Validation of the Vocabulary Size Test

The Vocabulary Size Test (VST) is one of the most commonly used assessment tools for measuring English vocabulary size in field language testing. Despite its common usage, only a limited number validity and reliability studies have been carried out with regard to VST. Besides, they were mostly predicated on Rasch model. This validation study has attempted reveal evidence construct VST, this end, item response theory (IRT) analyses performed based three-parameter logistic model (3PLM). assumptions IRT investigated via factor analysis (unidimensionality) Yen’s Q3 statistic (local independence). Detailed differential functioning (DIF) conducted Mantel-Haenszel, Lord's chi-square test, Logistic regression methods add internal structure check fairness as lack measurement bias. results showed that 3PLM fitted data better than one- two-parameter models. DIF indicated 10 items exhibited large (seven favoring males three females). further guessing effect was not negligible