Alternative approaches to analyzing individual differences in the rate of early vocabulary development

This paper examines individual differences in the rate of early lexical development with a specific interest in gender differences. Twenty-six children were assessed monthly from either 8, 9, or 10 months of age through 14 months of age, using the MacArthur Communicative Development Inventory: Words and Gestures. Individual differences in developmental trajectories of vocabulary comprehension and production were explored using two analytic approaches. The first involved traditional parametric statistics, while the latter utilized classification procedures. Both techniques demonstrated that the lexical development of girls outpaced that of boys. The inductive approach also revealed the presence of distinctive "fast" and "slow" trajectories for both comprehension and production that were not exclusively segregated by gender. Cases exhibiting fast trajectories were predominantly girls, but several boys also followed this developmental pattern. The opposite pattern emerged for the slow trajectories. There was strong correspondence between production and comprehension, but a few cases clustered into the fast development group on one measure and the slow group on the other. The identification of these outliers may offer an important tool for exploring mechanisms of language development. Validation of the clustering results was based on the prospective prediction of an external criterion variable, namely, lexical development at 21 months, and by replication on an independent sample.