ABSTRACT
In this chapter, we consider specifically the ratio of two proportions. We examine its applicability vis-à-vis the absolute difference, which was discussed in detail in Chapter 7. We examine confidence interval methods for the ratio of independent proportions, and the closely related rate ratio. For both measures, an asymptotic (delta) interval based on the standard error of the log-transformed measure takes a simple form. But this interval has limitations similar to those for the binomial proportion Wald interval. Better methods are available, in particular a score interval developed by Miettinen and Nurminen (1985). This is not of closed form; nevertheless it may be implemented in spreadsheet software using a fixed number of interval bisections. We defer detailed discussion of the more complex, less intuitive odds ratio to Chapter 11, including several additional arguments which repudiate the tendency to regard the odds ratio as the gold standard for comparison of proportions.
