ABSTRACT
Both reliability and validity are essential parts of the psychometric properties of a measuring instrument.1 The reliability of an instrument is concerned with the consistency of measurements: from time to time, from form to form, from item to item, or from one rater to another. On the other hand, the validity of an instrument is usually defined as the extent to which the instrument actually measures “what it is designed to measure” or “what it purports to measure.” Validity is therefore concerned with the relevance of an instrument for addressing a study’s purpose(s) and research question(s). Both reliability and validity are context-specific characteristics: for example, researchers are often interested in assessing if a measure remains reliable and valid for a specific culture, situation, or circumstance (e.g., a psychological test might be highly reliable and valid in a population of Caucasian adults but not in one of African American children). The conceptualization and specific definitions of reliability and validity have changed over time, as reflected in the various editions of Educational Measurement (Cronbach, 1971; Cureton, 1951; Feldt & Brennan, 1989; Haertel, 2006; Kane, 2006; Messick, 1989; Stanley, 1971; Thorndike, 1951). Table 25.1 contains a list of desiderata regarding reliability and validity of measuring instruments that should be followed in any empirical research report.
