RR = 0.085/0.035 = 2.4. Applications of Probability in Epidemiology Page 11 of 17 b. Odds ratios (OR) are commonly reported in the medical literature as the measure of association between exposure and outcome. Is this a risk ratio or rate ratio? Calculate the relative risk of injury among machine users compared to free-weight users. The odds ratio ((a/c)/(b/d)) looks at the likelihood of an outcome in relation to a characteristic factor. This prospective cohort study was used to investigate the effects of hormone replacement therapy (HRT) on coronary artery disease … Very rare outcomes (e.g., in the tables above) will give odds ratios that are extremely close to what the risk ratio … Consider an example from The Nurses' Health Study. Odds Ratio (OR) is a measure of association between exposure and an outcome. Bossuyta aDepartment of Clinical Epidemiology & Biostatistics, University of Amsterdam, Academic Medical Center, Post Office Box 22700, 100 DE Amsterdam, The Netherlands BioEpi540W 6. An odds ratio (OR) is a statistic that quantifies the strength of the association between two events, A and B. Unexposed 3-yr incidence (free-weights): 15/425 = 0.035. Calculate the odds ratio for injury among machine users (yes! Journal of Clinical Epidemiology 56 (2003) 1129–1135 The diagnostic odds ratio: a single indicator of test performance Afina S. Glasa,*, Jeroen G. Lijmerb, Martin H. Prinsc, Gouke J. Bonseld, Patrick M.M. A comparison of odds, the odds ratio, might then make sense. OR= ˇ 1 1 ˇ 1 ˇ 2 1 ˇ 2 Odds ratio for the Titanic example is OR= 3:76 0:37 = 10:16: This is very different from the relative risk calculated on the same data and may come as a surprise to some readers who are accustomed of thinking of odds ratio as of relative risk (Greenland, 1987). and generally odds ratios (which approximate risk ratios when the outcome is rare) are used. In epidemiological terms, the odds ratio is used as a point estimate of the relative risk in retrospective studies. Odds ratio in a matched study In a 1:1 matching, a case is paired with a control based on a similar characteristic (e.g. Odds("comparison of two complementary (opposite) outcomes"): In words, the odds of an event "E" is the chances of the event occurring in comparison to The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure. Odds ratio is the key statistic for most case-control studies. An odds ratio of 11.2 means the odds of having eaten lettuce were 11 times higher among case-patients than controls. Because the odds ratio is greater than 1.0, lettuce might be a risk factor for illness after the luncheon. Provided that the disease is uncommon (say <10%), this sampling approach gives an odds ratio that is a reasonably good estimate of the risk ratio. Assume a rare outcome so OR’s and RR’s are approximately equal: We cannot get the additive interaction directly but suppose we divide Additive interaction p 11 - p 10 - p 01 + p 00 by p 00 We get: RR 11 - RR 10 - RR 01 + 1 Important points about Odds ratio: Calculated in case-control studies as the incidence of outcome is not known age), and the exposure is assessed in this pair. Odds Ratio = (a/c) / (b/d) = ad /bc An Odds Ratio of unity means that cases are no more likely to be exposed to the risk factor than controls. Odds Ratio as an Estimate of Risk Ratio. 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