It is designed to guide professional practice and to be useful to a wide range of people who work with tests or test results. By using these standards, educators can make decisions about whether a particular test is technically adequate and appropriate for their purposes and whether they can reasonably reach conclusions based on the test results. Validity is evaluated by the extent to which the instrument measures what it purports to measure.
Adequate reliability depends upon a low magnitude of errors of measurement.
Such errors can be introduced, for example, by inconsistencies in the performance of those being tested or variability in their interest, motivation, or other emotional and physical states, which may be unrelated to the purpose of the test. Such influences make stability in test scores of young children very problematic. Norms are usually displayed in tables showing the performance of reference groups of students who have taken the test. Norms can be expressed in age or grade equivalents, standard scores, percentiles, and so on. Norms permit the comparison of the performance of new groups of test takers with the samples of students on whom the test was standardized.