**Core Concept**
The cross product ratio is a statistical measure used to compare the odds of an event occurring in two different groups. It is a key concept in epidemiological studies, particularly in the context of case-control studies.

**Why the Correct Answer is Right**
In a case-control study, researchers often compare the odds of exposure to a risk factor in cases (individuals with a specific outcome) versus controls (individuals without the outcome). The cross product ratio, also known as the odds ratio (OR), is calculated as the ratio of the odds of exposure in cases to the odds of exposure in controls. This measure helps researchers estimate the strength and direction of the association between the exposure and the outcome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the cross product ratio is not calculated in cohort studies, which involve following individuals over time to assess the development of outcomes.
**Option B:** This option is incorrect because the cross product ratio is not a measure of association, but rather a measure of the strength and direction of the association between two variables.
**Option C:** This option is incorrect because the cross product ratio is not a type of regression analysis, but rather a statistical measure used in case-control studies.

**Clinical Pearl / High-Yield Fact**
When interpreting the cross product ratio, keep in mind that an OR of 1 indicates no association between the exposure and the outcome, while an OR greater than 1 indicates a positive association, and an OR less than 1 indicates a negative association.

**Correct Answer:** D. Case-control study.