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The odds ratio (OR) is a measure of the magnitude of association between an exposure (smokeless tobacco use) and outcome (pulmonary fibrosis) and can be readily calculated from a 2x2 contingency table. OR = odds of exposure in cases / odds of exposure in non-cases = (45 / 15) / (20 / 40) = 6.0. The OR is defined as the ratio of exposed cases to unexposed cases divided by the ratio of exposed non-cases to unexposed non-cases, where cases represent patients with the outcome of interest. Mathematically, this is expressed as: (exposed cases / unexposed cases) / (exposed non-cases / unexposed non-cases). An OR of >1 indicates that the exposure is associated with higher odds of the outcome (risk factor). An OR of <1 indicates that the exposure is associated with lower odds of the outcome (protective factor). An OR of 1 indicates that the exposure is not associated with the odds of the outcome. Illustration A shows the 2x2 contingency table for this case. Incorrect Answers: Answer 1: 0.2 is obtained by the following calculation: odds of non-exposure in the cases / odds of non-exposure in non-cases = (15 / 45) / (40 / 20), where exposure status is reversed in both numerator and denominator of the OR equation. Answer 2: 0.5 is the odds that a non-case was a user of smokeless tobacco. This is the denominator of the OR equation. Answer 3: 2.5 is the risk ratio, which is another measure of association between exposure and outcome. Risk ratio = probability of pulmonary fibrosis given an exposure / probability of pulmonary fibrosis given no exposure = (45 / (45 + 20)) / (15 / (15 + 40)). Risk ratios are not valid for case-control studies because the true prevalence of the outcome is unknown. Answer 4: 3.0 is the odds that a case with pulmonary fibrosis was a user of smokeless tobacco. This is the numerator of the OR equation. Bullet Summary: The odds ratio is a measure of association between an exposure and outcome and is calculated as the ratio between the odds that a case was exposed and the odds that a non-case was exposed.
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