Relationship between the Vineland Adaptive Behaviour Scales and the Wechsler Adult Intelligence Scale IV in adults with intellectual disabilities.

BACKGROUND: Diagnoses of intellectual disability (ID) are based on three criteria: significant impairments in intellectual functioning, concurrent deficits in adaptive behaviour, and both being acquired in the developmental period. Adaptive behaviour was formally incorporated into the diagnosis in 1959; the rationale being that IQ and adaptive behaviour were different constructs that independently contribute to the diagnosis. However, there are limited data on the relationship between IQ and adaptive behaviour and especially so for adults who have ID. The aim of this study was to investigate this relationship on two widely used assessment tools: the Wechsler Adult Intelligence Scale - fourth edition (WAIS-IV) and the Vineland Adaptive Behaviour Scales (VABS).

METHOD: Data were extracted from the case files of 147 adults who had a formal diagnosis of ID based on the WAIS-IV and VABS. Internal consistency was computed and compared to general population data. Correlations between the WAIS-IV and the VABS were computed.

RESULTS: Internal consistencies for the tests when used with adults who have ID were generally good. The correlations between the WAIS-IV and VABS composite and subdomain scores were all low and similar to those reported for the general population.

CONCLUSION: These results suggest that the WAIS-IV and VABS are measuring different constructs that contribute to the diagnosis of ID. Unfortunately, by the time such data have been collected, tests are being revised. The Vineland has now been revised and is in its third edition, but no data on the relationship between IQ and the VABS were collected during test development. The WAIS-V is now in development, and so it is recommended that such work be incorporated into this process or shortly thereafter to ensure that the tools continue to access independent constructs.