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Estimating Premorbid Intelligence
from history, IQ testing and neuropsychological testing > Assessment > IQ Scores > IQ Classification > IQ Estimation


Estimating Premorbid Intelligence

Determining Change in Cognition Following Brain Injury

Richard A. Lanham, Jr., PhD and Thomas Misukanis, PhD
Brain Injury Source, Pediatric Issue, Volume 3, No. 3, Summer 1999


Neuropsychology, a specialty discipline within clinical psychology, is devoted to the study of brain-behavior relationships, the assessment of neurobehavioral impairment and its effects on functioning and methods of treatment. In cases of traumatic brain injury (TBI), where an individual is suspected of having experienced a disruption in cognitive functioning, a referral for neuropsychological evaluation often will be made in order to determine the nature and extent of any cognitive changes that may have occurred. Such information is valuable for tracking the course of recovery, guiding treatment and compensatory strategies and assisting in discharge planning. This article discusses an essential part of the neuropsychological evaluation: the methods and associated problems of estimating premorbid level of functioning against which post-injury test results are compared in an effort to establish the resulting cognitive impairments of the TBI.

Methods of Evaluation

In evaluating an individual with a suspected brain injury, the neuropsychologist makes use of a variety of psychometric tests. These tests either have been specifically developed to assess for cerebral impairment (e.g. Halstead-Reitan Neuropsychological Test Battery, HRB) or have been routinely employed in such a manner as to allow for the determination of deficient cognitive functioning (e.g. Wechsler Adult Intelligence Scale, now in its third edition). Without some standard of comparison, however, test results in and of themselves would be little more than meaningless numbers. In general, two standards of comparison are employed: 1) a normative or extra-individual standard of comparison where an individual’s performance is compared to that of others with whom some important attributes are shared and 2) a self-standard or intra-individual standard of comparison, where an individual’s performance is compared to pre-injury expectations regarding that individual’s level of ability.

Normative Standard

In employing the normative standard of comparison, an individual’s test scores are compared to the performance of a large group of people of interest, permitting score adjustment for such factors as age, sex or level of education. These comparisons give the neuropsychologist information about the individual’s current cognitive functioning relative to a particular normative group. For example, knowing that an individual performed at the 2nd percentile – thus performing worse than 98% of the sample against which the score is being compared – may be a strong indicator of cerebral impairment. The adequacy of the comparison between the individual being evaluated and the normative group being employed is crucial and places limitations on the type of conclusions that may be drawn and the certainty one may have in them. For instance, knowing that the above 2nd percentile performance occurred on the entrance examination for Mensa and that the "normative" comparison group was all current and past Mensa members, might alter the interpretation of the score as a reflection of brain impairment.


The second standard of comparison, the self-standard, compares an individual’s post-injury performance with some measure of pre-injury functioning. Ideally, one would directly compare post-injury test results against results obtained from those very same tests administered some time prior to the injury. While this situation potentially does exist for military personnel participating in the Defense and Veterans Head Injury Program (DVHIP), it is a rare occurrence for pre-injury testing to exist in nonmilitary settings. Even for ex-military persons, access to the Armed Services Vocational Aptitude Battery (ASVAB), the current version of the military classification test taken by all noncommissioned personnel upon entry into service, is restricted to protect the test’s integrity, much like the various civilian entrance examinations (e.g., SAT, MCAT, GRE). While the DVHIP is able to administer the ASVAB, with the permission of the military, six months post-injury as part of its ongoing clinical and research program (e.g., Grafman et al., 1988), in general clinical practice it would not be available for re-administration.

Most commonly, the self-standard of comparison involves estimating an individual’s pre-injury level of ability. Unfortunately, this standard is frequently applied incorrectly or neglected entirely. Lezak (1995) considers it critical that the neuropsychologist obtains or makes an educated estimate about the individual’s cognitive functioning prior to his/her injury. Similarly, Matarazzo (1990) wrote that too often the psychological practitioner relies solely on data gathered from test results and fails to consider and integrate other important sources of data. Using only the normative standard of comparison can result in erroneous conclusions.

For example, it is not unusual to assess an individual with a superior pre-injury level of cognitive ability whose post-injury scores now fall within the average range. While such results may be considered "within normal limits" from a statistical standpoint, these average scores clearly represent a loss in functioning for that individual. The impact of this change in functioning will ultimately depend on a variety of factors, including the interaction of the person’s constellation of cognitive strengths and weaknesses with the variety of occupational and environmental demands present in his/her life.

To illustrate, "average" ability in the area of information processing speed may be adequate for a person employed in a position that is repetitive or where rapid appraisal, integration and utilization of new information is not required. It may not be adequate, however, for someone working as an air traffic controller or a trauma surgeon. Because of the limitations evident in utilizing a strictly normative approach, it is important that the neuropsychologist also gather extra-test information in order to make an estimation of the individual’s premorbid level of cognitive functioning against which to compare current test performance.

Methods of Premorbid Estimation

Vanderploeg (1994) divides the methods of estimating premorbid functioning into four basic approaches: one based on historical data reflecting past achievements, two based on an individual’s post-injury test performance and one based upon demographic information.

The first of these methods involves information collected through clinical interview and review of available records. Typically, the clinical interview will cover such areas as educational achievements and work accomplishments; marriage, family and social functioning; and medical and psychological histories. While this information is often based upon self-report, which should be corroborated, it provides the neuropsychologist with the individual’s perspective on how his/her life has changed as a result of the injury. Given that it is not unusual for persons with brain injury to lack awareness of and insight into their cognitive and psychological difficulties, it is extremely beneficial and frequently crucial to speak with others who interact with the individual on a regular basis. Family members, co-workers and personal friends can provide valuable supplemental and corroborative information on how the individual’s functioning has changed since the TBI, as well as provide information on the individual’s level of awareness and insight.

Historical records, such as school transcripts, military C-file and employment records, particularly job performance evaluations, are helpful in establishing the capabilities the individual had prior to the injury (Kay, 1992; Matarazzo, 1990; McCaffrey et al., 1993; Sbordone, 1991). Academic records, such as school transcripts and results from standardized achievement tests, are another source of valuable information in establishing an individual’s premorbid level of cognitive ability. The patterns of performance present within an individual’s grades may reflect prior cognitive strengths or weaknesses. Caution should be exercised when reviewing school records. While good grades and high scores on academic tests are generally associated with average to above average cognitive ability, poor performance on these same measures does not necessarily reflect lower premorbid functioning. Other factors such as motivation, attitudes towards academic pursuits, the lack of a stable supportive home environment and other life events can result in actual academic achievement being lower than intellectual capability.

In reviewing records pertaining to work performance, the neuropsychologist attempts to determine the cognitive abilities that were required for the successful performance of a particular person. If the records indicate a history of successful work performance, then this information provides some indication of the types of capabilities the individual may have possessed. Military records often can provide similar job skills related information. Ideally, all of the collected historical data would converge to present a unified picture of how the individual functioned prior to their TBI. Some of the difficulties in estimating premorbid intellectual ability from historical data are the myriad of factors, other than intellectual ability, that influence an individual’s life achievements and the more subjective nature of the information being collected. Standardized achievement test results can assist greatly in making the above estimation process more objectively based. Unfortunately, obtaining such information is more the exception rather than the rule.

The remaining three methods of estimation attempt to address the need for a more objectively based procedure. Two of these methods make use of current, post-injury neuropsychological test results to estimate an individual’s prior level of cognitive functioning (Ciplotti & Warrington, 1995; Lezak, 1995). The first of these is referred to as the "hold" method of estimation. This approach is named from the deterioration quotient devised by Wechsler (1958). In this approach, the neuropsychologist makes use of test scores which are resistant to brain injury and frequently well preserved (thus the name "hold" tests) in order to estimate prior level of functioning. Abilities such as vocabulary (Yates, 1956) and reading (Nelson, 1978) are examples of cognitive functions, shown to be more resilient to disrupting effects of brain injury, that are typically used to predict premorbid intellectual ability.

As Ciplotti and Warrington (1995) point out, however, the use of these abilities to predict prior functioning may be problematic in cases where the individual has a long-standing learning problem such as dyslexia or where there exists damage to the parts of the brain directly involved in the performance of those functions. For example, performance on tests of reading or vocabulary would likely underestimate the prior ability of an individual who sustained left temporal lobe damage resulting in aphasia. In this instance, the estimation of the individual’s premorbid abilities would be skewed by a deficiency that was result of the injury. It also should be pointed out that implicit in the "hold" test method is the assumption that a single level of performance accurately reflects an individual’s pre-injury state. Research has shown that this assumption may not necessarily be true, at least not for segments of the population at the extremes of intellectual functioning (Matarazzo et al., 1988). While normative data do indicate significant variability in performance (i.e. strengths and weaknesses) across a range of cognitive abilities, research also has shown that a high degree of variability in performance may reflect the presence of brain impairment (Black, 1974; Simpson & Vega, 1971).

The second of these methods utilizing current test performance to estimate premorbid cognitive ability, the "best performance method" is an offshoot of the "hold" test approach. Rather than using tests which are pre-determined to be resistant to brain injury, the entire testing record is reviewed and the highest scores, in combination with historical data, are used to make predictions concerning the individual’s prior level of functioning (Lezak, 1995). While at times providing useful information, caution should be exercised using this approach in that I often results in an overestimation of pre-injury level of cognitive ability (Vanderploeg, 1994).

The last of the four methods employs an actuarial approach to the estimation of pre-injury ability, utilizing known relationships between demographic variables and performance on intelligence testing. Variables such as age, education and occupation are put into one of a variety of regression formulas that exist in the research literature to yield a predicted "IQ" score. This score is then compared to the individual’s actual test performance. In the event that current performance levels are significantly below predicted performance, such discrepancies may be suggestive of impaired cognitive functioning. There are numerous studies utilizing the actuarial approach to predict IQ scores (Barona, Reynolds & Chastain, 1984; Eppinger et al., 1987; Karzmark et al., 1985; Wilson, Rosenbaum & Brown, 1979) and performance on other neuropsychological measures (Karzmark et al., 1984). The actuarial approach has its limitations, particularly when the individual’s premorbid functioning lies at the extremes of ability where there exists much less accuracy in prediction (Ciplotti & Warrington, 1995; Sweet, Moberg & Tovian, 1990).


Clearly, the preferred method of determining alteration in functioning would be to reassess the individual on a battery of standardized tests that encompass a sufficient range of cognitive abilities administered prior to the injury and then examine any differences in performance. Unfortunately, this possibility rarely exists. As a result, a number of approaches have been developed over the years which neuropsychologists use to estimate a premorbid level of functioning against which to compare test results. It should be emphasized that none of these methods are without criticism. Consequently, it is ill advised to rely too heavily on any single method. While the above methods have their limitations, when used in combination they can provide fairly accurate estimations of premorbid functioning that allow the neuropsychologist to better determine what changes in cognitive function may have occurred as a result of the individual’s brain injury.

In conclusion, the following example is given to illustrate the combined use of the above methods. An individual who was involved in a motor vehicle crash is referred for a neuropsychological evaluation to determine the nature and extent of any cognitive impairment. Prior to the injury, this person had graduated college with excellent grades and had established a successful work history as a financial broker. Since the accident, she reports experiencing difficulties with forgetfulness, inattention, distractibility and irritability that are detrimentally affecting her work performance. The results of all the neuropsychological testing reveal that the vast majority of scores fall around the upper end of the average to high average range (75th percentile). None of the scores fall into what is commonly considered the impaired range. If a strictly normative interpretive approach were applied to the data, the results would be considered reflective of normal cognitive functioning. Upon closer inspection, however, the neuropsychologist notes that tests of attention and vigilance, speed of information processing, new verbal learning and executive functioning generally fall in the low average to the low end of the average range (15th to 30th percentiles). Further, school grades, standardized test scores such as the Scholastic Aptitude Test (SAT) and several actuarial methods of premorbid IQ estimation all suggest premorbid functioning falling within the high average to superior range of ability. Utilizing this information as the standard of comparison suggests that this individual is experiencing diminished functioning in attention, memory, processing speed and executive functioning relative to her other cognitive abilities – a pattern consistent with the known sequelae of TBI.

Estimating an individual’s pre-injury level of cognitive functioning is a complex but essential part of the neuropsychological evaluation. As in the above example, going from a high average/superior level of ability to an average level of ability reflects a loss of cognitive function which can have dire effects for an individual’s wage earning potential and his/her overall quality of life. Proper estimation of premorbid functioning ensures more accurate diagnosis of cognitive changes, which ultimately increases the chances of individuals with brain injury getting the services they need.

Richard A. Lanham, Jr., PhD, is the clinical and research neuropsychologist for the Traumatic Brain Injury Clinic at the Minneapolis VA Medical Center and a co-investigator in the defense and Veterans Head Injury Program (DVHIP). He holds a clinical assistant professor appointment with the Department of Psychology at the University of Minnesota and is in private practice with Minnesota Clinical and Neuropsychological Associates, P.A., Bloomington, MN.

Thomas Misukanis, PhD, is a clinical neuropsychologist in private practice with Minnesota Clinical and Neuropsychological Associates, P.A., Bloomington, MN. He also serves as an adjunct professor in the Department of Psychology at the University of Saint Thomas in St. Paul, MN.


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