Discussions of relative and absolute differences cannot ignore that there are two relative differences.
8 November 2012
While attempting to promote clarity in reporting of health inequalities, the article by King et al. does just the opposite.
A nearly universal problem with health inequalities research is the reliance on standard measures of differences between outcome rates without recognizing that such measures tend to be systematically affected by the prevalence of an outcome. A particular problem with reliance on relative differences is that as the prevalence of an outcome changes relative differences in experiencing it and relative differences in avoiding it tend to change systematically in opposite directions. For example, as mortality declines relative difference in mortality tend to increase while relative differences in survival tend to decrease; as receipt of beneficial healthcare procedures increases, relative differences in receipt of such procedures tend to decrease while relative differences in failure to receive them tend to increase.[2.3] The US National Center for Health Statistics (NCHS) has specifically recognized that determinations of whether a health inequality is increasing or decreasing will commonly turn on whether one examines relative differences in favorable outcomes or relative differences in the corresponding adverse outcomes.
Confusion, however, is widespread. Studies of racial inequalities in cancer outcomes commonly refer to relative differences in survival and relative differences in mortality interchangeably, often stating they are analyzing one while in fact analyzing the other, and invariably without recognizing that the two relative differences tend to change in opposite directions as overall survival rates change or that more survivable cancers tend to show larger relative differences in mortality, but smaller relative differences in survival, than less survivable cancers. Morita et al., relying on relative differences in vaccination rates as a measure of inequality, found that a school-entry Hepatitis B vaccination requirement that dramatically increased vaccination rates also dramatically decreased race/ethnic vaccination inequalities; NCHS, relying on relative differences in failure to be vaccinated, would have found dramatic increases in inequalities. See Table 4 of reference 7.
Despite increasing scholarly recognition of these issues,[8-11], the article by King et al., while seeming to be a comprehensive review of the manner in which health inequalities are reported, reflects no recognition that there even exist two relative differences with respect to every health and healthcare dichotomy, much less that the two relative differences tend systematically to yield different conclusions as to direction of changes over time. Further, King et al. stress the importance of reporting a relative and absolute difference in circumstances where the two measures yield different interpretations as to the directions of changes in equalities over time. Yet whenever a relative and absolute difference yield different conclusions as to directions of changes over time, the unmentioned relative difference necessarily will have changed in the opposite direction of the mentioned relative difference and in the same direction as the absolute difference. See Section B of reference 12. Few readers of the King article would imagine that the unmentioned relative difference would ever support an opposite conclusion of the mentioned relative difference much less that it would always do so.
A more serious issue in the discussion by King et al. of the reporting of both relative and absolute differences, or any like discussion, is that such discussions suggest that there can be more than one reality respecting whether the forces underlying the differences between rates at which advantaged and disadvantaged groups experience some outcome have increased or decreased over time. As discussed in Section D of reference 12, however, there can be only one reality as to whether those forces have increased or decreased. But neither the absolute difference, nor either of the two relative differences, can capture that reality unless appraised with recognition of the way these measures tend to be affected by the prevalence of an outcome.
The article does usefully show the vast scope of health inequalities research, essentially all of which has been fundamentally unsound for failure to consider the way the measures relied upon are affected by the prevalence of an outcome. Ignoring such issue can only promote similarly unsound research going forward.
1. King NB, Harper S, Young ME. Use of relative and absolute effect measure in reporting health inequalities: structured review. BMJ 2012;345:e544 doi: 10.1136/bmj.e5774
2. Scanlan JP. Can we actually measure health disparities? Chance 2006:19(2):47-51: http://www.jpscanlan.com/images/Can_We_Actually_Measure_Health_Dispariti...
3. Scanlan JP. Race and mortality. Society 2000;37(2):19-35: http://www.jpscanlan.com/images/Race_and_Mortality.pdf
4. Keppel K., Pamuk E., Lynch J., et al. (2005) Methodological Issues in Measuring Health Disparities. Vital Health Stat;2 (141): http://www.cdc.gov/nchs/data/series/sr_02/sr02_141.pdf
5. Mortality and Survival Page of jpscanlan.com: http://jpscanlan.com/mortalityandsurvival2.html
6. Morita JY, Ramirez E, Trick WE. Effect of school-entry vaccination requirements on racial and ethnic disparities in Hepatitis B immunization coverage among public high school students. Pediatrics 2008;121:e547: http://pediatrics.aappublications.org/content/121/3/e547.full
7. Scanlan JP. Applied Statistics Workshop, presented at the Institute for Quantitative Social Science at Harvard University, Cambridge, MA, Oct. 17, 2012: http://jpscanlan.com/images/Harvard_Applied_Statistic_Workshop.ppt
8. Carr-Hill R, Chalmers-Dixon P. The Public Health Observatory Handbook of Health Inequalities Measurement. Oxford: SEPHO; 2005: http://www.sepho.org.uk/extras/rch_handbook.aspx
9. 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
10. Eikemo TA, Skalicka V, Avendano M. Variations in health inequalities: are they a mathematical artifact? International Journal for Equity in Health 2009;8:32: http://www.equityhealthj.com/content/pdf/1475-9276-8-32.pdf
11. Bauld L, Day P, Judge K. Off target: A critical review of setting goals for reducing health inequalities in the United Kingdom. Int J Health Serv 2008;38(3):439-454.
12. Scanlan JP. Letter to Harvard University Oct. 9, 2012: http://jpscanlan.com/images/Harvard_University_Measurement_Letter.pdf
Competing interests: None declared
James P. Scanlan, Attorney at Law, 1529 Wisconsin Ave., NW, Washington, DC 20007
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