Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Applied Linguistics and English as a Second Language

First Advisor

Dr. YouJin Kim

Second Advisor

Dr. Scott Crossley

Third Advisor

Dr. Eric Friginal

Fourth Advisor

Dr. Andrea Révész


Despite the importance of English speaking skills in higher education contexts (Andrade 2009), there has been a lack of investigations into longitudinal development in English as second language (ESL) speakers’ oral proficiency in relation to their oral production features (complexity, accuracy, fluency: CAF) and individual differences in working memory (WM) and aptitude. Existing research examining the relationships between CAF measures and L2 oral proficiency mostly focused on monologic tasks although CAF measures might significantly vary between monologic and dialogic task types (Michel et al., 2012). The purpose of this dissertation is threefold. First, the study investigates whether CAF measures of ESL speakers’ monologic and dialogic oral performances predict development in their oral proficiency over time. Second, the dissertation examines whether ESL speakers’ WM and aptitude are predictive of their oral proficiency development. Third, the dissertation also examines whether the relationships between CAF measures and oral proficiency are mediated by the speakers’ WM and aptitude. In total, 60 ESL participants (matriculated and non-matriculated) performed both monologic and dialogic oral tasks at three different times over eight months. The participants’ oral proficiency was measured by TOEFL iBT speaking tests and communicative adequacy ratings of their monologic and dialogic speech. The results show that in monologic speech, high proficient ESL speakers produced more syntactically and lexically complex language, whereas in dialogic speech, they produced faster speech. The findings also indicate that although in both monologic and dialogic speech, the participants with lower phonation (compared to pauses) significantly developed their oral proficiency over time, in dialogic speech, the participants with longer turns (in-between pauses) had longitudinal development in oral proficiency. The dissertation also found that high proficient ESL speakers with higher aptitude used more familiar vocabulary in their monologic speech but shorter fluent runs and shorter clauses in dialogic speech. Overall, the study argues that high proficient speech in monologic versus dialogic modes have different linguistic benchmarks. The findings also offer insights into the processes of high proficient L2 speech production in monologic and dialogic tasks by suggesting the combined effects of ESL speakers’ aptitude and CAF features on their oral proficiency scores.


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