1 Department of Social and Preventive Medicine, University of Zurich, Sumatraastrasse 30, CH-8006 Zurich, Switzerland.
2 Department of Social and Preventive Medicine, University of Berne, Switzerland.
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Abstract |
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Methods This study is based on a longitudinal data set from the Swiss National Cohort, currently incorporating a probabilistic record linkage of the 1990 Swiss census, and all subsequent deaths until the end of 1997. The study population covers all Swiss nationals aged 25 years living in German speaking Switzerland, with 19.7 million person-years and 296 929 deaths observed. Educational gradients were analysed using standardized mortality ratios, multiple logistic regression, and the Relative Index of Inequality (RII).
Results There were sizeable gradients in mortality by education for all age groups and both sexes. The mortality odds ratio decreased by 7.2% (95% CI: 7.07.5%) per additional year of education for men, and by 6.0% (95% CI: 5.66.3%) for women. In men, we found a steady decrease of the gradient from 13.1% (95% CI: 11.914.4%) in the age group 2539 to 4.5% (95% CI: 4.05.0%) in the age group 75 years. For women in the age groups under 65 the gradients were smaller; over the age of 40 there was no decrease with increasing age. These results were fairly insensitive to variations in the parameters of record linkage.
Conclusions Despite a comparatively low overall mortality, Swiss men in the 1990s show larger relative gradients in mortality by education than men in other European countries in the 1980s, with the possible exception of younger men in Italy. In Switzerland there is a sizeable potential for further increasing overall life expectancy by reducing the mortality of those with a lower educational level. The results presented contribute to a reliable assessment of socioeconomic mortality differentials in Europe.
Accepted 9 July 2002
In the last decades of the 20th century, a resurgence of interest in health inequalities by socioeconomic status (SES) could be observed. During this time, several comprehensive reviews were published covering various European countries.111 An overview of the recent European situation can be gained from the publications of the Mackenbach group.1216 The reason for the renewed interest is the accepted view that in most countries an accentuation of inequalities has occurred over the last 20 years.1722 In addition, some recent studies found evidence of causes for socioeconomic health disadvantages in early life as well as for factors perpetuating such health disadvantages over generations.23,24 This lead to policy initiatives explicitly aimed at reducing SES-related health inequalities.25,26
A promising line of research is to compare the socioeconomic health differentials of many countries. The databases used by Kunst et al.12,13 were of varying quality: data from representative longitudinal studies were available for France, England and Wales, and the Nordic countries, while some data incorporated into their analyses were either cross-sectional or non-representative (Switzerland, Italy, Spain, Portugal). There is consensus about the limitations of cross-sectional data (e.g. descriptive instead of analytical approach, numerator/ denominator bias, ecological fallacy). Thus there is an interest in obtaining solid longitudinal evidence from other European countries.
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Research into mortality according to education |
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The reported inequalities in mortality by education are generally significantly larger for males than for females. However, not all studies have found a difference in educational mortality gradients by gender,27,31,36 and some have found a stronger gradient for females,30,39 or shown inconsistencies according to the time period4 or the life stage.38 Various explanations for the smaller gradient in women were provided: a different pattern of risk factors16 and hence differences in the distribution of causes of death,16,29 as well as in employment and family status.28 As a rule, risk ratios decrease with increasing age, though this pattern is inconsistent in females.38,39
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Research into health inequalities in Switzerland |
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Representative data on reported morbidity, symptoms, and risk factors in relation to education and other socioeconomic variables have been available from health surveys since the early 1980s,4751 the latest being the Swiss Health Survey of 1997 for which our team has prepared a report on SES by education, occupational group, and income.52
However, so far there has been no data set available for studying morbidity or mortality with a longitudinal design. This has changed with the inception of the Swiss National Cohort Study, of which this paper is the first report. The study population selected for the present analysis is representative of the German speaking part of Switzerland and covers more than 7 years of follow-up. This study is important for Switzerland as it is the first analysis of Swiss mortality data by educational class, and the first such analysis including women.
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Aims of this study |
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Material and Methods |
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Study population
We obtained all anonymous records of the Swiss census of 4 December 1990 from the Swiss Federal Office of Statistics. This database was used as a population register. Mortality analyses for foreign nationals living in Switzerland are seriously biased by numerator/denominator problems and migration effects.54,55 For this reason and in view of internationally different educational systems we excluded the death and census records of foreign nationals. Substantial differences in the systems of education between the Swiss cultural groups (53, ref. 56, p. 686) and in the proportions of successfully linked records (see section on record linkage) led us to restrict the present analysis to Swiss nationals living in predominantly German-speaking areas. As outlined above, university schooling is generally not completed before the age of 25. We therefore applied a lower age limit of 25 years. Of a total of 4.197 million Swiss nationals living in the German-speaking parts of the country, 1.276 million were younger than 25 years. Thus, the study encompassed 2.921 million people and 19.735 million person-years.
Deaths
We obtained all anonymous death records for the years 19901997 from the Swiss Federal Office of Statistics. Between 5 December 1990 (the day after the census) and 31 December 1997, 443 219 deaths were registered in Switzerland. Of these, we excluded 3628 deaths of children born after the 1990 census, 28 397 deaths of foreign nationals, and 6777 deaths of Swiss nationals aged <25 years at the date of the census. Of the remaining 404 417 deaths, 73.4% concerned individuals who had lived in the study area; of these 145 206 were male and 151 723 were female.
Record linkage
Death certificate records and census records were linked on the basis of coinciding key variables (sex, birth date, commune of residence, marital status, and religious denomination). A probabilistic record linkage method was used57,58 and several scenarios with different weighting of the key variables were executed in order to obtain optimal linkage quality. Linkage was done in two phases. In phase A, each death record was classified according to the number of potential counterparts in the census (Table 1) and then processed. Using strict criteria for linkage (no discordance in any key variable, no further record with the same combination of key variables) 73.3% of death records could be linked. Under these criteria, even a single minor discordance prevented linkage. In phase B, all death records not strictly linked in phase A were successively processed through passes 1 to 4. For 16.2% of death records this meant a search for a better link, while 10.5% still had no link in phase A. Inspection showed that strict linkage excluded some good links. In the default version we therefore focused on a threshold permitting linkage with one major or several minor discordances depending on pocket size (e.g. deviations leading to a record linkage in a small village might not be acceptable in a larger municipality). In this way, 88.6% of death records could be linked. Excluding only the improbable linkages (liberal criteria), 96.2% of the death records could be linked. Information on record linkage procedures and the quality of linkage are given in detail elsewhere.54 An investigation of the 11.4% of deaths not linked for the default version revealed that this proportion was higher for inhabitants of larger cities (27%), for younger people (24% in the ages 2534), those who were divorced (15%), and for women (13%).
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Measures of mortality
Age was determined at the time of the census, i.e. 4 December 1990. Mortality rates were calculated for 5-year age-sex classes using numbers of deaths and person-years at risk. Individuals registered in the census that could not be linked to a death record were assumed to be alive at the end of the follow-up. Based on the resulting age-specific rates, standardized mortality ratios for all educational categories were determined by the indirect method (basis: study population). Inequalities were quantified by comparing standardized mortality ratios between the different educational categories as well as by measures of slope such as the Relative Index of Inequality (RII).59,60 The RII adjusts for the variation in the size of the individual educational groups and therefore can be considered to be an appropriate inequality index for comparing countries with respect to the size of health inequalities by educational level.12 The association of mortality with the number of years of education was analysed using individual data and logistic regression with years of education and age as independent variables (age in years, linear and quadratic; the quadratic term was introduced to allow for a progressive increase of mortality with age).
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Results |
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International comparison
Table 5 compares the RII based on the four educational groups from Table 3
with results from five other European countries (table adapted from Kunst et al.12). For men aged 4559 years, the 1990s Swiss gradient in mortality by education is clearly the steepest, for men aged 3044 years it is exceeded only by Italy. For women, the Swiss gradients are comparable to other countries.
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Discussion |
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For our German Swiss study population, we found sizeable gradients in mortality by education for all age groups; the pattern of mortality was fairly consistent between ages 25 and 89. The significantly larger gradients for males than for females in those aged under 65 are in accordance with the majority of the literature (Introduction). As in other studies,22,27,33,38 the relative inequalities for males decreased with increasing age. For females over 45 this is not the case, a finding that is difficult to judge given the sparse and rather inconsistent results from other European studies.28,38
What are the reasons for the large mortality gradients by education in Swiss males in the 1990s compared with the data published by Kunst et al.?12 This difference could be the result of comparing Swiss data from the 1990s with data from other European countries from the 1980s. It cannot be ruled out that the steeper Swiss gradients reflect the general widening of socioeconomic differentials reported from several countries. However, given the stable economic situation of Switzerland and todays understanding of origins of social differences in health, such a substantial increase within only a decade seems rather unlikely. As mentioned, the Swiss studies focusing on the early 1980s are not methodologically comparable with our study (unlinked cross-sectional design, based on occupation instead of education). However, the reported risk ratios reached a similar size.
Are the results obtained here reliable? Incomplete census forms were quite rare in the 1990 Swiss census and therefore were not a major source of differential misclassification. A source of bias could be death certificates not linked at all or linked to the wrong census record. With a strict record linkage threshold, the probability of incorrect linkages could be minimized, but only at the price of losing a substantial number of probably correct linkages and a selection bias in favour of the least mobile groups, e.g. homeowners and the sick. Indeed, the more restrictive the linkage requirements, the steeper were the gradients. With liberal linkage criteria, the probability of incorrect linkage increases and the resulting non-differential misclassification will dilute the real differences. As a compromise, we chose a threshold balancing false-positively and false-negatively linked records, permitting the linkage of 89% of death records. There is still some room for bias because there were subgroups for which the fraction of unlinked deaths was larger. Thus, the results for city dwellers and for those aged <40 years may be less reliable. However, our sensitivity analyses indicate that these effects are likely to be small.
Switzerland is a country with high life expectancy. However, life expectancy is determined more and more by the mortality risks of the elderly. Unfavourable mortality in younger ages may be masked by a favourable situation in the older ages. This is the case in Switzerland, with elevated mortality rates for those aged 2049 years62 compared with other western European countries. Together with our finding of a particularly steep educational gradient in mortality in this age group, a focus for prevention efforts is clearly identified. As the differentials in mortality by education also apply to older age groups, there is sizeable potential for further increasing overall life expectancy by reducing the mortality of those with a lower level of education.
KEY MESSAGES
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Acknowledgments |
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References |
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