sought to analyze the importance of individual-level relative deprivation, as measured by the Yitzhaki index, for all-cause mortality in the Swedish population.
We used prospective longitudinal data from the Swedish population with baseline collection in 1990 and mortality follow-up until 2006. The Swedish Work and Mortality Data Base, maintained at the Centre for Health Equity Studies, was created to study how work, income and labour market position combine to impact morbidity and mortality. The data were based on a linkage of the National Population and Housing Censuses, the Total Population Register (RTB), the Longitudinal Data Base on Education, Income and Employment (LOUISE), the Cause of Death Register and inpatient registers. When restricting the data to individuals alive January 1 1990 and aged 25–64years, the data included a total of 4.7 million individuals, for whom we conducted a mortality follow-up until 2006 [see Additional file 1 for sample size]. From this cohort, we excluded all individuals who lacked income data, all individuals who had emigrated and for whom we do not have any death data (N=4 227 490). The occupation variable derives from the 1990 National Population and Housing Census, and 19% had either unclassifiable occupations or missing values, for example persons on sick leave, students or unemployed. These 19% were excluded when using occupation in the models. Ethical permission (no 02–481) was provided by the Regional Ethics Committee at Karolinska Institute in Stockholm.
The central exposure variable, the measure of relative deprivation was calculated using the Yitzhaki index , in a similar way as done in previous studies [5,13,15]. The index calculates the relative deprivation for each individual by summarizing the differences in income between that individual and all individuals with a higher income in that person’s reference group, taking the size of the reference group into consideration. Relative deprivation for the individual i is the sum of the income gap between i and j (yj – yi, where j has a higher income than i), divided by number of people in the reference group (N). Relative deprivation (i)=1/N ∑ (yj – yi), ∀ yj>yi.
The income measure used in the Yitzhaki index calculation is the disposable individual income, based on the individual’s total income from work and social benefits minus taxes. To simplify the interpretation of the results, the Yitzhaki index was rescaled by division of 10 000 SEK. When constructing reference groups for each individual, we made the assumption that individuals compare themselves to similar others. In the analysis, we tested different reference group settings in accordance with previous studies [5,13]: 1) Living region (divided into metropolitan areas 36%, large towns 33% and other areas including countryside 31%), 2) Occupation (classified in accordance with the Swedish Socioeconomic Classification (SEI) and divided into the categories higher non-manual employees, lower non-manual employees, qualified manual workers, unqualified manual workers, self-employed and farmers. The final two, self-employed and farmers were categorized together), 3) Age group (25–34, 35–44, 45–54, 55–64) and 4) a combination of these three.
In the analysis, we included age, marital status (married, unmarried, divorced and widowed), occupation (higher non-manuals, lower non-manuals, qualified manual, unqualified manual, self-employed and farmers) and income as covariates.
We used Cox regression modeling (SAS version 9.2) with survival time in years as the time scale, with a maximum of 16years. All analyses were stratified by gender, as previous studies have shown a gender difference in the importance of income and relative deprivation [7,19]. All people still alive in 2006 were censored. We calculated the crude model using the different reference group settings from our complete data set. Estimates are presented as hazard ratios (HR) with 95% confidence intervals. In the multivariate models, we adjusted for age group and marital status in the first model (data not shown). In Mod