Events per person year--a dubious concept.

Abstract

In 1982 a new measure was introduced in research into osteoporosis and is now used everywhere in the literature. The so called "fracture rate" relates the number of fractures (single in some patients, multiple in others) to the cumulative time of observation of all patients. This concept, however, has no sound basis. Counting events instead of patients usually violates basic statistical assumptions and invalidates the use of common statistical tests and estimators. Its clinical interpretation is rather dubious. The use of such a measure impedes the search for valid and clinically meaningful outcome criteria and should be abandoned.