An appropriate difference function to represent changes in a group, as reflected by total test scores, makes use of the pooled within persons regression (as distinct from the between persons regression) of posttest scores on pretest scores. Regressions within each individual, based on simple assumptions about his item scores, are also readily available.
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References
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WebsterH.Extension of a simple psychometric model to measure change.Berkeley: Center for the Study of Higher Education, Univer. of Calif., May, 1963.
