Of the 6,895 men at baseline in the study, data on all measures used in the analysis were available for 5,363 men. Compared with the 1,532 men who did not have complete data, men included in this study did not differ markedly on socioeconomic position (p=0.18) or age (p=0.25). The average age of men included in the analysis was 49.9 (SD=5.9) for the high socioeconomic group, 48.6 (SD=6.0) for the intermediate group and 49.7 years (SD=6.3) for the low group at the start of the follow-up period (Phase 3). The distribution of risk factors as a function of socioeconomic position is shown in . Socioeconomic position was associated with smoking, hypertension, and diabetes, but not with cholesterol.
Distribution of risk factors (Phase 3, 1991–93) as a function of socioeconomic position.
presents the results aimed at identifying the contribution of each risk factor in explaining social inequalities in CHD (analysis 1). Age-adjusted analysis using Cox regression shows that the relative risk for CHD in the low socioeconomic group was 1.66 (95% CI=1.20–2.29) compared with the high socioeconomic group. Adding smoking to this model reduced the association in the low group by 18%. Similarly, hypertension explained 14%, high cholesterol 3% and diabetes 6% of the association between socioeconomic and CHD. All the risk factors taken together explained 38% of the relative risk in CHD in the low socioeconomic group. We repeated this analysis on CHD events excluding “definite” angina. These results (not shown) show the four risk factors explain 40% of the relative risk in CHD in the low socioeconomic group.
Relative risk approach to assessing the role of risk factors in explaining social inequalities in CHD.a
presents the results from the relative and absolute approaches to examining the contribution of risk factors to social inequalities in CHD by standardizing the proportion of men with 1–4 risk factors (high-risk group) to be the same in the low and high socioeconomic groups (analysis 2). In the observed data, the rate ratio for CHD in the low socioeconomic group compared with the high group was 1.67 and the excess rate was 5.2 per 1000 person years. Overall, 67% (19426/29121) person-years in the high group and 76% (2565/3387) in the low socioeconomic group were in the high-risk group (1–4 risk factors). In the total study sample this percentage was 65% in the younger and 74% older age strata. Standardization implies setting these as the age-specific distributions of person-years in the two socioeconomic groups, leading to 20112 and 2338 person-years in the high and low groups respectively. The age-specific standardized distributions are used to calculate the expected number of CHD events in the two socioeconomic groups. This results in a CHD rate of 7.9 per 1000 person-years in the high socioeconomic group and 12.4 per 1000 in the low group, leading to a reduction in relative risk of 16%. The excess rate in the low SEP group changes from 5.2 to 4.5 per 1000 person-years, a reduction of 14%.
Adjusting for risk factors using standardization among men with at least one risk factor: Reduction in relative and absolute rates.
presents the impact of risk factors from a PAR perspective; here the calculations estimate the reduction in CHD if the risk factor were to be completely removed from the population (analysis 3). The relative risk estimates for the four risk factors show hypertension to be the risk factor most strongly associated with CHD in the total population (RR=1.84; 95% CI=1.51 to 2.25) and in the high socioeconomic group (1.80; 1.35 to 2.41) and intermediate group (2.16; 1.61 to 2.89). However, in the low socioeconomic group smoking has the strongest association (1.71; 0.93 to 3.14). Nevertheless, given the high prevalence of high cholesterol, both overall and in the three socioeconomic groups, cholesterol has the highest PAR: 25% (95% CI=15% to 36%) overall and in the three socioeconomic groups: high, 23% (8% to 38%); intermediate, 28% (12% to 43%); and low, 21% (−19% to 56%). The formula for the calculation of PAR shows it to be influenced by both the relative risk estimate and the prevalence. Hence, when stratifying by socioeconomic status, smoking has a PAR of 16% (−3% to 36%) in the low group and 0.1% (−3.3% to 4%) in the high group. The PAR associated with having at least one risk factor (i.e., 1–4 risk factors) was 46% (33% to 57%) overall. However, this varies as a function of socioeconomic position, ranging from 58% (13% to 91%) in the low group to 41% (23% to 57%) in the high group.
Relative risk and population attributable risk of CHD associated with risk factors, by socioeconomic position.