Calculating achievement composite scores for regression discrepancy models

Regression discrepancy-model equations fail to account for the common practice of obtaining more than one achievement score for a discrepancy area (e.g., reading comprehension). This article presents the equations required to calculate composite achievement scores based upon two achievement scores. Equations to calculate the composite's reliability and correlation with IQ are also given. Calculating composite scores offers a method to combine achievement scores from the same discrepancy area to produce a single standard score that is often more comprehensive and reliable than either individual score, and more suitable for regression equations. Composite scores may not always be necessary in discrepancy assessment, but can help overcome problems with interpretation of regression results.