Considerations in the use of difference scores to identify learning-disabled children

Although difference scores are widely used in classifying children as learning-disabled, their psychometric properties are often not well understood. Such scores generally contain more error than single test scores. Reliability and standard error of measurement figures for several combinations of ability and achievement measures are presented. The rates and types of errors that occur when such scores are used to classify children as learning-disabled are discussed. Three recommendations for using difference scores are given: (a) combinations of ability and achievement tests that yield difference score reliabilities higher than .80 should be used when classifying children; (b) scores should be reported as a band of scores (± one standard error of measurement) to inform decision-makers regarding the amount of error estimated to be in the score, and (c) the criterion score for classifying the learning disabled should be set after consideration of the rate and types of errors likely to occur.