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The lexical diversity of children with specific language impairment (SLI) (ages 3 years 7 months to 7 years 3 months) was compared to that of normally developing same-age peers and younger normally developing children matched according to mean length of utterance in words (MLUw). Lexical diversity was calculated from spontaneous speech samples using D, a measure that uses repeated calculations of type-token ratio (TTR) to estimate how TTR changes as the speech samples increase in size. When D computations were based on 250-word samples, developmental differences were apparent. For both children with SLI and typically developing children, older subgroups showed higher D scores than younger subgroups, and subgroups with higher MLUws showed higher D scores than subgroups with lower MLUws. Children with SLI did not differ from same-age peers. At lower MLUw levels, children with SU showed higher D scores than younger typically developing children matched for MLUw. The developmental sensitivity of D notwithstanding, comparisons using 100-utterance samples, in which the number of lexical tokens varied as a function of the children's MLUws, and comparisons between 250- and 500-word samples revealed the possible influence of sample size on this measure. However, analysis of the effect sizes using smaller and larger samples revealed that D is not affected by sample size to the degree seen for more traditional measures of lexical diversity.
KEY WORDS: specific language impairment, vocabulary, lexical diversity, lexicon, language sample
A recent focus of research has been the development of a profile of the language abilities of children with specific language impairment (SLI). Attempts have been made to assess all areas of these children's language, including phonology, morphosyntax, pragmatics, and vocabulary development. Often these children's grammatical abilities are considered to be weaker than their lexical abilities, although even the latter appear to be below age-level expectations (see Leonard, 1998, for a recent review). This interpretation has been based on comparisons using formal tasks of both production and comprehension and on comparisons using spontaneous speech measures.
Most of the measures used to assess children's vocabulary abilities in spontaneous speech are estimates of their lexical diversity. These measures are intended to reflect the variety and specificity of words that a child chooses to use and, less directly, the breadth of topics that can...