This study developed a vocabulary size test for Japanese learners of English as a foreign language (EFL) using the New JACET List of 8,000 Basic Words (VST-NJ8). Data from the vocabulary size tests were interpreted to assess, diagnose, and predict learners’ lexical proficiency. Yet, existing tests for Japanese EFL learners involve two critical issues regarding unrepresentative contents and inflated estimation of vocabulary size due to methodological restrictions. For these reasons, a large-scale item bank was created in Study 1 by sampling 800 target words and 2,400 distractors from the New JACET List of 8,000 Basic Words. The data from 2,155 test-takers were analyzed using the three-parameter logistic (3PL) model of the item response theory to obtain the invariant parameters of difficulty, discriminability, and pseudo-guessing probability for every item. In Study 2, an automatic test assembly was conducted in consideration of the number of items to be included, the distribution of parts of speech, and the test information functions estimated by the 3PL model. The item characteristics were argued for validation in terms of the content, substantive, structural, generalizability, and external aspects of construct validity. The administration, scoring, and interpretation methods of the VST-NJ8 are fully provided.
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