A Rasch-based validation of the Vocabulary Size Test

The primary purpose of this study was to provide preliminary validity evidence for a 140-item form of the Vocabulary Size Test, which is designed to measure written receptive knowledge of the first 14,000 words of English. Nineteen native speakers of English and 178 native speakers of Japanese participated in the study. Analyses based on the Rasch model were focused on several aspects of Messick’s validation framework. The findings indicated that (1) the items and examinees generally performed as predicted by a priori hypotheses, (2) the overwhelming majority of the items displayed good fit to the Rasch model, (3) the items displayed a high degree of unidimensionality with the Rasch model accounting for 85.6% of the variance, (4) the items showed a strong degree of measurement invariance with disattenuated Pearson correlations for person measures estimated with different sets of items of 0.91 and 0.96, and (5) various combinations of items provided precise measurement for this sample of examinees as indicated by Rasch reliability indices >0.96. The Vocabulary Size Test provides teachers and researchers with a new instrument that greatly extends the range of measurement provided by other measures of written receptive vocabulary size.