Modeling Longitudinal Trajectories of Word Production With the CDI

The MacArthur-Bates Communicative Development Inventories (CDI) are widely used, parent-report instruments of language acquisition. Here, we focus on the word-inventory sections of the instruments, and show two different approaches to modeling CDI data, based on real-world needs. First, we show that Words & Gestures data collected out-of-age-normed-range can be robustly adjusted to Words & Sentences scores. Second, we demonstrate a novel application of Gompertz growth curves to longitudinal CDI data, especially when the same timepoints were not collected between individuals (i.e., an accelerated longitudinal design). Gompertz curves provide a "growth rate" or an "age at maximum growth" parameter that can be used to summarize vocabulary development. We compare these parameters between healthy developing children in two longitudinal cohorts, as well as a cohort of children with a diagnosis of speech disorder, language disorder, or learning or reading disability, who we show to have lower growth rates. We hope these analyses and results inform future work on longitudinal CDI analyses.