Observed Upper-Bound Patterns in Clausal Density and Embedding Depth: A Corpus-Based Study of Chinese EFL Learners’ Argumentative Writing

This corpus-based study examines how syntactic complexity develops, and whether it approaches an upper boundary, in intermediate-to-advanced L2 English writing, using clausal density (CD) and embedding depth (ED) as the main metrics. A quantitative analysis of 566 annotated compositions from a Chinese EFL learner corpus (CET-4 and CET-6) reveals a clear but bounded developmental path. Although higher-proficiency writers produce sentences with significantly greater CD and ED, both groups exhibit a similar upper boundary within this corpus, with maximum observed values reaching CD=5 and ED=3. Notably, while CET-6 writers exhibit higher CD and ED than CET-4 writers at the group level, writing scores within each proficiency level do not significantly predict either metric. This dissociation, where group-level development does not translate into within-level linear prediction, suggests that beyond a moderate threshold, further clausal elaboration decouples from holistic writing quality. These results suggest that, within the present corpus, clause-level complexity may approach an upper boundary, thereby raising questions about the assumption that syntactic complexity functions as a consistent linear predictor of L2 writing proficiency.