Breadth and depth of ESL learners' lexical knowledge; exploring its interplay with written language proficiency

This thesis adopts a mixed methods approach to investigate the interplay between lexical knowledge and written language proficiency among learners of English as a Second Language (ESL). To achieve its objectives, the study examines how written language ability relates to a battery of size and depth lexical measures. The Word Associates Test (WAT), the Vocabulary Levels Test (VLT), the Vocabulary Profile (VocabProfile) tool and written compositions were used to produce quantitative data on the interplay between learners' breadth and depth of vocabulary knowledge on the one hand, and writing proficiency on the other hand. Purposive sampling was used to identify ESL participants in order to ensure that data generated would be capable of producing relevant insights to address research questions. Stratified random sampling was used to select 40 written language samples from the International Corpus of Learner English (ICLE) to ensure topic consistency with 18 essays from ESL students. This allowed for a comparative analysis between lower proficiency (ESL) and higher proficiency (ICLE) students. Following written assessments, the study employed the stimulated reconstruction procedure to obtain emic perspectives on the rationale behind the lexical choices that ESL learners made during the WAT. Quantitative findings obtained highlight aspects of both size and depth of lexical knowledge as important factors in the interplay between vocabulary knowledge and written language skills. Qualitative findings highlight the potential for multiple factors that could affect individual learners' trajectories. Taken together, the findings from quantitative and qualitative data deepen lexical insights by highlighting the complex interplay between lexis and writing. The study draws on the Dynamic Systems Theory (DST) as a lens for interpreting and reconciling these findings. To that effect, it offers methodological contributions by highlighting the relevance of DST to ESL developmental processes, which is a relatively new theory in the field of Applied Linguistics.