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Canadian Medical Association Journal
April 7'98

Correction: Annual visits to GPs by elderly patients

CMAJ 1998;158:871

My letter "Annual visits to GPs by elderly patients" (CMAJ 1998;158[3]:299 [full text / résumé]) included an incorrect statement about odds ratios. The correct explanation of this measure is given here.

The odds for any event, p/(1 ­ p), where p is the probability for the event, can never be negative, because p can never be greater than 1. Consequently, the odds ratio, which is calculated by dividing the odds for an event in the index category by the odds for the event in the reference category, can range from zero to infinity, but it can never be negative.

Murray M. Finkelstein, PhD, MD, CM
Assistant Professor
Department of Family and Community
Medicine
Mount Sinai Hospital
Toronto, Ont.

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