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Misinterpreting patterns of relative differences in mortality
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James P. Scanlan, Attorney James P. Scanlan, Attorney at Law
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jps{at}jpscanlan.com James P. Scanlan
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Wilkinson and Pickett [1] address associations between income inequality and overall population health and between income inequality and health disparities and discuss whether underlying processes are similar with respect to both associations. They conclude that, while narrower income differences are associated with better overall health, the narrowing of income differences may do little to reduce health disparities because narrower income differences are associated with improved health of both the wealthy and the poor. Without any examination of data, there is reason to expect narrower income differences to be associated with better overall health at least because of the diminishing marginal utility of the disproportionate resources of the wealthy in areas with high income inequality and possibly also (as Wilkinson has argued in a number of places) because the greater social cohesion in a more egalitarian society may improve the health of both advantaged and disadvantaged groups. There are even more obvious reasons to expect narrower income differences to be associated with smaller health disparities. But the study of these issues is complex. The study of the size of health disparities in different settings is particularly complicated by the fact that the standard measures of differences between rates at which advantaged and disadvantaged groups experience or avoid some outcome tend to be systematically associated with the overall prevalence of the outcome. Most notably, the less common an outcome, the greater tends to be the relative difference between rates of experiencing it and the smaller tends to be the relative difference between rates of failing to experience it.[2-5] Wilkinson and Pickett, however, rely on gradients in mortality rates (a function of relative differences in such rates) as their principal indicator of the size of health disparities, without any recognition of the extent to which lower mortality will tend to be associated with steeper gradients (larger relative differences) in mortality rates or less steep gradients in survival rates. Wilkinson and Pickett also touch briefly on the distinction between relative and absolute differences. They do so somewhat confusingly, however, by observing that [i]f ill health is reduced in all income groups but is more reduced in poor that in wealthy groups, absolute differences will be smaller [but] relative differences may be undiminished. Typically, researchers use a phrase like more reduced to mean a greater relative reduction rather than, as Wilkinson and Pickett apparently mean, a greater absolute reduction. In any case, however, consideration of absolute differences does not further the discussion here since absolute differences tend also to be correlated with the overall prevalence of an outcome (usually in a way that is the inverse of the correlation of the relative difference in rare outcomes like mortality,[2- 5] though in a more complicated way with regard to relative differences in commoner outcomes [3-8]). As Wilkinson has done previously,[9,10] Wilkinson and Pickett note findings that comparatively egalitarian (and healthy) societies like Sweden do not have smaller mortality differentials than less egalitarian societies.[11,12] But the principal authors of such studies, which found Sweden and Norway to have shown larger than average relative differences in mortality rates between advantaged and disadvantaged groups, have themselves recently recognized that there tends to be a systematic relationship between low levels of an outcome and high relative differences in rates of experiencing the outcome.[13] Thus, those authors have essentially called into question the meaning of the comparatively larger relative differences in mortality in Sweden or other countries with low overall mortality that received attention in their earlier studies.[14] In an ironic way, Wilkinson and Pickett seem correct that narrower income differences may tend to increase health disparities as the authors measure those disparities. That is, if narrow income differences are associated with better overall health, in places with narrow income differences, overall mortality rates will tend to be low, with a corresponding tendency for relative differences in mortality rates to be large (though relative differences in survival rates to be small) notwithstanding a tendency for narrower income differences also to reduce health inequality. But determining whether narrower income differences could in fact cause health disparities to be larger in some meaningful sense that is, in a way that is not, solely for statistical reasons, a function of low overall mortality involves a more complicated inquiry. And it is one that, like all other inquiries into health disparities, there is little value in undertaking without an understanding of the ways measures of differences between rates of experiencing or avoiding an outcome tend to be systematically correlated with the overall prevalence of the outcome. References: 1. Wilkinson RG, Pickett KE. Income inequality and socioeconomic gradients in mortality. Am J Public Health 2008;98:699-704. 2. Scanlan JP. Race and mortality. Society 2000;37(2):19-35 (reprinted in Current 2000 (Feb)): http://www.jpscanlan.com/images/Race_and_Mortality.pdf 3. Scanlan JP. Can we actually measure health disparities? Chance 2006:19(2):47-51: http://www.jpscanlan.com/images/Can_We_Actually_Measure_Health_Disparities.pdf 4. Scanlan JP. Measuring health disparities. J Public Health Manag Pract 2006;12(3):293-296, responding to Keppel KG, Pearcy JN. Measuring relative disparities in terms of adverse events. J Public Health Manag Pract 2005;11(6):479483: http://www.nursingcenter.com/library/JournalArticle.asp?Article_ID=641470 5. Scanlan JP. The Misinterpretation of Health Inequalities in the United Kingdom, presented at the British Society for Populations Studies Conference 2006, Southampton, England, Sept. 18-20, 2006: http://www.jpscanlan.com/images/BSPS_2006_Complete_Paper.pdf 6. Scanlan JP. Can We Actually Measure Health Disparities, presented at the 7th International Conference on Health Policy Statistics, Philadelphia, PA, Jan 17-18, 2008: PowerPoint Presentation: http://www.jpscanlan.com/images/2008_ICHPS.ppt; Oral Presentation: http://www.jpscanlan.com/images/2008_ICHPS_Oral.pdf 7. Scanlan JP. Measurement Problems in the National Healthcare Disparities Report, presented at American Public Health Association 135th Annual Meeting & Exposition, Washington, DC, Nov. 3-7, 2007: PowerPoint Presentation: http://www.jpscanlan.com/images/APHA_2007_Presentation.ppt;Oral Presentation: http://www.jpscanlan.com/images/ORAL_ANNOTATED.pdf; Addendum (March 11, 2008): http://www.jpscanlan.com/images/Addendum.pdf 8. Scanlan JP. Perceptions of changes in healthcare disparities among the elderly dependant on choice of measure. Journal Review Feb. 12, 2008 (responding to Escarce JJ, McGuire TG. Changes in racial differences in use of medical procedures and diagnostic tests among elderly persons: 1986-1997. Am J Public Health 2004;94:1795-1799): http://www.journalreview.org/view_pubmed_article.php?pmid=15451752&specialty_id=0 9. Wilkinson R. The politics of health. Lancet 2006;368:1229-1230. 10. Scanlan JP. Why we should expect Nordic countries to show large relative socioeconomic inequalities in mortality. Lancet Oct. 7, 2006 (responding to Wilkinson R. The politics of health. Lancet 2006;368:1229- 1230: http://www.thelancet.com/journals/lancet/article/PIIS0140673606695019/comments?action=view&totalComments=1 11. Cavelaars AE, Kunst AE, Geurts JJ, et al. Differences in self reported morbidity by educational level: a comparison of 11 western European countries. J Epidemiol Community Health 1998;52:219227. 12. Mackenbach JP, Kunst AE, Cavelaars AE, Groenhof F, Geurts JJ. Socioeconomic inequalities in morbidity and mortality in western Europe. The EU Working Group on Socioeconomic Inequalities in Health. Lancet 1997;349:16551659. 13. Houweling TAJ, Kunst AE, Huisman M, Mackenbach JP. Using relative and absolute measures for monitoring health inequalities: experiences from cross-national analyses on maternal and child health. International Journal for Equity in Health 2007;6:15: http://www.equityhealthj.com/content/6/1/15 14. Scanlan JP. Reconsidering a landmark study. Lancet Feb. 25, 2008 (responding to Mackenbach, JP, Kunst, AE, Cavelaars, et al. Socioeconomic inequalities in morbidity and mortality in western Europe, Lancet 1997; 349: 1655-59): http://www.thelancet.com/journals/lancet/article/PIIS0140673696072261/comments?action=view&totalComments=1 |
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