Emerging European recognition of the Scanlan's Rule in health disparities research: Will the United States be left behind?

Since 1987, in many articles and presentations at scientific conferences, I have explained the statistical pattern whereby the rarer an outcome the greater tends to be the relative difference in experiencing it and the smaller tends to be the relative difference in avoiding it and the implications of such pattern for health disparities research. Among the most notable implications is that as adverse outcome like mortality decline, relative differences in experiencing them tend to increase while relative differences in avoiding them tend to decrease. As beneficial health procedures become more widespread relative differences in receipt of such procedures tend to decrease while relative differences in failing to receive them tend to increase. This thinking has had little influence in the United States. The National Center for Health Statistics (NCHS) has merely responded by recommending that all relative differences be measured in terms of adverse outcomes, thus causing many healthcare disparities that previously were deemed to be decreasing now to be deemed to be increasing. But NCHS has yet to address the fundamental issue of whether either relative difference can identify changes in disparities that reflect something besides changes in overall prevalence. In Europe, however, increasing numbers of scholars are recognizing that relative differences in experiencing or avoiding an outcome are systematically affected by the overall prevalence of an outcome and the need to take overall prevalence into account in disparities research. This recognition puts Europe much closer to giving health disparities research a sound statistical foundation than the United States.