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professional statisticians and psychometricians

Some children, indicated by blue lines in Figure 70.2, showed a steep rate of expressive vocabulary development, with expressive vocabularies of nearly 700 words at the last time point. Other children, indicated by green lines in Figure 70.2, showed a steady increase in vocabulary development, with expressive vocabularies of 400 to a bit more than 600 words at the last time point. Still other children, indicated by gold lines in Figure 70.2, showed a slow increase in vocabulary development. The remaining children, indicated by red lines, showed a flat rate of vocabulary development, with little change in the number of words they produced over the 2-year period.

The four color-coded clusters illustrated in Figure 70.2 were produced via statistical cluster analysis based on the trajectory of the individual children’s vocabulary development. However, the average age of the children within each of the four clusters did not differ; neither did their level of cognitive development nor their degree of autistic traits. Most markedly, all children were undergoing the same therapy. Therefore, these data show just how variable the course of language development can be for autistic children, even when the children are all receiving 15–20 h per week of early behavioral intervention, including speech-language therapy.