motivation, and order of assessment (Koegel et al., 1997). There are difficulties analyzing age equivalents across different tests because of lack of equality in intervals (Mervis and Robinson, 1999). Deviation IQ scores may not extend low enough for some children with autism, and low normative scores may be generated from inferences based on very few subjects. In the most extreme case, a young child tested with the Bayley Scales at 2 years and a Leiter Scale at 7 years might show an IQ score gain of over 30 points. This change might be accounted for by the change in test (i.e., its emphasis and structure), the skill of the examiner, familiarity with the testing situation, and practice on test measures—all important aspects of the measurement before response to an intervention can be interpreted.
Because researchers are generally expected to collect IQ scores as descriptive data for their samples, the shift to reporting IQ scores as outcome measures is a subtle one. For researchers to claim full “recovery,” measurement of a posttreatment IQ within the average range is crucial and easier to measure than the absence of autism-related deficits in social behavior or play. IQ scores, at least very broadly, can predict school success and academic achievement, though they are not intended to be used in isolation. Indeed, adaptive behavior may be a more robust predictor of some aspects of later outcomes (Lord and Schopler, 1989b; Sparrow, 1997). Furthermore, an IQ score is a composite measure that is not always easily dissected into consistent components. Because of the many sources for their variability and the lack of specific relationship between IQ scores and intervention methods, IQ scores on their own provide important information but are not sufficient measures of progress in response to treatment and certainly should not be used as the sole outcome measure.
Similar to findings with typically developing children, tests of intellectual ability yield more stable scores as children with autistic spectrum disorders become older and more varied areas of intellectual development can be evaluated. Although the process of assessment can be difficult (Sparrow, 1997), various studies have reported on the reliability and validity of appropriately obtained intelligence test scores (Lord and Schopler, 1989a). Clinicians should be aware that the larger the sampling of intellectual skills (i.e., comprehensiveness of the test or combination of tests), the higher will be the validity and accuracy of the estimate of intellectual functioning (Sparrow, 1997).
There are several important problems commonly encountered in the assessment of children with autism and related conditions. First, it is common to observe significant scatter, so that, in autism, verbal abilities