Classification of Learning Disabilities: an Evidence- Based Evaluation

The purpose of this paper is to review research on the classification of learning disabilities (LD). We begin by briefly reviewing the nature of classification research. Then we discuss the evolution of definitions of LD, making explicit the classification hypotheses from which these definitions derive. An extensive review of the evidence for these hypotheses will be provided for the three components of classification implicit in the federal definition of LD: discrepancy, heterogeneity, and exclusion. We will show that classification hypotheses involving discrepancy and exclusion as embedded in federal (and state) policy have at best weak validity, often representing inaccurate and outdated assumptions about LD. There is evidence for heterogeneity of LD, but some reorganization of the types of LD identified in the federal definition may be necessary. Throughout the paper we identify alternative approaches to classification and identification, including weaknesses in any psychometric approach to the identification of LD. We suggest that classifications based on inclusionary definitions that specify attributes of different forms of LD are more desirable than current exclusionary definitions. Inclusionary definitions permit a focus on identification procedures that are intervention oriented as well as a focus on prevention, both of which are desirable and could contribute to improved results in remediating LD.