Cross‐sectional Versus Longitudinal Data: An Empirical Comparison of Mean Differences in Academic Growth

Three sources of data for studies of growth (matched longitudinal, unmatched longitudinal, and cross‐sectional) were compared using mean achievement test scores collected from 32,000 students tested repeatedly from 1961 to 1967. The principal results were that matched longitudinal data yielded significantly higher means than unmatched longitudinal data, and that a school's dropout rate was highly related to the discrepancy between cross‐sectional and matched longitudinal data ( r = .85). Seven sources of differences among the three types of data were considered: age differences, cohort differences, time changes, equating errors, retest effects, cohort change effects, and selection effects. Methodological guidelines for future research in studies of growth and change are presented.