Purpose: The current study examined (a) the relation between morphologically complex word (MCW) use (words containing at least one derivational morpheme such as prefixes and suffixes) and teachers' ratings of writing quality, (b) average change in MCW use in writing across the school year, and (c) differential change in MCW among students with varying language abilities and linguistic backgrounds including students with developmental language disorders (DLDs) and multilingual learners (MLs).
Method: Expository writing samples (writing for the purpose of explaining or educating) were collected in October and May from 824 fifth-grade students, including 109 with DLD and 170 who were MLs receiving English as a second language service. Students' written responses were coded for the use of MCW. Pearson product-moment correlations and two-level hierarchical linear models were employed to investigate the association between MCW usage and writing quality, as well as increases in MCW usage over the course of the academic year, taking into account the nested structure of students within classrooms.
Results: The relation between students' MCW use and teachers' writing quality ratings was moderately strong (r = .47). Student use of MCW in expository writing showed significant change from fall to spring across all students. However, the amount of change in MCW use across the school year was significantly lower for MLs (effect size [ES] = .09) and students identified with DLD (ES = .10).
Conclusions: The relation between MCW use and teachers' writing quality ratings highlights the utility of MCW as a written language measure for progress monitoring or assessment. The presence of differential change and potential Matthew effects for MLs and students with DLD substantiates the need for further exploration of instructional components that support the increased use of complex vocabulary.