 people(14 822 men and 18 729 women) who were granted DP in a working population of 2.5 million people; this compared to a stable incidence of approximately 19 000 per year in the 1970s.6
Medical certification for granting DP has become one of the major paths to public aid in modern welfare states. In periods where the number of DP increases rapidly, there is often a general concern that the programmes are in crisis.1 Policymakers and analysts have explained the increased programme size in different ways. The sociological traditions take the perspective of disability as a social role, and discuss what leads individuals to adopt the disabled role.7,8 Economic theory takes the perspective of the welfare-maximizing rational man and suggests that people will define themselves as disabled when the benefits of that role are greater than those derived from work.1 Background factors such as the increasing number of women in paid work, pressure from physicians facing people with illness and pressure from both the legal and the economic realm have also been discussed.1
The diagnoses most often applied for DP are musculoskeletal, psychiatric and cardiovascular.5,9 The proportion of recipients with psychiatric disorders seems to increase10 and these disorders are of special concern because they affect people in early adulthood.11
Low level of education,12 low socioeconomic status (SES),1315 physically strenuous work,16 too quick a pace at work, unpleasant working conditions,17 unemployment18 and poor physical condition19 have been found to be determinants of DP in different population samples. Significant differences in prevalence of DP by area deprivation have also been shown.20 Sociologists have noted that there is an increasing health-related selection out of the workforce, especially affecting people with low SES, which seems to be an increasing problem connected to working life and employment conditions in Western societies.21
Most epidemiological studies on DP have used narrow cohorts, included only one gender or investigated the effects of few specific risk factors. A total population study addressing causes of inequalities in risk of receiving a DP by SES controlled for a wide range of confounders has, to our knowledge, not been performed. The main objective of this 10-year follow-up study was to examine SES and education level (which may serve as a proxy for SES) as predictors of DP.
Materials and Methods
Subjects
A comprehensive health survey, the Nord-Trøndelag Health Study (HUNT), was conducted in Nord-Trøndelag county, central Norway, in 19841986.22 All inhabitants 20 years residing in the county were invited to participate in the study. A total of 74 599 people participated, accounting for 88.1% of the adult population. In addition to filling in questionnaires each participant was screened for a number of health measures. The analyses in this study were restricted to men (n = 32 194) and women (n = 30 175) aged 2066 years without DP at baseline. The upper age limit was set at 66 years as retirement pension is available to everyone at the age of 67 in Norway.
Disability pension
Information on later uptake of DP up to 1995 was obtained from the National Insurance Administration database. The eligibility criteria for granting DP in Norway were established by law in 1967. The pension is intended to secure the income of people who have had their earning ability permanently impaired by at least 50% due to illness or disease, injury, or disability. In addition, five conditions must be met: the applicant must have been a member of the national insurance programme for at least 3 years (everybody who is a resident in Norway is a member); the applicant must be between 16 and 67 years; the illness or disease, injury or disability must be the main cause for impaired earning ability (excludes primary social causes); the applicant must have undergone appropriate medical treatment and rehabilitation in order to improve his/her earning ability; and the earning ability must be impaired long-term and by at least 50%. These medical criteria have been essentially unchanged since 1967. However, there was a slight tightening of the criteria in 1991, emphasizing that medical conditions should be the main reason for the disability, explicitly excluding social problems as a cause. For all practical purposes DP has been a one-way event, usually lasting until retirement pension age at 67 or death.
Socioeconomic status and other variables
All independent variables in this study were taken from the HUNT-Study questionnaire, except civil status which was taken from the national register in the survey summons file. Methodological studies have shown the high validity of health questions in the survey.23
Socioeconomic status was measured by two different approaches. In the first approach people were classified based on their position in the labour market.24 Standard occupational class codes were not available in HUNT, but due to the similarity between the occupational classification in HUNT and the Erikson, Goldthorpe and Portocarero (EGP) scheme, an approximation was possible with a reclassification. A comparison between this method and a standard method has shown the applicability of this procedure.25 However, for women the EGP scheme based on own occupation is a less reliable measure of SES.26 Thus, additional analyses using a social class grouping based on husbands occupation were applied. Women living alone were still classified according to own occupation.
In the second approach the population was stratified according to the highest education level achieved. Education level serves as a proxy for SES and is probably the best measure for SES among women in this setting.27,28 Since education level can provide a gradient scale for both genders and serve as a one-dimensional measure of SES, we used education level as a proxy for SES in the multivariate Cox regression analyses for both genders.
Any long-standing health problem was monitored by asking Do you suffer from any long-standing limiting somatic or psychiatric illness, disease, or disability? The answer categories were yes and no. This variable is used in many international studies27 and corresponds closely to the main eligibility criteria for DP in Norway.
Employment status was reclassified into the following categories: employed (full or part time), unemployed, homemaker, other (student, unclassified and other pensions than DP).
The other variables originally had four to seven answer categories in the health survey questionnaire. Citations to validation studies or to theoretical context for these variables are given below. In the analyses these variables were classified as follows:
Occupational risk factors:16,29,30
low job control (little or no ability to plan own work versus considerable or full ability to plan own work),
high physical demands (often or always worn out versus seldom or never worn out),
high demands in concentration and attention (often or always worn out versus seldom or never worn out),
low job satisfaction (less than good satisfaction versus good to very good satisfaction).
Psychosocial risk factors:31,32
separated or divorced (separated or divorced versus unmarried, married or widow/er),
loneliness (often or very often lonely versus some times or less often lonely),
low subjective well-being (extremely, very or fairly dissatisfied with life versus yes and no, fairly, very, or extremely satisfied).
Health perception:33
perceived health less than good (perceived health less than good versus good or very good).
Health-elated lifestyle factors:3436
lack of physical exercise (physical exercise less than once a week versus once a week or more),
smoking (current smoker versus not current smoker),
high alcohol consumption (have drunk excessively versus may have or have not drunk excessively).
Follow-up and endpoints
Each participant contributed person-years from the year of study entry (The HUNT Study 19841986) until the year of being granted DP (n = 7322), reaching 67 years (n = 6057), death (n = 1195) or emigration (n = 368) before these events, or the end of follow-up on 31 December 1994 (n = 47 427). Median follow-up time was 9.7 years (mean, 8.7 years). Every citizen in Norway is given a unique national identity number of 11 digits at the time of birth, which contains information on birth date and gender. This identity number enabled individual linkage between collected information in the HUNT Study, the register of DP at the National Insurance Administration and the register of deaths at Statistics Norway which were used to determine vital status (alive, emigrated, dead).
Statistics
When comparing the incidence of DP in the study county with the total country, age adjustment by direct standardization was applied. The Cox proportional hazards model37 was used to calculate age- and multivariate-adjusted relative risk estimates (hazard ratio with 95% CI) of receiving DP according to SES and educational level, using the highest class and level as reference. In the final model the data-set was stratified by gender and age (2049 years and 5066 years) due to the interaction and exponential increasing effect of age on risk of receiving a DP.38 We considered all variables in the HUNT Study, which was not originally designed for these analyses, as potential determinants of DP. The variables in the final model (Table 4) were selected by empirical and statistical approaches. First, all variables were tested individually as independent variables, then possible interactions and confounding were explored. In the final stratified model education level and age were entered into the model, then all other variables were selected by forward likelihood-ratio statistics. The multivariate relative risks (hazard ratios)(mRR) should be interpreted as the risk of receiving a DP for people exposed, adjusted for all variables in the model. All statistical analyses were performed using the statistical software SPSS for Windows version 10.0.
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Figure 1 shows the incidence of DP per 1000 person-years at risk in the study county compared to the total country 19741998. The incidence rates of the study county followed the national rates closely. The incidence varied considerably with an overall increasing trend.
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Table 5 presents the crude hazard ratio of receiving a DP according to education level successively adjusted for age, any long-standing health problem, occupational factors, psychosocial factors, health perception and lifestyle factors. A large attenuation of the hazard ratio after inclusion of an explanatory variable indicates the importance of this variable for the exposed group. An increasing hazard ratio, as observed for women aged 5066 years when controlling for any long-standing health problem and occupational factors, implies that the factor controlled for was more prevalent in the groups with high education.
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The incidence of DP varied considerably in this Norwegian population during the follow-up, but followed the national trend towards an overall increase. Low education level, physically demanding work, low job control, self-perceived health less than good, and long-standing health problems strongly influenced the risk of receiving a DP.
The strength of this study is that it covers a total Norwegian population of over 60 000 people with participation rates at 85% among men and 90% among women. Use of the National Insurance Administrations register data ensured complete data on DP in the follow-up. However, the participation rate varied, 65% for men aged 2024 years being the lowest. A comprehensive non-responder study was performed after HUNT I.39 The low participation in the youngest age groups was explained by people being too busy, not interested, or studying outside the county. In that study there was no significant selection according to morbidity, and there was no consistent association between non-response and education level in the age groups selected for this study.
The relatively high proportion of people who could not be allocated to a social class, reflects missing data on occupation and education level for participants not returning the second questionnaire by mail after the initial screening day. Analyses have shown that there was no significant selection according to health among non-responders according to these variables.
Global morbidity and health measures from the HUNT database recorded at baseline were preferred in this study; Compared to medical diagnostic data, these variables are much more comprehensive and presumably better suited for this total population study. Data from the National Insurance Administration comprised medical diagnoses applied when DP was granted, but these data were not recorded at baseline.
In a 10-year follow-up period, the impact of the exposure variables may decrease over time. To check this hypothesis we performed analyses splitting up the material for people granted DP into three equal time periods after the initial survey. For the main independent variable, education level, there was no consistent pattern of losing explanatory power over time. For unemployment reported at baseline, for example, a condition that may easily change, there was no consistent pattern of losing explanatory power either. These sub-analyses indicate that the variables selected in the Cox proportional hazards model kept their predictive power for the entire study period.
The methods applied in this study are more frequently used when studying associations between exposure and disease. The use of the term risk factor may then be appropriate. When DP is the outcome, the endpoint is undesirable for society on both human and economic grounds, but may be wanted or necessary for the individual. In this study we therefore primarily use the word determinant about the exposure variables. Use of the term determinant may also be looked upon as a precaution, since we studied associations rather than causal relationships.
The incidence of DP has varied considerably in Norway since the early 1980s.6 The variations reflect the strong influence of non-medical determinants of disability, which to a large extent may be associated with conditions in the labour market. No rapid changes in morbidity can explain these variations in a developed country. Thus, the increasing use of psychiatric/ musculoskeletal diagnoses observed in disability statistics may account for a high degree of medicalization of processes leading to early retirement from work.
The social gradient in risk of receiving a DP was higher for people below 50 years compared to older people. This reflects great problems for young people with less education in the labour market, and a strong and maybe increasing health-related selection out of work in these cohorts affecting people with low SES.21 The overall age distribution of the 7322 people receiving a DP was very skewed. Receiving a DP was much more frequent among older people, and with increasing age it becomes more evenly distributed according to SES. The striking difference between women over and below 50 years (Table 5), may partly be due to legislation. DP has rarely been granted to homemakers without a personal income, and homemakers were more prevalent among older women with low education. Further, a weak association between job demands and older women was found. This result suggests that occupational risk factors were more evenly distributed according to SES among women over 50 years compared to other groups.
All determinants were measured at the individual level, and might lead to better understanding of the processes leading to disability. This might result in more specific intervention strategies for preventing subjects at high risk becoming disabled; intervening at intra-personal, inter-personal and organizational levels in health and rehabilitation.40 This would involve interventions dealing with education, job control and self-perceived health, particularly in younger people. High-risk interventions are important, have been frequently applied and often proposed in the struggle against increasing DP rates in many countries.41,42 However, the efficacy of the type of interventions that have been tried has been disappointing.41 This may be because they failed to target the correct risk factors/determinants. Or, they failed to understand and work with the contextual factors which may be far more important in determining unemployment and DP receipt than individual level factors.21,4345
Acknowledgments
The Norwegian Research Council financed the study. The Nord-Trøndelag Health Study (The HUNT Study) is a collaboration between the HUNT Research Centre, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Verdal, Norwegian Institute of Public Health, Oslo, and the Nord-Trøndelag County Council.
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