What is the odds ratio for the following?Screening test resultsDiagnosis Total DiseasedNot diseasedPositiveabNegativecdTotala+bb+d
**Core Concept**
The question is testing the understanding of epidemiological measures, specifically the **odds ratio (OR)**, which is a measure of association between an exposure and an outcome. The odds ratio is calculated from a **2x2 contingency table**, which is used to summarize the relationship between a binary exposure and a binary outcome.
**Why the Correct Answer is Right**
The odds ratio is calculated as the ratio of the odds of exposure among the cases to the odds of exposure among the controls. In this table, the odds of a positive test result among the diseased is a/b, and the odds of a positive test result among the non-diseased is c/d. The odds ratio is therefore (a/b) / (c/d), which simplifies to **ad/bc**.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the formula for calculating the odds ratio from a 2x2 contingency table.
**Option B:** This option is also incorrect for similar reasons, as it does not correctly represent the relationship between the exposed and unexposed groups.
**Option D:** This option is incorrect as it represents a different epidemiological measure.
**Clinical Pearl / High-Yield Fact**
The odds ratio is a useful measure of association in case-control studies, where the incidence of disease cannot be directly measured. It is also useful in cross-sectional studies where the prevalence of disease is measured.
**Correct Answer:** ad/bc