The definition of instrument validity is the extent to which an instrument measures what it is supposed to. Validity is established by correlating the scores with a similar instrument. Also, expert review establishes validity.
There are four types of validity.
1. Criterion (Concurrent or Predictive) Validity focuses on how well the instrument compares with external variables considered to be direct measures of the characteristic or behavior being examined. Intelligence test scores used to predict future performance are an example of criterion validity.
2. Content Validity is concerned with how well the content of the instrument samples the kinds of things about which conclusions are to be drawn. For example, a test of school readiness can be examined to see whether the content relates to knowledge and skills expected of first grade children following the kindergarten program.
3. Construct Validity involves the extent to which certain explanatory concepts or qualities account for performance. For example, a personality test can be studied to see how well theoretical implications of the typologies account for the actual results obtained.
4. Face Validity is often used to indicate whether the instrument, on the face of it, appears to measure what it claims to measure. Here the key question is whether the persons making use of the instrument accept it as a valid measure in the everyday sense of the word?
For more information, see Handbook in Research and Evaluation for Education and the Behavioral Sciences by Stephen Isaac and William B. Michael.