MacArthur-Bates Communicative Development Inventories (MB-CDIs)

The MacArthur-Bates CDIs ask a parent what their child currently understands and says. That approach is deliberate: a toddler will not perform on demand for a stranger, but the person who spends every day with them knows which words are there.

Three forms cover different stages. Words and Gestures, for 8 to 18 months, works through hundreds of early vocabulary items with separate columns for understood and said, then asks about communicative and symbolic gestures. Words and Sentences, for 16 to 30 months, records words the child produces, early grammar, and their three longest utterances. The CDI-III, for 30 to 37 months, is a single sheet covering expressive vocabulary and grammar. Results convert to percentiles.

Licensing Notes: Purchasing requires Qualification Level B. Forms are consumables sold in packs. Reproduction requires publisher permission. Source:

Learn More: Three things worth knowing. Short forms exist covering vocabulary only, for when full administration is too long — the full forms take 20 to 30 minutes or more for a parent to complete. Web-CDI offers a computerized adaptive version for expressive vocabulary. And the CDIs are explicitly usable with older children who have developmental delays, matched to developmental rather than chronological age, which is how they appear in clinical trials for genetic syndromes.