i, 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 Model 2 we adjusted for age group, marital status and occupation, and in Model 3 we also added income.
The descriptive analyses in Table Table11 show the number of deceased by age group, marital status and occupational group. The number of deceased is higher among males than females. The proportion of deceased during the follow-up also varies across marital status and occupation, with higher rates among divorced, widows/widowers and unqualified manual workers. Descriptive analyses also show the median relative deprivation within reference groups used. For an individual, having a lower value on the relative income deprivation index means having a higher income within the reference group. Having a value of 0, means having the highest income within the specific reference group. The median relative income deprivation differs depending on whether living region, age group or occupation has been used. The lowest value was found when combining these three, since the income diversity is smaller when reference groups become more specific.
In order to illustrate the possibility of non-linearity in the association between relative deprivation and mortality, we calculated HR for continuous as well as quartiled relative deprivation (Table (Table2).2). Cox regression showed that, among men, relative deprivation was significantly associated with mortality regardless of the definition of reference groups for which we calculated relative deprivation. The crude HRs for mortality by 10 000 SEK unit increase in relative deprivation are between 1.069 (95% confidence intervals [CI]: 1.066 -1.072) and 1.109 (95%CI: 1.107-1.111), depending on the reference group. The crude HRs for women range between 0.986 (0.982-0.990) and 1.118 (1.116 – 1.120). The crude HR of the highest versus the lowest quartile is between 1.414 (95%CI: 1.392-1.437) and 2.503 (95%CI: 2.474-2.532) among men, and between 0.958 (95%CI: 0.937 - 0.980) and 2.082 (95%CI: 2.041 - 2.123) among women, depending on the reference group used. Adjusting for age group, marital statu