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What is the Discrepancy Model

  • Ever been told that a student is “fine” when you know they’re not?

  • Ever been told that the student gets good grades so they must be doing “okay?”

If you are doing diagnostic assessments to see if a student is showing signs of dyslexia, you cannot just check to see if they are performing at “grade level.”

What is the discrepancy model, and why is it dangerous when assessing a student for dyslexia?

For many years, clinicians used to diagnose dyslexia based on a ‘discrepancy’ or difference between the student’s IQ or intelligence, and their output, meaning how they were performing academically on certain tests.

We do NOT use this model anymore, or should not be using this model, although many clinicians and even educators are still using the ‘gap’ between intelligence and output to diagnose dyslexia. In an educational setting, it looks more like determining if a student is performing WAY below their peers or well below grade level. Here’s why that’s a problem:

“Individuals with dyslexia are commonly misdiagnosed or even missed entirely. Part of the problem is unreliability in diagnosis that occurs for definitions that feature a single indicator, such as IQ-achievement discrepancy or RTI (Response to Intervention)” -Richard K. Wagner, “Why Is It So Difficult To Diagnose Dyslexia and How Can We Do It Better?” (Dec 2018).

Wagner goes on to say that, just like when cardiovascular disease is diagnosed, a variety of risk factors are examined in a patient over many years to determine if they are showing patterns of behavior consistent with this diagnosis. It would be foolish to take one number on one or two blood tests and make the diagnosis. He then says the following:

“A promising solution to this problem is the use of hybrid models that combine multiple indicators or criteria, thereby reducing the effects of measurement error” (Wagner 2018).

What are these criteria? There is no black and white universal law when diagnosing dyslexia, but Wagner says we need to look at family history of dyslexia, phonological difficulties, reading comprehension worse than listening comprehension, decoding difficulties, and more.

The diagnostic process needs to be comprehensive, look at multiple criteria, spanning through the student’s entire life, and not attempt to make a decision on if their output (performance) is matching their IQ level or their grade level.


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