## **Core Concept**
The specificity of a screening test is the proportion of true negatives (people without the disease who test negative) among all actual negatives (people without the disease). It measures the test's ability to correctly identify those without the disease.

## **Why the Correct Answer is Right**
To calculate specificity, we use the formula: Specificity = TN / (TN + FP), where TN is the number of true negatives and FP is the number of false positives. From the given data: TN = 600 (Not diseased and Negative test) and FP = 200 (Not diseased but Positive test). So, Specificity = 600 / (600 + 200) = 600 / 800 = 0.75.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because its calculation does not match the formula for specificity.
- **Option B:** This option is incorrect because, similar to Option A, it does not correctly represent the specificity calculation based on the provided data.
- **Option D:** This option is incorrect as it does not align with the correct calculation of specificity.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that specificity and sensitivity are inversely related in many cases, and the ideal test would have both high sensitivity and specificity. However, in practice, enhancing one often reduces the other. Understanding the clinical context and the consequences of false positives versus false negatives helps in choosing the appropriate test characteristics.

## **Correct Answer:** B. 0.75.