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The use of discrepancy formulas for diagnosing dyslexia and other learning disabilities

One particular diagnostic issue that bears examination is the use of discrepancy formulas for diagnosing dyslexia and learning disabilities. The standard definition of a learning disability is the presence of a significant discrepancy between a student’s cognitive potential and their documented academic achievement. This is a discrepancy model for conceptualizing learning disabilities. A significant problem occurs when specific school jurisdictions attempt to rigidly codify this model through the use of arithmetic formulas to identify such a discrepancy. Although a discrepancy model makes sense from a clinical perspective, many professionals in the field argue strongly against the use of a rigid discrepancy formula. There are many flaws in the use of a rigid discrepancy formula.

One problem is that there is no consistent agreement as to how much of a discrepancy is enough. I met with a parent earlier this year. Upon changing school systems, she found that her child was no longer eligible for services because the new district used a different discrepancy formula that required a wider discrepancy.

Another significant flaw in the use of discrepancy formulas is that it assumes that the scores used to obtain an indication of a student’s cognitive ability are rigidly valid. Dyslexic students are notably inconsistent in performance, which tends to lower scores on IQ tests. Also, the same processing deficits that impact academic performance impact performance on tests of potential. Careful analysis of the performance of dyslexics on cognitive testing often reveals much greater cognitive potential than the formal scores reflect. It is therefore erroneous to look solely at scores on IQ tests as an indication of a dyslexic student’s cognitive potential.

Perhaps the most pernicious aspect of the use of discrepancy formulas is that they require a student to fail before identifying a child as dyslexic and providing intervention. Given our understanding of the processing deficits that underlie dyslexia and the strong familial history, there is no justification for allowing students to fall years behind their peers in reading skills before identifying them as dyslexic and providing them with intervention.

An analogy I like to use is to that of a medical doctor treating a patient with a family history of hemophilia. A physician is aware of the genetic nature of the disease and the physiological manifestations. No physician would require a hemophiliac to lose a certain amount of blood before providing medical help, but that is analogous to what discrepancy formulas do. They require the student to fail and for damage to be done before providing intervention.

Finally, use of discrepancy formulas often serve to under recognize bright dyslexics and hard-working dyslexics, who often are able to compensate to a degree that a wide enough discrepancy never develops. It also serves as a disincentive for providing appropriate early intervention.

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