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Education And Debate

Events per person year—a dubious concept

BMJ 1995; 310 doi: (Published 18 February 1995) Cite this as: BMJ 1995;310:454
  1. Jurgen Windeler, lecturera,
  2. Stefan Lange, research assistantb
  1. a Institut fur Medizinische Biometrie, Ruprecht-Karls-Universitat Heidelberg, Germany
  2. b Abteilung Medizinische Informatik und Biomathematik, Ruhr-Universitat Bochum, Germany
  1. Correspondence to: Dr Windeler, Institut fur Medizinische Biometrie, Ruprecht-Karls-Universitat, Im Neuenheimer Feld 305, 69120 Heidelberg, Germany.
  • Accepted 30 September 1994

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.

The concepts of design and analysis of randomised clinical trials seem to be well known to most researchers in clinical medicine. Randomisation, double blinding, definition of a primary end point, and prior calculation of power and sample size are widely accepted criteria for the quality of a clinical trial. There are additional problems, however, one of them being the handling of drop outs and missing information about them. Despite the favoured concept of intention to treat1 2 these problems have not gained adequate attention or—what is worse—have produced inadequate and invalid solutions.

We came to know one “solution” when we reviewed several trials concerning the treatment of osteoporosis,3 4 5 6 7 8 9 10 but there are other topics of research in which a similar procedure can be observed.11 12

The problem

Suppose that a clinical trial is performed to compare a new drug versus placebo with some binary end point. This may be death, myocardial infarction, recurrence of cancer, or any other criterion of success or failure. In the case of osteoporosis this is the occurrence of new (vertebral) fractures. We will assume a three year treatment and observation period with the primary end point of the trial being …

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