**Core Concept**
Measurement validity in the context of social and preventive medicine refers to the extent to which a measurement tool accurately assesses the concept it is intended to measure. Validity is a crucial aspect of epidemiological research, ensuring that conclusions drawn from data are reliable and applicable to the population being studied.

**Why the Correct Answer is Right**
An indicator is valid if it measures the thing it is supposed to measure. This means that the measurement tool accurately reflects the concept or phenomenon it is intended to assess. For instance, a blood pressure monitor is valid if it accurately measures blood pressure, not some other physiological parameter. The validity of an indicator is determined by its ability to accurately capture the intended information, making it a fundamental principle in research design and data analysis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because an indicator that also measures the thing it didn't suppose to measure is an example of a confounding variable, which can lead to biased results. This is not a characteristic of a valid indicator.
**Option C:** Overvaluation when measured by different methods may indicate a lack of consistency or reliability in the measurement tool, but it does not necessarily imply that the indicator is valid. Validity is about accuracy, not consistency.
**Option D:** Undervaluation when measured by different methods may suggest that the indicator is not sensitive enough, but it does not necessarily imply that the indicator is invalid. Validity is about accuracy, not sensitivity.

**Clinical Pearl / High-Yield Fact**
In research design, validity is often assessed using various techniques, including face validity, content validity, criterion validity, and construct validity. Understanding the different types of validity is essential for designing and evaluating research studies.

**✓ Correct Answer: B. It measures the thing it expected to measure**