Vocabulary Size of Thai Graduate Students in Different Disciplines and Their Opinions of Its Influences on the Use of AI in English Language Learning

This study investigates the vocabulary size of Thai graduate students across science and non-science disciplines, alongside their opinions on the effects of vocabulary size on their use of AI tools in their English language learning. A total of 217 students from a public Thai university completed the Updated Vocabulary Levels Test (Webb et al., 2017) and engaged in semi-structured interviews. Quantitative data were analyzed using descriptive statistics, revealing that most students had a low vocabulary size. The Mann-Whitney U test showed significantly higher performance among science students at the 1,000-, 4,000-, and 5,000-word levels. Qualitative data from interviews showed that vocabulary size may not influence most types of AI-assisted language learning activities, their trust in it, or their reliance on AI for language learning, except for the choice of prompt language. These findings offer implications for English vocabulary instruction and highlight the importance of integrating AI-assisted language learning tools with lexical development in higher education contexts.