The most important consideration in evaluating and selecting an assessment tool is its validity. Validity refers to the extent to which a test measures what it purports to measure. A similar process is undertaken in determining the validity of tests and assessment techniques. An instrument’s validity is dependent upon a number of factors, including the population or sample for which it is used, its internal properties, and its relationship to other variables. The “ultimate validity” of a test can never be proven. Rather, ongoing use of an instrument contributes to accumulated evidence of validity. Content validity is the degree to which test items represent the performance domain the test is intended to measure. Content validity can be established in a variety of ways including expert judgments, and certain logical and empirical procedures. Criterion-referenced validity refers to the ability of a test to systematically demonstrate a relationship to external criteria.