Measurement theory defines validity with respect to the intended purpose of assessment, meaning that a test is valid only for that purpose. However, the type of test does not determine the nature of the evidence that is required to establish validity for the test. For example, a test used for screening program applicants to a school can be considered valid if the obtained scores relate to future performance in the school. A test that purports to measure a trait, such as intelligence, should discriminate among people who vary in intelligent behavior. A test designed to measure mastery in a course of study should distinguish among groups of people who differ in their content knowledge covered by that course, e.g., novice, intermediate and experts.
Content validity connotes the extent to which an instrument adequately samples a content domain.
In measurement theory, the term content domain refers to a population of tasks, problems or observable behaviors that define an area of interest. A test is simply a sample of problems from that domain. The problem of establishing the content validity of a test involves determining whether the test contains a representative sample of items from a legitimate domain. The evidence for content validity comes from a rational analysis of the items in the sample. Content validity is NOT established through statistical analysis, but rather through an appeal to reason and subjective judgment.