Learning disability diagnosis and placement decisions require estimates of the discrepancy between a child's ability to achieve and actual achievement. This paper reviews current estimation techniques and their resultant effects on the learning disability classroom's composition. An alternate estimation methodology, using z-score conversions, is presented. The methodology is generalized beyond specific tests. Z-score conversion tables are presented for the Stanford-Binet, WISC-R, WRAT, and the PIAT. A table to estimate the probability levels for differences between ability and achievement tests is presented. The significance of the difference between a child's scores on the ability and achievement measures can be evaluated using the tables presented. A concrete example, using the appropriate tables, is provided.