When measuring health disparities, what is more important: absolute or relative differences among populations?
Neither relative nor absolute differences are useful measures of health disparities because both tend to be affected by the overall prevalence of an outcome. Solely for reasons related to the shapes of the underlying risk distributions, the rarer an outcome the greater tends to the relative differences in experiencing it and the smaller tends to be the relative difference in avoiding it. Thus, for example, as mortality declines, relative differences in mortality tend to decrease while relative differences in survival tend to decrease. Absolute differences also tend to change systematically as the prevalence of an outcome changes. Roughly, as rare outcomes become more common, absolute differences tend to increase; as common outcomes become even more common absolute differences tend to decrease. See my Can we actually measure health disparities? Chance2006:19(2):47-51 (http://www.jpscanlan.com/images/Can_We_Actually_Measure_Health_Disparities.pdf) as well as the Measuring Health Disparities page (http://jpscanlan.com/measuringhealthdisp.html) and Scanlan’s Rule page (http://jpscanlan.com/scanlansrule.html) of of jpscanlan.com, especially the Relative Versus Absolute sub-page of the former (http://jpscanlan.com/measuringhealthdisp/relativevabsolutediff.html).
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Both absolute and relative differences are important in assessing health disparities. In the same sense that absolute and relative risk are both important in assessing the effectiveness of any health care treatment, it is important to consider both of these in assessing differences in health outcomes. Which is more important depends to some degree on what the baseline rates are, as small relative differences translate into large absolute differences with high baseline mortality rates and small absolute differences equate to large relative differences at low baseline mortality rates. Comparison of absolute rates implies an additive model for examination of health disparities whereas relative differences imply a multiplicative model (increased relative risk). Relative risks are probably a better measure of health disparities because biologically it seems more likely that, to the extent that being uninsured or being of a certain racial group is a risk factor for worse outcomes, this effec
