A common practice in the field of learning disabilities is analysis of ability-achievement discrepancies. The reliability of such difference scores is an important statistic in such decision making. In the study, selected ability and achievement devices were administered to a sample of 99 fourth grade low achievers (half of whom were identified as learning disabled). The reliability of various difference scores was then analyzed. In all cases, the reliabilities were moderately high. Reliabilities of differences for devices normed on the same population and differences for devices normed on different populations were comparable. (Author/DB)