This study compared a unidimensional model of vocabulary and a two-factor model comprising vocabulary breadth and depth in a second language (L2). A total of 167 Chinese Grade 4 and 5 primary school children (Meanage = 9.96 years old) learning English as an L2 participated in this study, and they were tested on four English vocabulary tests. Our results of confirmatory factor analyses indicate that vocabulary breadth and depth were not two distinct dimensions, and the unidimensional model was supported. Theoretical and practical implications were discussed.

In lexical development, the specificity of phonological representations is important. The ability to build phonologically specific lexical representations predicts the number of words a child knows (vocabulary breadth), but it is not clear if it also fosters how well words are known (vocabulary depth). Sixty-six children were studied in kindergarten (age 5;7) and first grade (age 6;8). The predictive value of the ability to learn phonologically similar new words, phoneme discrimination ability, and phonological awareness on vocabulary breadth and depth were assessed using hierarchical regression. Word learning explained unique variance in kindergarten and first-grade vocabulary depth, over the other phonological factors. It did not explain unique variance in vocabulary breadth. Furthermore, even after controlling for kindergarten vocabulary breadth, kindergarten word learning still explained unique variance in first-grade vocabulary depth. Skill in learning phonologically similar words appears to predict knowledge children have about what words mean.