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It is functional impairment that should be the true target of our evaluations. From my perspective as a forensic neuropsychological evaluator, my primary goal is to collect data allowing me to provide an explanation of an individual’s daily functional impairments. My review of the medical records, observation and psychological testing are all directed at generating data to provide explanations for the liabilities or weaknesses as well as strengths or assets an individual brings to every day living so as to better appreciate their lives. Traditionally the evaluation of symptom presentation and severity has formed the foundation for making diagnostic and prognostic determinations. However, there is an emerging research literature demonstrating that symptom count and to some extent even symptom severity are not particularly good predictors of daily functional impairment. Thus, a diagnosis of a specific condition does not automatically equate with a particular level of functional limitation in every day life.

In our new textbook, Assessing Impairment: From Theory to Practice (Springer, 2009), Jack Naglieri and I address these issues indepth. Until very recently functional impairment was not a major focus in medical and clinical diagnosis or treatment for either physical or mental health problems. This new interest has been sparked by an emerging body of literature beyond the forensic setting demonstrating that symptoms and functional impairment need to be considered separately in making diagnostic decisions, evaluating response to treatment, determining maximum medical improvement and future prognosis. In fact, there are a wide range of findings suggesting that the lives of individuals who do not meet specific symptom criteria may be just as impaired and disrupted as the lives of individuals who meet various criteria. Further, many who may meet symptom count for a specific diagnosis may not be significantly impaired. Though addressing impairment is a requirement for most medical and mental health diagnoses, it still remains the case that it is often an after thought for clinical as well as forensic diagnosticians. The manner in which symptoms and impairments contribute to disability, handicap and deficits in adaptive functioning are just being defined and understood. It is still the case that there is no agreement on even the simplest of nomenclature issues about impairment. The term impairment is used differently by medical, mental health and educational professionals.

To illustrate the complexities of evaluating impairment, consider two individuals whom I recently evaluated. As part of the evaluation I assessed their self-reports of depressive symptoms. Both reported a moderate level of depressive symptoms. Yet, when I asked about their daily lives, two very different pictures emerged. One individual was minimally engaged in activities of every day living, sitting at home, not working and isolated from others. The other individual was working, made an effort to participate with others and to those around them did not appear to be significantly depressed. In a number of recent studies when impairment levels are set as part of the diagnostic process, the percentage of individuals meeting those diagnoses decreases significantly.

The foundation of a neuropsychological assessment has traditionally been the administration of standardized tests. It has often been argued that such tests are more reliable and valid than symptom reports. Yet an additional emerging body of literature is demonstrating that neuropsychological tests may share a similar weakness as diagnoses and symptom complaints in predicting functional impairment.

In a recent study published in the Archives of Clinical Neuropsychology, Laurence M. Binder, Grant Iverson and Bryan Brooks reviewed normative studies of neuropsychological variability in test performance by healthy adults on a broad battery of instruments. Surprisingly, test score scatter or the difference between high and low scores demonstrated that these very normal individuals often had large discrepancies between best and worst scores. Yet traditionally this type of discrepancy has been used to confirm diagnoses and to predict or explain impairment. When abnormality was defined as a score more than one standard deviation below the average on test batteries with at least twenty different measures the great majority of normal individuals in this large study had one or more abnormalities. Restricting the sample to participants with above average intelligence or high educational levels and using more conservative definitions of abnormality such as two standard deviations below the mean (2^nd percentile or lower) did not eliminate the presence of abnormal scores. These authors concluded that abnormal performance on some proportion of neuropsychological tests in a battery is psychometrically normal. They further concluded that abnormalities do not necessarily signify the presence of acquired brain dysfunction because low scores and large intra-individual variability are often characteristic of healthy, unaffected adults. Abnormalities more than one standard deviation below the mean occurred in one out of four tests in more than one sixth of the normal sample.

Cautions against over-interpreting isolated low or high scores for that matter have been present in the scientific literature for over twenty years. The scientific literature has demonstrated that there are large discrepancies commonly between IQ scores, various indices of IQ and subtest scores. It is also the case that there is neither published research nor test manuals providing information regarding the base rates of large discrepancy scores when all discrepancies are considered simultaneously as is done in clinical practice.

If it is the case that symptom count or severity, diagnosis and neuropsychological test scores are not overly robust predictors of daily functional impairment then where does that leave us? It would appear that to truly appreciate the lives of any individual following a traumatic event one must carefully examine their pre and post-accident functional behavior in addition to counting symptoms, administering neuropsychological tests and making diagnoses. At this point the majority of the variance in explaining functional differences between injured individuals remains unidentified. However, this does not mean that symptoms, neuropsychological tests and diagnoses are unimportant. Rather, these emerging lines of research help us once again appreciate the complexity of human function and behavior and the continued gap between the needs of the legal system and the science of forensic assessment.