The present study aimed to validate a 70-item Korean bilingual version of the Vocabulary Size Test (VST) using Rasch modeling. The goal was to assess the applicability of this Korean version of the VST for Korean learners of English in an English as a foreign language (EFL) context by examining validity evidence based on Messick’s framework. Specifically, the study focused on the content, substantive, and external aspects of construct validity. However, the findings provided weak evidence supporting the utility of the VST as a measure of receptive vocabulary for Korean EFL learners. The test was deemed too easy and lacked the ability to effectively differentiate among varying levels of second language proficiency, with many test items exhibiting unexpected behavior. Additionally, the VST showed a weak correlation with another measure of second language proficiency. In light of these findings, the study offers specific recommendations for improving the test's validity and usefulness.

The current study investigated the usage of the bilingual Vocabulary Size Test (VST) within Korean EFL environment. Thirty-two university students with an intermediate to high proficiency participated in this study. The students were given a Korean bilingual version of VST and reported their official English scores. The findings of this study are as following: (1) The VST scores showed a significant relationship with students’ proficiency scores which indicates that students with a bigger size of vocabulary had a higher proficiency of English. (2) When VST scores were divided into each frequency level, no particular frequency band demonstrated a significant relationship with English proficiency. Also, the difficulty of each level did not show a consistent pattern. Based on the results of VST, the influence of Korean EFL environment such as loanwords and the test-oriented education system was discovered. This study provides implications for teaching vocabulary in EFL environment and suggests the need for developing of a vocabulary size test for Korean context.