The concurrent and predictive validity of the Dutch version of the Communicative Development Inventory in children with Down Syndrome for the assessment of expressive vocabulary in verbal and signed modalities

• The assessment of expressive vocabulary in Down Syndrome should incorporate signs.
• The N-CDI can be adapted to also include parental report on signed vocabulary.
• The adapted N-CDI is a valid measure of expressive vocabulary in children with DS.

The expressive vocabulary of children with Down Syndrome (DS) is generally measured with parental reports, such as the Communicative Development Inventory (CDI), given that standardized tests for assessing vocabulary levels may be too difficult for most young children with DS. The CDI provides important insight into the parents’ perception of their child’s vocabulary development. The CDI has proven to be a valid measurement of expressive vocabulary, spoken and gestural, in typical and atypical populations. The validity in children with DS is not well established and signed vocabulary is often not included. This longitudinal study examined the concurrent and predictive validity of the Dutch version of the CDI (N-CDI) in children with DS between 2;0 and 7;6 years old to assess spoken and signed vocabulary. N-CDI scores were assessed on strength of association with mental age, an expressive vocabulary test and spontaneous language analyses in a play setting with parents at T1 and T2 (1.5 years later), and a therapy setting with speech language pathologists at T1. The results of the present study show that the N-CDI is a valuable and valid measurement of expressive vocabulary in children with DS. Strengths and weaknesses of several assessment methods for expressive vocabulary are discussed.