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JAMA Guide to Statistics and Methods
July 3, 2018

Odds Ratios—Current Best Practice and Use

Author Affiliations
  • 1Department of Health Management and Policy, Department of Economics, University of Michigan, Ann Arbor
  • 2National Bureau of Economic Research, Cambridge, Massachusetts
  • 3Division of Health Policy and Management, School of Public Health, University of Minnesota, Minneapolis
  • 4Center for Health Services Research in Primary Care, Durham Veterans Affairs Medical Center, Durham, North Carolina
  • 5Department of Population Health Sciences, Duke University School of Medicine, Durham, North Carolina
  • 6Division of General Internal Medicine, Department of Medicine, Duke University School of Medicine, Durham, North Carolina
JAMA. 2018;320(1):84-85. doi:10.1001/jama.2018.6971

Odds ratios frequently are used to present strength of association between risk factors and outcomes in the clinical literature. Odds and odds ratios are related to the probability of a binary outcome (an outcome that is either present or absent, such as mortality). The odds are the ratio of the probability that an outcome occurs to the probability that the outcome does not occur. For example, suppose that the probability of mortality is 0.3 in a group of patients. This can be expressed as the odds of dying: 0.3/(1 − 0.3) = 0.43. When the probability is small, odds are virtually identical to the probability. For example, for a probability of 0.05, the odds are 0.05/(1 − 0.05) = 0.052. This similarity does not exist when the value of a probability is large.