Reliability, while indispensable only tells you that the test is measuring something consistently. It does not prove that you are measuring what you intend to measure. A mis-manufactured 33 inch yardstick will consistently tell you that 33 inch boards are 33 inches long. Unfortunately, if what you’re looking for is a board is one full yard long, then your 33 inch yardstick though reliable is misleading you by 3 inches.
What you need is a valid yardstick. Validity tells you whether the test (or yardstick) is measuring what you think it’s supposed to be measuring.
A test, as we said, is a sample of a person’s behavior but some tests are more clearly representative of the behavior being sampled than others. A typing test, for example, clearly corresponds to an on the job behavior. At the other extreme, there may be no apparent relationship between the items on the test and the behavior. This is the case with projective personality tests. Thus, in the Thematic perception test, the psychologists asks the person to explain how he or she interprets an ambiguous picture. The psychologists use that interpretation to draw conclusions about the person’s personality and behavior. In such tests, it is more difficult to prove that the tests are measuring what they are said to measure in this case some trait of the person’s personality that they have valid.
The accuracy with which a test, interview and so on measures what it purports to measure or fulfills the function it was designed to fill.
Test validity answers the question. Does this test measure what it’s supposed to measure? Put another way, validity refers to the correctness of the inferences that we can make based on the test. For example, if Jane gets higher score on a mechanical comprehension tests than Jim can we be sure that Jane possesses more mechanical comprehension than Jim? With employee selection tests, validity often refers to evidence that the test is job related – in other words, that performance on the test is a valid predictor of subsequent performance on the job. A selection test must be valid since, without proof of validity, there is no logical or legally permissible reason to continue using it to screen job applicants. You would not be too comfortable taking the GRE, if you didn’t think about your score. Equal employment law calls for use of valid tests. In employment testing, there are two main ways to demonstrate a test’s validity: criterion validity and content validity.
A type of validity based in showing that scores on the test (predictors) are related to job performance (criterion).
Demonstrating criterion validity means demonstrating that those who do well on the test also do well on the job, and that those who do poorly on the test poorly on the job. Thus, the test has validity to the extent that the people with higher test scores perform better on the job. In psychological measurement, a predictor is the measurement (in this case, the test score) that you are trying to relate to a criterion like performance on the job. The term criterion validity reflects that terminology.
A test that is content valid is one that contains a fair sample of the tasks and skills actually needed for the job in question.
Employers demonstrate the content validity of a test by showing that the test constitutes a fair sample of the content of the job. The basic procedure here is to identify job tasks that are critical to performance, and then randomly select a sample of those tasks to be tested. In selecting students for dental school, many schools give applicants chunks of chalk, and ask them to carve something that looks like a tooth. If the content you choose for the test is a representative sample of what the person needs to know for the job, then the test is probably content valid. Clumsy dental students need not apply.
Demonstrating content validity sounds easier than it is in practice. Demonstrating that (1) the tasks the person performs on the test are really a comprehensive and random sample of the tasks performed on the job and (2) the conditions under which the person takes the test resemble the work situation is not always easy. For many jobs, employers opt to demonstrate other evidence of a test’s validity – most often, criterion validity.