Error in the estimation of intellectual ability in the low range using the WISC-IV and WAIS-III

The error, both chance and systematic, in the measure of true intellectual ability in the low IQ range is quantified and combined to find an overall confidence interval. The chance error was due to: lack of stability, scorer error and lack of internal consistency. The systematic error was due to: the Flynn effect, a floor effect and that error apparent from the lack of agreement between the WISC-IV and WAIS-III. For low Full Scale IQs the WAIS-III can only be considered accurate to within 18 points above the measured IQ and 28 points below, and the WISC-IV to 16 points below the measured IQ and 25 points above it. The implications for the diagnosis of intellectual disability are considered.