Mapping Vocabulary Size Across Frequency Levels: Case of a Japanese Learner of English

Vocabulary knowledge has been consistently identified as central to second language proficiency and as a strong predictor of language performance. This study investigated the vocabulary size of a Japanese Learner of English (JLE) by examining her performance on the Vocabulary Size Test (VST) alongside a language needs assessment. Drawing on the Lexical Threshold Hypothesis and research on vocabulary size and lexical coverage, the study explored how the informant’s vocabulary knowledge was distributed across frequency levels and how her instructional background and language needs were reflected in her vocabulary size estimate. Results indicated that she currently has a vocabulary size of 7,400-word families, obtained the highest scores at high-frequency word levels, and had variable and declining performance at mid- and low-frequency levels. While this size suggests sufficient coverage for basic spoken discourse and simpler texts, it falls below the thresholds associated with unassisted comprehension of advanced academic and literary texts. Item-level analysis revealed that incorrect responses may have been shaped by classroom-emphasized meaning-inferencing strategies, which contrasted with her reported profile. Moreover, it was found that the majority of the incorrectly answered items involved nouns, particularly abstract concepts or institutional terms that are less likely to appear in everyday conversations. These findings highlight the importance of interpreting vocabulary size estimates in relation to the learners’ profiles and needs, and suggest that mid- and low-frequency vocabulary development should be prioritized to support academic and professional language use.