Content Validity
Whether a test's items adequately represent the entire content domain of the construct. Do the questions cover all aspects of what you're trying to measure?
Content validity (APA Standards, 2014) asks: "Does the test cover the full breadth of the construct?" A Big Five Conscientiousness test should cover organization, diligence, self-discipline, and dutifulness — not just one aspect.
Content validity is typically established through expert judgment: do psychologists agree the items represent the domain? For example, a job knowledge test should cover all essential job duties, not just easy-to-test topics.
Weakness of MBTI: low content validity for Thinking-Feeling dimension. "Do you prefer to decide based on logic (T) or values (F)?" doesn't represent the actual theoretical distinction Jung proposed. The items are too narrow and don't capture the depth of the construct.
Strength of Big Five: high content validity. Items systematically cover multiple aspects of each trait (Conscientiousness includes organization, dutifulness, achievement-striving, self-discipline, and cautiousness — the full domain).
Source: APA Standards for Educational and Psychological Testing.