# Odds ratio adbc odds ratio 6 x 7 42 35 4 x 3 12cases

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the ratio of the cross-products. Odds ratio = ad bc Odds ratio = 6 x 7 = 42 = 3.5 4 x 3 12 Cases Controls Exposed 6 3 Nonexposed 4 7 Total 10 10 Gordis, Epidemiology, 5 th edition, Page 224 Examples of calculating Odds Ratios in Case- control studies
Matched-Pairs Case-Control study: Controls are selected by matching each one to a case according to variables that are known to be related to disease risk, such as sex, age, or race (individual matching or matched pairs). The results are then analyzed in terms of case-control pairs rather than for individual subjects. If exposure is dichotomous (a person is either exposed or not exposed), four types of case-control pairs or combinations are possible . Examples of calculating Odds Ratios in Case- control studies
Concordant pairs (pairs with the same exposure experience) 1. Pairs in which both the case and the control were exposed 2. Pairs in which neither the case nor the control was exposed Discordant pairs (pairs with different exposure experience) 3. Pairs in which the case was exposed but the control was not 4. Pairs in which the control was exposed and the case was not Gordis, Epidemiology, 5 th edition, Page 225 Examples of calculating Odds Ratios in Case- control studies
Gordis, Epidemiology, 5 th edition, Page 225 Examples of calculating Odds Ratios in Case- control studies Exposed Not Exposed Exposed a b Not Exposed c d Control Case
Examples of calculating Odds Ratios in Case-control studies Calculation of the odds ratio in such a matched- pair study is based on the discordant pairs only ( b and c ). The concordant pairs are ignored, because they do not contribute to our knowledge of how cases and controls differ in regard to past history of exposure. The odds ratio for matched pairs is therefore the ratio of the discordant pairs. The odds ratio for the preceding 2 × 2 table is as follows: Odds ratio (matched pairs) = b/c
Examples of calculating Odds Ratios in Case-control studies
Gordis, Epidemiology, 5 th edition, Page 226 Examples of calculating Odds Ratios in Case- control studies Exposed Not Exposed Exposed 2 4 Not Exposed 1 3 Control Case The concordant pairs are ignored in the analysis of matched pairs. There are four pairs in which the case was exposed and the control was not and one pair in which the control was exposed and the case was not. Hence, the odds ratio for matched pairs is as follows: Odds ratio = b/c = 4/1 = 4
Conclusion This chapter has introduced the concepts of absolute risk, relative risk, and odds ratio. In Chapter 12 we will discuss another important aspect of risk: the attributable risk.
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