Willson and Reynolds (in this issue) challenged my thesis that the regression-based discrepancy method (RDM) is not a valid tool to detect aptitude-achievement discrepancies. In this response, I show that the statistical and theoretical counterarguments of Willson and Reynolds are based on a misreading of the statistical models presented. Furthermore, I demonstrate that the regression adjustment, which is largest for lower correlations, is the direct source of the lack of validity of the RDM procedure. Nevertheless, RDM can be considered a valid method to measure an achievement component that is unrelated to intelligence.
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References
1.
Cronbach, L.J., & Furby, L. (1970). How should we measure "change"—or should we?Psychological Bulletin,74, 68-80.
2.
McLeod, J. (1979). Educational underachievement: Toward a defensible psychometric definition. Journal of Learning Disabilities , 12, 42-50.
3.
Reynolds, C.R. (1984). Critical measurement issues in learning disabilities . The Journal of Special Education, 18, 451-476.
4.
Rogosa, D.R., Brandt, D., & Zimowski, M. (1982). A growth curve approach to the measurement of change. Psychological Bulletin,92, 726-748.
5.
Van den Broeck, W. (2001a). The concept of developmental dyslexia: Toward a behaviorally grounded diagnosis. Manuscript submitted for publication.
6.
Van den Broeck, W. (2001b). The reliability of aptitude-achievement discrepancy measures. Manuscript in preparation.
7.
Willson, V.L., & Reynolds, C.R. (1984). Another look at evaluating aptitude-achievement discrepancies in the diagnosis of learning disabilities . The Journal of Special Education, 18, 477-487.
8.
Zimmerman, D.W., & Williams, R.H. (1982). The relative error magnitude in three measures of change. Psychometrika, 47, 141-147.
