The measure of how well the test matches an accepted test (“gold standard”) outside the study. There are two types of criterion validity: Concurrent validity – The measure of how well the test being studied and the “gold standard” test measure the same thing at the same time. Predictive validity -- The measure of how well the test being studied predicts some practical result that the “gold standard” will find later.

The extent to which the test "agrees" with previous tests that measure the same concept

(is the measure associated with expected behaviors?)