Measures of differences between the rates at which two groups experience an outcome tend to change in some manner as a result of changes in the overall prevalence of an outcome. Most notably, as an outcome declines, relative differences in experiencing it tend to increase and relative differences in avoiding it tend to decline. Other measures such as absolute differences between rates and odds ratios also tend to change as the overall prevalence of an outcome changes, though in more complicated ways. The presentation would illustrate these tendencies and discuss the obstacles they raise for efforts to determine whether health disparities are increasing or decreasing and whether these obstacles can be overcome.