Establishing Measurement Equivalence and Invariance in Longitudinal Data With Item Response Theory

If measurement invariance does not hold over 2 or more measurement occasions, differences in observed scores are not directly interpretable. Golembiewski, Billingsley, and Yeager (1976) identified 2 types of psychometric differences over time as beta change and gamma change. Gamma change is a fundamental change in thinking about the nature of a construct over time. Beta change can be described as respondents' change in calibration of the response scale over time. Recently, researchers have had considerable success establishing measurement invariance using confirmatory factor analytic (CFA) techniques. However, the use of item response theory (IRT) techniques for assessing item parameter drift can provide additional useful information regarding the psychometric equivalence of a measure over time that is not attainable with traditional CFA techniques. This article marries the terminology commonly used in CFA and IRT techniques and illustrates real advantages for identifying beta change over time with IRT methods rather than typical CFA methods, utilizing a longitudinal assessment of job satisfaction as an example.