The effect of selection criteria on outcome studies of children with pervasive developmental disorders (PDD).

The objective of this paper is to see whether regression to the mean biases the differences on outcome observed between children with Asperger syndrome and autism. Children with pervasive developmental disorders (PDD) and normal nonverbal IQ were divided into those with Asperger syndrome and autism. It was hypothesized that IQ in the autistic children would fall at the 2-year outcome assessments since the autistic group represented a subgroup selected on the basis of extreme scores on a single measure. In fact, the Asperger syndrome group experienced a significant drop in nonverbal IQ contrary to expectation. Serial measurements of nonverbal IQ at inception indicated that this drop was probably a function of high initial nonverbal IQ scores that fall over time due to increasing complexity of problem-solving tests. The results are discussed in relation to outcome studies of high functioning children with PDD.