Best Performance Method in Neuropsychological Assessment


Posted on 3rd April 2008 by Gordon Johnson in Uncategorized

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There are two fundamental premises upon which the statistical application of the science of neuropsychology is based: The first is that a determination can be made of what a given individual’s premorbid abilities were. The second is that an individual is giving consistent best effort throughout the test battery. Neither assumption works perfectly, but the extent to which these two assumptions work well enough, will determine whether legitimate statistical based diagnostic conclusions can be incorporated into the assessment. In today’s blog, we will discuss the Premorbid Ability assumption. In tomorrow’s, the consistent best effort issue.

Premorbid ability. By premorbid ability, we mean a given individuals abilities prior to the onset of the accident or disease process. If no assumptions can be made about premorbid ability, no diagnosis about “cause” can be made by a neuropsychologist. All they are capable of saying is that a given individual has certain weaknesses and disabilities, but no definitive diagnosis can be made. Thus, some method of assessing premorbid ability is essential.

Most neuropsychologists don’t look at enough information in determining pre-morbid IQ. They base far too much of their assessment with respect to pre-morbid ability on the test battery itself. In our earlier example of the person with the IQ of 135 post the accident, that is less of a problem. Clearly a person who has a post-morbid IQ of 135, was very superior before the onset. But most cases are not so clear cut. A previously brilliant person may not continue to have a very superior IQ after the accident. If certain deficits bring the person down into an IQ range of 110 or so, we would likely need to look for other evidence to determine IQ.

One way is by looking at the areas where they still have strengths. If their average scores are in the 130 or above area, and there are a few scores in areas we might suspect would be effected by the injury, then it might be easy to say this person was very superior before. But again, that is the easy pattern to spot. Most profiles are not that obvious.

Another method is to look at certain subtest scores, where it is believed that a given ability is unlikely to be substantially effected by the given injury. Reading scores are often thought to be an ability that is rarely changed significantly by a mild or moderate injury. Thus, a neuropsychologist might say that a person with a “very superior” reading score and a much lower current IQ, had pathological deficits, based on the retained ability to read at a high level.

All of these methods work far better with someone with a very high IQ. When you are dealing with people in the average range, IQ’s of 90-110, it becomes much more difficult to make such assumptions about premorbid IQ from subtest scores.

Another method is to assume IQ based on a assessment of that person’s educational level. So a person with a college degree would be assumed to have a higher IQ than someone without. The obvious flaw in such logic, that some brilliant people don’t go to college, isn’t even the most significant problem. The significant problem is that it groups all college graduates together. Ask anyone who went to college. Not all of their classmates were of equal intelligence and ability.

Another method, one I believe to be considerably better than the first two, is called the ‘best performance method.” The best performance method is based upon the assumption that a person’s highest areas of achievement are the best indicators of premorbid ability. If these areas of highest achievement are in contrast to significantly lower subtests scores that may point to pathology.

Of course, there is considerable disagreement as to how to apply the “best performance method.” Many neuropsychologists dismiss it as they interpret this method as applying only to the best performance on individual tests, within the full battery of tests. That would mean if the person got 99% in arithmetic or vocabulary, that would mean that such person is in the 99%. It is easy to poke holes in a restricted use of the “best performance method” because we all have normal variances in what we are good at.

However, another interpretation of the best performance method is that it makes a full assessment – not just of the scores on the given battery of tests – but also the person’s real world performances or achievements. For example, if a person has graduated from medical school, one assumes that they are very near the top of the pre-morbid ability level. Likewise, if they have risen to the top of any profession, they would be assumed to be near the top.

In my opinion, the overall preferred method, which of course is harder to reduce to statistical probabilities, is to use of the real world “best performance method”. Such method considersall factors, school records, work performance records, areas of retained strength on the test. If someone got a math score of 700 and a verbal score of 700 on the SAT when applying to college, they clearly were way above average at that time. If they went on to graduate from a competitive law school or medical school, we must almost assume that they were at the superior or likely very superior level.

If the scores were good, but not great, if they graduated from college with more than a B average and went on to have a successful career, we can’t assume they were only average. Whether they are high average or superior is open to interpretation but that is what professionals are supposed to do: make subjective interpretations of complex multi-faceted variables, to reach conclusions.

Who a person was before injury is far more complex than how well they do now on a reading score. Only if neuropsychologists look at not the basic outline of a person’s premorbid life, but level of achievement within that life, will neuropsychology be able to identify the true areas of acquired deficits and disability.

Tomorrow the concept of “consistent best effort.”

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