A comparison of methods for calculating a severe discrepancy

Given the substantial rise in the number of students identified as learning-disabled, increasing attention has centered on methods for determining a severe discrepancy between ability and achievement. Using scores from 86 learning disabilities referrals, we compared four such methods (a z-score discrepancy, an estimated true score discrepancy, an unadjusted regression procedure, and an adjusted regression procedure). Each student was evaluated with the WISC-R, PIAT, and K-ABC. A high degree of agreement was found between z-score and estimated true score difference approaches. Less agreement was found between the unadjusted regression procedure and the other methods. It was concluded that the four methods cannot be used interchangeably in the calculation of severe discrepancies. Of the four methods that were analyzed, the unadjusted regression procedure selected the smallest percentage of students.