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Inherent in many definitions of developmental dyslexia (DD) is the notion of a discrepancy between actual achievement and'potential'. Discrepancy definitions, however, have been controversial with problems associated with validity and measurement being persistent as well as ubiquitous. Thus, the aim of the current review is twofold: first, to delineate the problems associated with construct and criterion-related validities of DD definitions; and second, to explore issues associated with measurement including the unreliability of discrepancy scores, measurement error, the correlation between intelligence and reading, regression to the mean, the use of cut-off scores, and the use of test norms. Given the extent of these issues, it is difficult to have confidence in the principles associated with reading disability research and current clinical practice. Thus, there is an urgent need to revaluate the premises associated with DD definition, in order for future growth and development to occur in this field.

" Whatever exists at all exists in some quantity. To know it thoroughly involves knowing its quantity as well as its quality"(Thorndike, 1918). Thorndike's (1918) statement provides a basis by which to conceptualise research that has occurred in the field of reading disability and developmental dyslexia (DD). Throughout the decades there have been attempts to develop methods of quantification and operationalization of DD, as well as a concurrent quest to understand the nature of this disorder.