Evaluation of two methods for estimating Premorbid Intelligence on the WAIS-III in a clinical sample.

Two methods of predicting premorbid WAIS-III Full Scale IQ, namely a demographic measure (DP IQ) and the Oklahoma Premorbid Intelligence Estimate-3 (OPIE-3), were assessed in three traumatic brain injury (TBI) groups of varying injury severity and normal controls. On the DP IQ, average scores were in keeping with the general population mean across all groups but range was severely restricted. OPIE-3 IQ scores were not subject to severe range restriction and were comparable with the general population mean in the control and least severely brain injured group. They were significantly lower relative to controls in the severe and extremely severe TBI groups, with differences averaging around 7 and 10 points, respectively. OPIE-3 (Best) proved particularly robust in more severely injured subjects. Effect sizes for the combined TBI sample relative to controls were greatest for OPIE-3 measures that included two non-verbal subtests and were least for that involving vocabulary only. Implications of the findings for clinical practice and further research are discussed.