Language Measure	Age tested	Description
Auditory Comprehension	All three test ages	The auditory comprehension subtest of the Preschool Language Scales-4 (Zimmerman, Steiner, & Pond, 2002) was used to measure children’s understanding of spoken language. The task requires children to demonstrate their understanding of language by performing specific commands given by an examiner. Standardized scores were used in analysis.
Expressive Vocabulary	All three test ages	The Expressive One-Word Picture Vocabulary Test (Brownell, 2000) was used to assess expressive vocabulary. For this task, children are asked to provide the words that label a series of pictured items shown one at a time on separate pages. Standardized scores were used in analysis.
Morphosyntactic measures (MLU, conjunctions, pronouns)	All three test ages	A 20-minute language sample was obtained at each test age, transcribed, and submitted to SALT analysis. At kindergarten the sample was based on five related themes, presented to the child in a series of prompts. At the younger ages the sample was obtained from a parent-child play session involving a standard set of toys. Each sample was transcribed by two (kindergarten) or three (48 and 36 months) independent viewers. When disagreement was found in how an utterance was transcribed, it was resolved by discussion among transcribers. Mean length of utterance in morphemes (MLU), conjunctions (excluding and), and pronouns were assessed.
Syllable Counting	Kindergarten	Syllable counting assesses sensitivity to syllable structure within words. Children saw and heard a man on a computer monitor say a word and were asked to count the number of syllables in the word by tapping them on the table. The percentage of correct answers (out of 48) was used in analysis.
Final Consonant Task	Kindergarten	In the Final Consonant Choice task, children saw and heard a male speaker produce a target word which the child needed to repeat correctly. Three more words were then presented in a similar fashion. The child’s task was to select the word out of the three that had the same ending sound as the target word. The percentage of correct answers (out of 48) was used in analysis.
Real-word Utterances	48 and 36 months	The numbers of utterances containing at least one real word were counted for the entire 20-minute language sample obtained from children, and served as a dependent measure of language advancement.