REFINING AND VALIDATING PAUL NATION’S VOCABULARY SIZE TEST FOR O-LEVEL CANDIDATES IN PAKISTAN: AN ITEM DISCRIMINATION AND PREDICTIVE VALIDITY STUDY

Abstract

This study investigates the quality and predictive potential of Paul Nation’s Vocabulary Size Test (VST) among O-level students in Pakistan. The primary objectives were to (1) conduct item discrimination analysis to evaluate how effectively individual items of the VST distinguish between students with different levels of vocabulary proficiency, (2) assess the internal consistency reliability of the refined test, and (3) examine the predictive validity of the test in relation to students’ English academic performance. The study employed convenience sampling and involved 716 O-level students from various schools in Faisalabad. The original VST developed by Nation and Beglar (2007) was administered, and item analysis was carried out using facility and discrimination indices. Items with facility values between 0.30 and 0.70 and discrimination values of 0.40 or above were retained, resulting in a refined 41-item version of the test. Reliability analysis yielded a Cronbach’s alpha of 0.86, indicating high internal consistency. To evaluate predictive validity, a simple linear regression was conducted using scores from the refined VST and English mock exam results obtained from a subsample of 30 O-level students. The results showed a strong, statistically significant relationship (R² = .69, p < .001), supporting the test’s predictive utility. These findings suggest that the refined VST is both a reliable and valid instrument for assessing vocabulary size and predicting academic performance in English among O-level learners. Collectively, the findings support the refined VST as a reliable, valid, and operationally efficient instrument suitable for immediate use by schools and testing providers for diagnostic assessment, placement decisions, and early identification of learners at risk, while also offering researchers a standardized and empirically validated tool for investigating vocabulary knowledge and its relationship with academic outcomes at the O-level.