Some Results on Reliability for the Longitudinal Measurement of Change: Implications for the Design of Studies of Individual Growth

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Abstract
When changes in educational or psychological status are being measured, every subject in the sample must be observed on several chronologically successive occasions. In the pursuit of such longitudinal data, traditional researchers have been content to administer only a pre-test and a post-test (thus collecting two waves of data on each subject). More recently, however, methodologists have argued that multiwave data (i.e., more than two waves) must be collected for the effective measurement of change. Multi-wave data allows a suitable mathematical model to be fitted to each of the individual growth records as a way of summarizing the growth of each subject. Subsequent investigations of between-individual differences in growth can then be based on the results of these fits.