Model-Based Person Fit Statistics Applied to the Wechsler Adult Intelligence Scale IV.

An important task in clinical neuropsychology is to evaluate whether scores obtained on a test battery, such as the Wechsler Adult Intelligence Scale Fourth Edition (WAIS-IV), can be considered "credible" or "valid" for a particular patient. Such evaluations are typically made based on responses to performance validity tests (PVTs). As a complement to PVTs, we propose that WAIS-IV profiles also be evaluated using a residual-based M-distance ( ) person fit statistic. Large values flag profiles that are inconsistent with the factor analytic model underlying the interpretation of test scores. We first established a well-fitting model with four correlated factors for 10 core WAIS-IV subtests derived from the standardization sample. Based on this model, we then performed a Monte Carlo simulation to evaluate whether a hypothesized sampling distribution for was accurate and whether was computable, under different degrees of missing subtest scores. We found that when the number of subtests administered was less than 8, could not be computed around 25% of the time. When computable, conformed to a distribution with degrees of freedom equal to the number of tests minus the number of factors. Demonstration of the index in a large sample of clinical cases was also provided. Findings highlight the potential utility of the index as an adjunct to PVTs, offering clinicians an additional method to evaluate WAIS-IV test profiles and improve the accuracy of neuropsychological evaluations.