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• Role of the prevalence of an outcome in the size of rate differences
  James P. Scanlan
  Published on: 4 June 2007

• Published on: 4 June 2007

  Role of the prevalence of an outcome in the size of rate differences

  • James P. Scanlan, Attorney

  Dear Editor

  Martikainen et al.[1] note that relative, and in some cases absolute, socioeconomic differences in mortality have increased in the past 15-25 years in some European countries and the US, and find that over the period 1971 to 2000 such increases also occurred in Finland. The authors, however, overlook the statistical tendency whereby the rarer an outcome, the greater the relative difference between ra...

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  Dear Editor

  Martikainen et al.[1] note that relative, and in some cases absolute, socioeconomic differences in mortality have increased in the past 15-25 years in some European countries and the US, and find that over the period 1971 to 2000 such increases also occurred in Finland. The authors, however, overlook the statistical tendency whereby the rarer an outcome, the greater the relative difference between rates of experiencing it and the smaller the relative differences between rates of avoiding it.[2-6.] Thus, with respect to the observed increasing relative socioeconomic differences in mortality rates in many countries in recent decades (presumably including Finland), a key factor is that mortality has been declining. In such circumstances, one should expect increasing relative differences in mortality (though declining relative differences in survival rates). But without more, neither the increasing relative difference in mortality rates nor the declining relative difference in survival rates should be regarded as indicating meaningful changes in health inequality.

  In situations where absolute differences in mortality are increasing during times of a general decline in mortality, there may be more reason to regard such increase as indicating a meaningful worsening of health inequality.[2,3] But one would have to explore the specifics of the situations to draw such conclusion with any confidence.

  The tendency for relative differences in mortality to be larger where an outcome is rarer is also a reason why one would expect relative educational and occupational social class difference to be larger in younger than older age groups,[3] a pattern that the authors note and support with data in Table 3.. But one would generally expect relative differences in survival to be larger among the older age groups, though the article does not provide the data to determine whether that is the case. So one ought not to regard the larger relative mortality differences in mortality among the young to suggest anything meaningful

  It is hard to know whether these purely statistical factors affect the measuring of the roles of education and occupation in the observed mortality differences. But for reasons noted in the preceding paragraphs it is usually difficult to draw meaningful inferences about any changes relating to health inequalities without considering the implications of those factors. And, given the influence of the prevalence of an outcome in the size of differences in experiencing or avoiding it, it is important that a study of patterns of change in any aspects of inequalities in mortality show the various mortality rates at the points in time used in the analyses.

  References

  1. Martikainen P, Blomgren J, Valkonen T. Change in the total and independent effects of education and occupational social class on mortality: analyses of all Finnish men and women the period 1971-2000. J Epidemiol Community Health 2007;61:499-505.

  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_Disparities.pdf.

  3. Scanlan JP. The misinterpretation of health inequalities in the United Kingdom. Paper presented at: British Society for Population Studies Annual Conference 2006, Southampton, England, Sept. 18-20, 2006: http://www.jpscanlan.com/images/BSPS_2006_Complete_Paper.pdf.

  4. Scanlan JP. Measuring health disparities. J Public Health Manag Pract 2006;12(3):294 [Lttr]: http://www.nursingcenter.com/library/JournalArticle.asp?Article_ID=641470.

  5. Scanlan JP. Race and mortality. Society 2000;37(2):19-35: http://www.jpscanlan.com/images/Race_and_Mortality.pdf.

  6. Scanlan JP. Divining difference. Chance 1994;7(4):38-9,48: http://jpscanlan.com/images/Divining_Difference.pdf.

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  Conflict of Interest:
  None declared.

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