Measuring Disparities: Relative and Absolute Differences
The COVID-19 epidemic has amplified health inequities, calling
attention to racial and socioeconomic disparities in COVID-19 cases,
hospitalizations, and deaths. This post describes two common ways to
measure health disparities, rate ratios and rate differences. We
discuss how to calculate these measures, and how different analysis
choices impact our understanding of disparities.
We limit our discussion to rate ratios and rate differences, two measures of between group disparities, but refer readers to here, here and here for further discussion of other important considerations for measuring disparities, including measures of total disparities rather than between-group disparities.
Generally, we employ the most “well-off” or privileged group as the reference group, as this group usually has a more favorable group rate (the rate reflecting better health status or less risk). Examples of common reference groups include non-Hispanic whites, men, the able bodied population, U.S. born population, commercially insured people, non-segregated neighborhoods, non-impoverished neighborhoods, etc. However, decisions about the appropriate reference group should be based on theory and reflect the historical and cultural context of the hypothesis tested. For the following explanation, we’ll use non-Hispanic whites as our reference group, and measure racial disparities in COVID-19 incidence rates in Philadelphia.
Rate (Absolute) Differences
Rate Ratios (Relative Differences)
Rate ratios, also known as risk ratios or relative differences, provide a measure of the relative magnitude of the disparity. A rate ratio is calculated by dividing the rate of interest by the reference rate.
Taking the same numbers as we used for the rate difference, the rate ratio equals the rate among non-Hispanic Blacks (2,550/100,000) divided by the rate among non-Hispanic whites (1,369/100,000), or 1.86. The case rate among non-Hispanic rates is 1.86 times the rate among whites, or 86% higher among Blacks than whites.
Large relative disparities can mask small absolute differences, particularly when overall rates are low: for example, if the Black death rate is 15/100,000 and non-Hispanic white rate is 5/100,000, the relative disparity will show a Black rate 3 times that of the white rate, but only an absolute difference of 10 deaths per 100,000 persons. Analogously small relative differences can mask large absolute differences if the rate is high. To draw conclusions about disparities and changes in disparities, it is therefore best to measure both absolute and rate differences concurrently.
Testing Site Metrics
- Fully vaccinated: % of the total population that has been fully vaccinated (taking into consideration the nunber of doses required for each vaccine).
- Partially vaccinated: % of the total population that has been only partially vaccinated (e.g., one dose of an mRNA-based vaccine).
- At least partially vaccinated: % of the total population that has been either partially or fully vaccinated.
- Doses per capita: total number of administered vaccine doses per 1000 individuals.
- Hesitancy: % of the adults in the city who describe themselves as “probably not” or “definitely not” going to get a COVID-19 vaccine once available to them.
- Strong Hesitancy: % of the adults in the city who describe themselves as “definitely not” going to get a COVID-19 vaccine once available to them.