1. The disparity between premature mortality rates for highest and lowest socioeconomic status groups has increased from 1991 to 2016 in Canada.
Evidence Rating Level: 2 (Good)
Study rundown: Around the world, the disparity in health outcomes based on socioeconomic status (SES) is growing over time. Although this increasing divide is well documented in the United States and Europe, attempts to quantify this widening gap in Canada have been lacking, due to important SES information not appearing on death records. By linking these death records to Canadian census data, the current study was able to assess the trends in the association between SES and premature mortality.
This retrospective cohort study found that from 1991 to 2016, overall premature mortality rates dropped for all socioeconomic groups, with the exception of women with no high school diploma. The absolute mortality inequalities related to income decreased in men over time but increased in women, whereas education-related inequalities increased in both men and women. The ratio of the highest and lowest income quintile relative risk increased by 1.33 times in men and 1.45 times in women. Taken altogether, the disparity in health outcomes due to SES is indeed growing.
A central strength of this study was that it minimized survivorship bias by examining premature mortality, defined as dying younger than 75 years of age. However, a limitation of this study was that it relied on census data involving self-reported income and education levels, which may be a source of selection bias. Overall, this study demonstrates that although premature mortality has been decreasing over time, not all SES levels experience decreases equally, which underlies the need for interventions to prevent these health outcome inequalities from widening even further over time.
Relevant Reading: Increasing Disparities in Mortality by Socioeconomic Status
In-Depth [retrospective cohort study]: In total, 16 285 045 Census records were sampled. Data was collected from the Canadian Census Health and Environment cohorts for the years 1991, 1996, 2001, 2006, and 2011, and mortality was measured in the 5-year period following each census cohort. In the first four cohorts, 20% of households were mandated to complete the long-form census, and in the final cohort, 30% of households were asked to voluntarily fill out the National Household Survey. The primary outcome was premature mortality, whereas the exposure was SES, specifically household income and education level measures.
The study found that between 1991 and 2016, the absolute premature mortality rate for decreased for nearly all SES level, with 1187 and 443 fewer premature deaths per 100 000 for men and women respectively. However, women without a high school diploma had a non-significant reduction, with only 36 fewer deaths per 100 000. When assessing the difference in absolute risk per 100 000, the gap between highest and lowest income quintiles decreased for men (2478 to 1915 premature deaths) but increased for women (1008 to 1085 deaths). As well, the relative risk ratio for highest and lowest income increased from 2.10 (95% CI 2.02-2.17) to 2.79 (95% CI 2.66-2.91) for men, and increased from 1.72 (95% CI 1.63-1.81) to 2.50 (95% CI 2.36-2.64) for women. Additionally, the gap between highest and lowest education levels increased for men and for women (1713 to 1843 deaths and 792 to 1119 deaths respectively). The education relative risk ratio grew from 1.98 (95% CI 1.90-2.07) to 2.60 (95% CI 2.48-2.72) for men and 1.82 (95% CI 1.69-1.95) to 2.31 (95% CI 2.18-2.44) for women.
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