Comparing productive vocabulary measures from the CDI and a systematic diary study

Expressive vocabulary data gathered during a systematic diary study of one male child's early language development are compared to data that would hav e resulted from longitudinal administration of the MacArthur Communicative Development Inventories spoken vocabulary checklist (CDI). Comparisons are made for (I) the number of words at monthly intervals (9; 10. 15 to 2;0.15) , (2) proportion of words by lexical class (i.e. noun, predicate, closed cl ass,'other'), (3) growth curves. The CDI underestimates the number of words in the diary study, with the underestimation increasing as vocabulary size increases. The proportion of diary study words appearing on the CDI differ ed as a function of lexical class. Finally, despite the differences in voca bulary size, logistic curves proved to be the best fitting model to charact erize vocabulary development as measured by both the diary study and the CD I. Implications for the longitudinal use of the CDI are discussed.