Epi
Odds Ratio
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Odds Ratio
See Also
Relative Risk
Weighted Mean Difference
Definition
Odds Ratio
Similar to
Relative Risk
when the condition at risk is rare
Odds Ratio is typically used instead of
Relative Risk
when the disease
Incidence
is unknown
Used when comparing discrete, dichotomous data points (e.g. death, complication, disease), especially in meta-analysis
Ratio of probability of an event in an exposed group to that in an unexposed group
Evaluation
Odds Ratio
Calculation
Odds Ratio = (P(disease I exposure) / P(healthy I exposure)) / (P(disease I no exposure) / P(healthy I no exposure))
Number Needed to Treat
can also be calculated based on Odds Ratio and expected event rate
Example
Exposure to Substance X
Disease Y Contracted in 25
Disease Y NOT Contracted in 75
NO Exposure to Substance X
Disease Y Contracted in 10
Disease Y NOT Contracted in 190
Probabilities
Odds of disease if exposed = (P(disease I exposure) / P(healthy I exposure)) = 25 / 75 = 0.334
Odds of disease if not exposed = (P(disease I no exposure) / P(healthy I no exposure)) =10 /190= 0.0526
Conclusion
Odds Ratio = (odds of disease if exposed) / (odds of disease if not exposed) = 0.334/0.0526 = 6.35
Patients exposed to Substance X have a more than 6 fold increased odds of developing disease Y than if unexposed
Interpretation
Odds Ratio may also be used to describe effectiveness of a medication that prevents the development of a disease
Odds Ratio of <1 suggests benefit of a treatment (if the
Confidence Interval
s do not cross 1)
Resources
Wikipedia
https://en.wikipedia.org/wiki/Odds_ratio
References
Hersh (2014) Evidence Based Medicine, AMIA’s CIBRC Online Course
Szumilas (2010) J Can Acad Child Adolesc Psychiatry 19(3): 227–9 +PMID: [PubMed]
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2938757/
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