Elsevier

Obstetrics & Gynecology

Volume 98, Issue 4, October 2001, Pages 685-688
Obstetrics & Gynecology

An odd measure of risk: use and misuse of the odds ratio

https://doi.org/10.1016/S0029-7844(01)01488-0Get rights and content

Abstract

OBJECTIVE:

To determine how often the odds ratio, as used in clinical research of obstetrics and gynecology, differs substantially from the risk ratio estimate and to assess whether the difference in these measures leads to misinterpretation of research results.

METHODS:

Articles from 1998 through 1999 in Obstetrics & Gynecology and the American Journal of Obstetrics and Gynecology were searched for the term “odds ratio.” The key odds ratio in each article was identified, and, when possible, an estimated risk ratio was calculated. The odds ratios and the estimated risk ratios were compared quantitatively and graphically.

RESULTS:

Of 151 studies using odds ratios, 107 were suitable to estimate a risk ratio. The difference between the odds ratio and the estimated risk ratio was greater than 20% in 47 (44%) of these articles. An odds ratio appears to magnify an effect compared with a risk ratio. In 39 (26%) articles the odds ratio was interpreted as a risk ratio without explicit justification.

CONCLUSION:

The odds ratio is frequently used, and often misinterpreted, in the current literature of obstetrics and gynecology.

Section snippets

Methods

The American Journal of Obstetrics and Gynecology and Obstetrics & Gynecology were searched from 1998 and 1999. Articles were selected using the search term “odds ratio” within PubMed (National Library of Medicine; http://www.ncbi.nlm.nih.gov/PubMed/). Each article was evaluated and classified by study type: cohort, case-control, randomized clinical trial, cross-sectional, or meta-analysis. The statistical method used to generate the odds ratios was classified as logistic regression or other

Results

During 1998–1999, 77 articles in the American Journal of Obstetrics and Gynecology and 77 articles in Obstetrics & Gynecology contained “odds ratio” when searched with PubMed. Three articles were excluded. One used the term “odds ratio” to describe a prevalence ratio; another did not contain an odds ratio computation; and the third was a review article with no information about how the odds ratio was computed. One hundred (66.2%) of the remaining 151 articles were cohort studies; 29 (19.2%)

Discussion

The odds ratio appears so often in clinical research reports because of its useful mathematical properties. The coefficients of logistic regression models convert readily into odds ratios, and the odds ratio is the measure of association derived from case-control data. There are situations (eg, comparing two proportions, both of which are close to one) when the odds ratio makes more “sense” than the risk ratio. However, in everyday situations we think more often in terms of risk ratios. We

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