Assessing the suitability of fractional polynomial methods in health services research: a perspective on the categorization epidemic

Objective: To show how fractional polynomial methods can usefully replace the practice of arbitrarily categorizing data in epidemiology and health services research. Methods: A health service setting is used to illustrate a structured and transparent way of representing nonlinear data without arbitrary grouping. Results: When age is a regressor its effects on an outcome will be interpreted differently depending upon the placing of cutpoints or the use of a polynomial transformation. Conclusions: Although it is common practice, categorization comes at a cost. Information is lost, and accuracy and statistical power reduced, leading to spurious statistical interpretation of the data. The fractional polynomial method is widely supported by statistical software programs, and deserves greater attention and use.