Do children with Williams syndrome really have good vocabulary knowledge? Methods for comparing cognitive and linguistic abilities in developmental disorders

Abstract

The comparison of cognitive and linguistic skills in individuals with developmental disorders is fraught with methodological and psychometric difficulties. In this paper, we illustrate some of these issues by comparing the receptive vocabulary knowledge and non‐verbal reasoning abilities of 41 children with Williams syndrome, a genetic disorder in which language abilities are often claimed to be relatively strong. Data from this group were compared with data from typically developing children, children with Down syndrome, and children with non‐specific learning difficulties using a number of approaches including comparison of age‐equivalent scores, matching, analysis of covariance, and regression‐based standardization. Across these analyses children with Williams syndrome consistently demonstrated relatively good receptive vocabulary knowledge, although this effect appeared strongest in the oldest children.

• Williams syndrome
• receptive vocabulary
• cognitive skills
• methodology

Notes

1. If an appropriate transformation cannot be found, an alternative approach is to use polynomial regression in which a quadratic term (the square of the predictor) or a cubic term is included in the regression. However, while this may significantly improve the fit of the model relative to the linear model, it is worth bearing in mind that psychometric functions tend to be monotonic and so rarely approximate a quadratic or cubic function. For example, if children reach ceiling by a particular age, a quadratic function would predict that scores would start going down again in older children.