# Content Validity (Logical or Rational Validity)

Statistics Definitions > Content Validity

## What is Content Validity?

When you create a test or questionnaire for a particular subject, you want the questions to actually measure what you want them to. For example, the AP Physics exam should cover all topics actually taught to students and not unrelated material like English or biology. This matching between test questions and the content the questions are supposed to measure is called content validity. If some of the test questions are measuring something else, this can create bias.

## Content Validity Example

More formally, that “something” you are trying to measure is called a construct. A construct can be (almost) anything. Simple constructs include height, weight and IQ. More complicated constructs include: ability to perform well at a certain job; competency with wide-ranging subject areas like physics or U.S. history, and ability to evaluate other people’s psychiatric condition.

Examples of measurements that are content valid:

• Height (construct) measured in centimeters (measurement).
• AP Physics knowledge (construct) measured by the AP exam (measurement).

Examples of measurements that have debatable content validity:

• The Bar Exam is not a good measure of ability to practice law (see here).
• IQ tests are not a good way to measure intelligence (see here and here).

## Differences with Face Validity and Internal Consistency

Content validity is also similar to face validity. However, they both use different approaches to check for validity. Face validity is an informal way to check for validity; anyone could take a test at its “face value” and say it looks good. Content validity uses a more formal, statistics-based approach, usually with experts in the field. These experts judge the questions on how well they cover the material.

Content validity and internal consistency are similar, but they are not the same thing. Content validity is how well an instrument (i.e. a test or questionnaire) measures a theoretical construct. Internal consistency measures how well some test items or questions measure particular characteristics or variables in the model. For example, you might have a ten-question customer satisfaction survey with three questions that test for “overall satisfaction with phone service.” Testing those three questions for satisfaction with phone service is an example of checking for internal consistency; taking the whole survey and making sure it measures “customer satisfaction” would be an example of content validity.