This article describes the strengths and weaknesses of different models of classification (primarily monothetic and polythetic) and then focuses on cluster analysis as a method of pursuing a polythetic model. Design features that need to be incorporated in a cluster analytic study are discussed in relation to theory formulation, internal validity,…

This review of the history of learning disabilities (LD) suggests that creation of the LD category has served scientific, if not always, policy purposes. It finds that 40 years of research have produced inventive, creative tools that benefit both investigation and intervention and, most importantly, a better understanding of individual…