Expressive vocabulary data gathered during a systematic diary study
one male child's early language development are compared to data that
would have resulted from longitudinal administration of the MacArthur
Communicative Development Inventories spoken vocabulary checklist
(CDI). Comparisons are made for (1) the number of words at monthly
intervals (9;10.15 to 2;0.15), (2) proportion of words by lexical class
noun, predicate, closed class, ‘other’), (3) growth curves.
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 differed as a function of
lexical class. Finally, despite the differences in vocabulary size, logistic
curves proved to be the best fitting model to characterize vocabulary
development as measured by both the diary study and the CDI.
Implications for the longitudinal use of the CDI are discussed.