Validity refers to the degree to which an item is measuring what it’s actually supposed to be measuring.
According to City, State and Federal law, all materials used in assessment are required to be valid (IDEA 2004). Additionally, it is important for the evaluator to be familiar with the validity of his or her testing materials to ensure appropriate diagnosis of language disorders and to avoid misdiagnosing typically developing children as having a language disorder/disability. A variety of measures contribute to the overall validity of testing materials.
Construct validity is the most important of the measures of validity. According to the American Educational Research Associate (1999), construct validity refers to “the degree to which evidence and theory support the interpretations of test scores entailed by proposed uses of tests”. In other words, does the test accurately measure what it claims to measure? Does a language assessment accurately measure language ability? If the language assessment claims to diagnose a language disorder, does it diagnose a language disorder when a child truly has one? For example, the PLS-5 claims that it assesses the development of language skills. The PLS-5 has to meet the standards set by the law and can be considered valid if it assesses language skills of the target population with an acceptable level of accuracy.
Content validity is based on expert opinion as to whether test items measure the intended skills. Content validity is widely cited in commercially available test manuals as evidence of the test’s overall validity for identifying language disorders. However, it is important to note that content validity is not based on any empirical data with concrete evidence proving its validity.
Concurrent validity is derived from one test’s results being in agreement with another test’s results which measure the same ability or quality. For example, during the development phase of a new language test, test designers will compare the results of an already published language test or an earlier version of the same test with their own. If the results match, as in if the child is found to be impaired or not with both tests, the test designers use this as evidence of concurrent validity. In theory, the test against which a new test is compared should be considered the “gold standard” for the field. In practice, test designers usually only use another invalid test as the standard against which it is compared. As a result, the concurrent validity only proves that it is equally inaccurate. High concurrent validity is only meaningful when it is compared to an accurate test.
American Educational Research Association, American Psychological Association & National Council on Measurement in Education. (1999) Standards for educational and psychological testing. Washington, DC: American Educational Research Association.
Individuals with Disabilities Education Improvement Act of 2004, H.R.1350,108th Congress (2004)