Understanding Neuropsychological Statistics in Diagnosing Brain Injury


Posted on 2nd April 2008 by Gordon Johnson in Uncategorized

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Yesterday’s blog threw out a few numbers to illustrate some basic starting principles about neuropsychology. As an aid to our further discussion of this neuropsychology, today I will give some basic numerical principles to help in further understanding the numeric part of neuropsychological assessment.

First, neuropsych scores are typical given in one of three scoring methods: Standard score, percentile score and T scores. T scores are a little bit too complicated to try to explain to a laymen, so I will limit this discussion to standard scores and convert them to percentile scores.

Most people are somewhat familiar to standard scores, because IQ’s are given in them. Yesterday I used the example of our successful professional who had a post accident IQ of 135. An IQ of 100 is perfectly in the middle. Something below 70 is evidence of significant impairment. Each time you move down the standard score grid by 10 points, it represents a significant drop.

Here are the basic categories of Standard scores, with their percentile equivalents.

Very superior — 130 and above — 98% and above
Superior __ 120 to 129 — 92% to 97%
High Average — 110 to 119 — 76% to 91%
Average — 90 to 109 — 25% to 75%
Low Average — 80 to 89 — 8% to 24%
Borderline — 70 to 79 — 3% to 7%
Impaired — below 70 — 2% and below

T scores use the same basic concept, and again using 10 points as the break point, but with a T score, the mid point is 50. Some neuropsychologists may disagree as to the exact point that separates these categories, but this is certainly representative of the concept.

The second term to understand in terms of understanding the statistical analysis done by a neuropsychologist is the concept of “deviations”. While I am incapable of synthesizing the dozens of different explanations of this concept into one cohesive definition, in essence, when you move from one category like very superior, to superior, you have moved one deviation. When you move from very superior to high average, that would be two deviations. Movements of two deviations are deemed to be significant.

Yesterday’s example of an IQ score of 135, which was very superior, to an average processing speed score of 100, is a movement of three standard deviations. That could be quite significant, but of course is only one factor to be looked at in doing a full blown “assessment.”

Tomorrow: assessing premorbid IQ and other ability levels.

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