**Core Concept**
The strength of association between two variables in epidemiological studies is a crucial aspect of understanding the relationship between exposure and outcome. It quantifies the likelihood of an outcome occurring in the presence of a particular exposure.

**Why the Correct Answer is Right**
The odds ratio (OR) is a measure of the strength of association between an exposure and an outcome. It represents the ratio of the odds of an outcome occurring in the exposed group versus the non-exposed group. The OR is calculated as the ratio of the odds of exposure among cases to the odds of exposure among controls. This measure is particularly useful in case-control studies and is often used to quantify the strength of association between a risk factor and a disease. The OR value ranges from 0 to infinity, with values closer to 1 indicating a weak association and values far from 1 indicating a strong association.

**Why Each Wrong Option is Incorrect**
**Option A:** The p-value is a measure of statistical significance, indicating the probability of observing the results by chance, but it does not quantify the strength of association between variables.

**Option B:** The coefficient of regression, also known as the regression coefficient, measures the change in the dependent variable for a one-unit change in the independent variable, but it does not directly quantify the strength of association between exposure and outcome.

**Option C:** The alpha value, also known as the significance level, is the threshold for rejecting the null hypothesis, but it does not measure the strength of association between variables.

**Clinical Pearl / High-Yield Fact**
When interpreting the odds ratio, remember that an OR of 1 indicates no association, an OR > 1 indicates a positive association, and an OR < 1 indicates a negative association. Additionally, the confidence interval (CI) around the OR can provide further insight into the precision of the estimate.

**✓ Correct Answer: D. Odds ratio. The odds ratio is a measure of the strength of association between an exposure and an outcome.**