Other early researchers agreed on this latter point, including Rutter (1966b), who stated that many commonly used measures of intelligence are “usually unsuitable” (p. 91) for children with autism given their reliance on verbal subtests. However, in another chapter in the same book, Rutter (1966a) reported that 71% of children with autism in his sample had MR, a statistic obtained in a study that utilized the Wechsler Intelligence Scale for Children, a commonly used measure of intelligence with many verbal subtests (see Rutter & Lockyer, 1967, which is a report of the same study). Thus, despite the recognition that certain methods of intelligence determination were inappropriate for children with autism, they were still used; and data from these studies were then cited by subsequent authors. (pp. 72-73, emphasis in original)
Of course, that's only one of many types of bias highlighted in Edelson's paper. It's a pretty good read.