1.5.2 Discrepancy definitions
The label is given if there is a discrepancy between perceived potential to learn to read (as indicated by general ability) and actual level of reading achievement.
The most common way of diagnosing dyslexia is to look for a discrepancy between someone's general ability as measured by an IQ assessment and his or her performance on standardised measures of reading and spelling. However, there are many variations in the procedures for measuring a discrepancy between potential and actual reading ability, and the precise boundaries that may be chosen as ‘cut-offs’ will also vary depending on the purposes of the measurement. Different criteria may be used, for example, in research studies (where strict statistical boundaries may be needed for scientific reasons) than in educational or other settings (where diagnoses may be used to guide more personal and/or practical decisions).
Notice that this approach to defining dyslexia reflects a psychometric perspective.
Discrepancy based definition and diagnosis of dyslexia assumes that an IQ assessment indicates a person's potential for learning to read. However, IQ is only weakly related to reading achievement, and assessment will involve some ‘measurement error’, so any predictions of ‘expected’ reading achievement will be insensitive at best. Keith Stanovich (1991) has argued for a different approach to discrepancy based diagnosis of dyslexia, where reading ability is compared to listening comprehension rather than IQ. Despite the academic support for such an approach, IQ based assessment remains the ‘gold standard’ method for identifying dyslexia, but educational psychologists do not simply look for a discrepancy in scores. Instead they examine a person's performance on each IQ subtest as well as overall performance. This information is combined with evidence from other sources, such as the person's case history, before dyslexia is identified.
An overall discrepancy between IQ and reading ability will identify a broad range of people with specific reading difficulties, but depending on the population studied, reading problems will in some cases arise entirely from social, emotional or cultural influences. This means that specific reading difficulties are not to be equated with dyslexia, as this term refers to a broader developmental syndrome proposed to have a (biological) basis.
Discrepancy definitions reflect the statistical approach to defining ‘abnormality’. Other criteria are usually required for any meaningful definition of ‘abnormal’ functioning – and the methods used for assessment, their reliability and their validity are crucial issues.
If someone's reading improves through special help and there is no longer a discrepancy, would that make him or her less dyslexic? If we accept that dyslexia has a biological basis which impacts on skills other than literacy, then even if the reading difficulties ‘go away’ the underlying cause of the dyslexia and other associated symptoms may remain.
Box 4: Definitions
Definition by exclusion: A definition that identifies a person as having a condition if there is no other known reason that can account for their symptoms.
Discrepancy definitions: A definition that identifies a person as having a condition by virtue of a perceived discrepancy between potential and actual ability.
Positive indicators: A symptom or characteristic that can be used to identify a condition by its presence.
Congenital word blindness: The term used by Hinshelwood in 1917 to describe dyslexia-like difficulties in children.