Assessment of sentence sophistication in L2 spoken production: Expansion of verbs and argument structure constructions

Despite the vast body of literature concerning automatic analyses of L2 production, there is limited research on L2 spoken production. Furthermore, investigations into L2 sentence production abilities have predominantly been centered around sentence length. To provide more sophisticated descriptions of L2 syntactic sophistication, this study examines the validity of using the expansion of verbs and constructions as an index of L2 oral production abilities, drawing upon the principles of usage-based approaches to language development. Employing natural language processing tools, we analyzed 790 speech samples produced by L2 English learners using frequency-based and construction-focused indices to assess their predictive validity in relation to the learners' speaking proficiency. Our findings revealed that advanced L2 learners tend to exhibit an increased use of diverse construction types and a wider range of verb types within each construction. Notably, among the individual construction types observed, the transitive and there-expletive constructions exerted the most substantial influence on the variance in speaking proficiency. These findings suggest a close relationship between the development of verbs and constructions in the spoken domain and factors related to language usage. The results provide an opportunity to evaluate L2 sentence production skills in speaking within the theoretical framework of usage-based approaches.