Purpose Composite measures of children's use of tense and agreement morphology differ in their emphasis on accuracy, diversity, or productivity, yet little is known about how these different measures change over time. An understanding of these differences is especially important for the study of children with specific language impairment, given these children's extraordinary difficulty with this aspect of grammar.

Method We computed 3 types of composite scores from spontaneous speech samples obtained from 17 preschoolers with specific language impairment before, during, and after their participation in a language intervention study. These measures were the Finite Verb Morphology Composite (a measure of accuracy), the Tense Marker Total (a measure of diversity), and the Productivity Score (a measure of productivity).

Results The 3 measures differed in their growth trajectories. Sample size did not alter the linear or quadratic nature of growth of any composite, although it did affect the absolute values found for the Tense Marker Total and Productivity Score.

Conclusion Even when sample size is controlled, early growth can be seen in tense and agreement accuracy with relatively limited diversity and productivity, whereas later growth in diversity and productivity can occur with very little change in accuracy.