Estimation of Premorbid Intelligence: A Review of Recent Developments

Abstract

Whether for clinical, medico-legal or research purposes, the detection and quantification of intellectual impairment in the individual case is problematic. As there are substantial individual differences in intellectual ability in the general population, simply comparing a client’s IQ test performance with the relevant test norms will be of little value. It is therefore necessary to compare current performance against an individualised comparison standard (Lezak, 1983). This standard is easily determined if test results are available from a period preceding the point at which neurological disorder or behavioural change raised the suspicion of impairment. However, such information is rarely available. It is therefore necessary to estimate an individual’s ‘expected’ or ‘premorbid’ level of performance.