Expired CME Article

Odds Ratios and Risk Ratios: What’s the Difference and Why Does It Matter?

Authors: Anthony J. Viera, MD, MPH


Odds ratios (OR) are commonly reported in the medical literature as the measure of association between exposure and outcome. However, it is relative risk that people more intuitively understand as a measure of association. Relative risk can be directly determined in a cohort study by calculating a risk ratio (RR). In case-control studies, and in cohort studies in which the outcome occurs in less than 10% of the unexposed population, the OR provides a reasonable approximation of the RR. However, when an outcome is common (iÝ10% in the unexposed group), the OR will exaggerate the RR. One method readers can use to estimate the RR from an OR involves using a simple formula. Readers should also look to see that a confidence interval is provided with any report of an OR or RR. A greater understanding of ORs and RRs allows readers to draw more accurate interpretations of research findings.

Key Points

* When an outcome in a research study is common (eg, occurs in more than 10% of the unexposed group), the odds ratio will tend to overestimate the risk ratio.

* One method to estimate the odds ratio involves using a simple formula.

* A confidence interval should be provided along with any report of an odds ratio or risk ratio.

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1.Katz KA. The (relative) risk of using odds ratios. Arch Dermatol 2006;142:761–764.
2.Bland JM, Altman DG. The odds ratio. BMJ 2000;320:1468.
3.Holcomb WL, Chaiworapongsa T, Luke DA, et al. An odd measure of risk: use and misuse of the odds ratio. Obstet Gynecol 2001;98:685–688.
4.Gordis L. Epidemiology, ed 2. Philadelphia, W.B. Saunders, 2000.
5.Sonis J. A closer look at confounding. Fam Med 1998;30:584–588.
6.Viera AJ, Kshirsagar AV, Hinderliter AL. Lifestyle modifications to lower or control high blood pressure: is advice associated with action? J Clin Hypertens 2008;10:105–111.
7.McNutt L, Wu C, Xue X, et al. Estimating the relative risk in cohort studies and clinical trials of common outcomes. Am J Epidemiol 2003;157:940–943.
8.Zhang J, Yu KF. What’s the relative risk? A method of correcting the odds ratio in cohort studies of common outcomes. JAMA 1998;280:1690–1691.
9.Robbins AS, Chao SY, Fonseca VP. What’s the relative risk? A method to directly estimate risk ratios in cohort studies of common outcomes. Ann Epidemiol 2002;12:452–454.
10.Greenland S. Model-based estimation of relative risks and other epidemiologic measures in studies of common outcomes and in case-controls studies. Am J Epidemiol 2004;160:301–305.
11.Fletcher RW, Fletcher SW. Clinical Epidemiology: The Essentials, ed 4. Philadelphia, Lippincott Williams & Wilkins, 2005.
12.Kraemer HC, Kupfer DJ. Size of treatment effects and their importance to clinical research and practice. Biol Psychiatry 2006;59:990–996.
13.Braithwaite W, Cole P, Feinstein AR, et al. The role of epidemiology in decision-making. The Annapolis Center, 1999. Available at: http://www.annapoliscenter.org/skins/default/display.aspx?mode=user&ModuleId=8cde2e88–3052-448c–893d-d0b4b14b31c4&action=display_page&ObjectID=c69722a1–5eca-41ba-a492–757235a0218f. Accessed January 28, 2008.
14.Harris RE, Chlebowski RT, Jackson RD, et al. Breast cancer and nonsteroidal anti-inflammatory drugs: prospective results from the women’s health initiative. Cancer Res 2003;63:6096–6101.
15.Citrome L. Show me the evidence: using number needed to treat. South Med J 2007;100:881–884.
16.Rothman KJ. Epidemiology: An Introduction. New York, Oxford University Press, 2005.
17.Szklo M, Nieto FJ. Epidemiology: Beyond the Basics, ed 2. Sudbury, Jones and Bartlett, 2007.
18.Streiner DL, Norman GR. PDQ Epidemiology, ed 2. Hamilton, B.C. Decker, 1998.
19.Norman GR, Streiner DL. PDQ Statistics, ed 2. Hamilton, B.C. Decker, 2003.
20.Guyatt GH, Rennie D. Users’ Guides to the Medical Literature: A Manual for Evidence-based Clinical Practice. Chicago, IL, AMA Press, 2001.
21.Straus SE, Richardson WS, Glasziou P, et al. Evidence-Based Medicine: How to Practice and Teach EBM, ed 3. Edinburgh, Elsevier Churchill Livingstone, 2005.