On the estimation of premorbid intellectual functioning: validation of Nelson & McKenna's formula, and some new normative data.

Abstract

The Schonell Graded Word Reading Test (SGWRT), the Wechsler Adult Intelligence Scale (WAIS) and Raven's Standard Progressive Matrices (RSPM) were administered to 78 healthy subjects aged between 18 and 84 years. Regression equations were derived for predicting Verbal, Performance and Full-Scale IQs measured with the WAIS on the basis of a subject's score on the SGWRT, and for predicting total score on the RSPM on the basis of age and score on the SGWRT. Discrepancies between obtained and predicted scores were approximately normally distributed; z scores are presented for these discrepancies for each of the predicted variables. The equation derived for predicting Full-Scale IQ was in good agreement with the equation derived by Nelson & McKennais sample. 'Significant' discrepancies (z score greater than 1.645) between obtained and predicted Full-Scale IQs were found in approximately half of a group of patients with confirmed organic cortical damage. These results constitute a validation of Nelson & McKenna's formula for estimating premorbid IQ, and in addition provide new normative data which may be used for estimating premorbid Verbal or Performance IQ, or score on the RSPM.