To study the time-related change in educational achievement, scores on knowledge tests, or personal attributes is a primary reason for collecting longitudinal data. Change is a fundamental concept in educational, sociological, and psychological settings. For some areas of research, such as developmental psychology or areas involving program evaluation, change is a central aspect of study. In other areas, change may not be the central aspect of study, but it can still be of concern. The term longitudinal is sometimes used to suggest situations in which change is investigated over long periods of time (e.g., decades), but we are using the term more generally to describe any setting in which one is interested in change and in which (1) the same people are observed repeatedly over time, (2) commensurable measurements (including parallel tests) are used, and (3) the timing for each measurement is known (Lloyd, 2010; McArdle & Nesselroade, 2003).