title: Interpretation of odds and risk ratios.
abstract: BACKGROUND
  Odds ratio and risk ratio are measures of association used to describe the efficacy of interventions and disease determinates; however, they are not interchangeable measures of association.
  
  
  OBJECTIVE
  To illustrate that interpretation of the odds ratio as a risk-based measure of efficacy can be misleading.
  
  
  METHODS
  None.
  
  
  METHODS
  A meta-analysis reported, the odds ratio and the risk ratio as measures of vaccine effect. Example data were obtained from a meta-analysis of the risk of infection with Tritrichomonas fetus (T. fetus), in trials that assessed whole-cell killed T. fetus vaccination in beef heifers.
  
  
  RESULTS
  When risk was used as the measure of disease frequency, the summary risk ratio was 0.82 (95% CI = 0.7-1.01), a 18% decrease in risk of infection. When odds were used as the measure of disease frequency and the summary odds ratio was 0.41 (95% CI = 0.2-0.84), a 59% decrease in odds of infection.
  
  
  CONCLUSIONS
  Problems arise for clinicians or authors when they interpret the odds ratio as a risk ratio. In the example provided, the efficacy of protective interventions was overestimated. In the case of disease determinates that increase the occurrence of disease, the interpretation of the odds ratio as a risk ratio would also lead to overestimation of the effect. It is important not to use the terms risk or probability of disease when the odds are the measure of disease frequency.
authors.affiliation: Veterinary Diagnostic and Production Animal Medicine, College of Veterinary Medicine, Iowa State University, Ames, IA 50010, USA. oconnor@iastate.edu
authors.forename: A M
authors.initials: AM
authors.lastname: O'Connor
mesh_terms: D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology / Q000469:parasitology*; D003627:Data Interpretation, Statistical; D005260:Female; D016017:Odds Ratio; D011529:Protozoan Infections, Animal / Q000453:epidemiology*; D012307:Risk Factors; D016835:Tritrichomonas foetus