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## Odds Ratio

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

### Odds Ratio (C0028873)

Definition (NCI) The ratio of the odds of an event occurring in one group to the odds of it occurring in another group, or to a sample-based estimate of that ratio.
Definition (NCI_NCI-GLOSS) A measure of the odds of an event happening in one group compared to the odds of the same event happening in another group. In cancer research, odds ratios are most often used in case-control (backward looking) studies to find out if being exposed to a certain substance or other factor increases the risk of cancer. For example, researchers may study a group of individuals with cancer (cases) and another group without cancer (controls) to see how many people in each group were exposed to a certain substance or factor. They calculate the odds of exposure in both groups and then compare the odds. An odds ratio of one means that both groups had the same odds of exposure and, therefore, the exposure probably does not increase the risk of cancer. An odds ratio of greater than one means that the exposure may increase the risk of cancer, and an odds ratio of less than one means that the exposure may reduce the risk of cancer.
Definition (MSH) The ratio of two odds. The exposure-odds ratio for case control data is the ratio of the odds in favor of exposure among cases to the odds in favor of exposure among noncases. The disease-odds ratio for a cohort or cross section is the ratio of the odds in favor of disease among the exposed to the odds in favor of disease among the unexposed. The prevalence-odds ratio refers to an odds ratio derived cross-sectionally from studies of prevalent cases.
Concepts Quantitative Concept (T081)
MSH D016017
English Cross Product Ratio, Cross-Product Ratio, Cross-Product Ratios, Odds Ratios, Odds, Relative, Ratio, Cross-Product, Ratio, Risk, Ratios, Cross-Product, Ratios, Risk, Relative Odds, Risk Ratio, Risk Ratios, RELAT ODDS, odd ratio, odds ratio, risk ratio, Odds Ratio, relative odds
Swedish Oddskvot
Finnish Kerroinsuhde
French OR (Odds Ratio), Rapport de chances, Risque relatif approximatif, Odds ratio
Russian REZUL'TATOV PEREKRESTNYKH KOEFFITSIENT, KOEFFITSIENT RAZLICHII V CHASTOTE SLUCHAEV, RISKA KOEFFITSIENT, КОЭФФИЦИЕНТ РАЗЛИЧИЙ В ЧАСТОТЕ СЛУЧАЕВ, РЕЗУЛЬТАТОВ ПЕРЕКРЕСТНЫХ КОЭФФИЦИЕНТ, РИСКА КОЭФФИЦИЕНТ
Japanese オッズ比, 乗積比, 危険比
Italian Odds relativo, Rapporto crociato, Odds ratio
Czech podíl šancí, poměr rizik, poměr šancí
Polish Wskaźnik ryzyka, Iloraz szans
Norwegian Odds ratio, Forholdet mellom to odds, Kryssproduktforhold, Oddsforhold, OR
Portuguese Razão de Diferença, Odds Ratio, Razão de Produtos Cruzados, Taxa de Risco, Razão de Chances
German Cross-Product Ratio, Odds Ratio, Risikoquotient
Dutch Oddsratio, Ratio, cross-product, Risicoratio
Sources
Derived from the NIH UMLS (Unified Medical Language System)

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