Validity of the Concept of Dyslexia: Alternative Approaches to Definition and Classification

This chapter reviews a series of studies addressing the hypothesis that dyslexia represents a disorder that is discrete and separable from other forms of poor reading ability. When dyslexia is defined as a discrepancy between intelligence test scores and reading achievement, comparative studies of poor readers who meet discrepancy and low achievement definitions do not provide strong support for the validity of this classification hypothesis. These findings are consistent when other approaches to defining and measuring aptitude and reading ability are used as well as across a range of definitions of severity of reading disability.