C-Test construct validity: Evidence from nonparametric item response theory

Abstract

C-Test is a gap-filling test designed to measure first and second language proficiency. Over the past four decades, researchers have shown the fit of C-Test data to parametric item response theory (IRT) models, but no study so far has shown the fit of C-Tests to nonparametric IRT models. The purpose of this study is to contribute to the ongoing C-Test validation project by providing evidence of fit to the Mokken scale analysis as a widely used nonparametric IRT model. A six-passage C-Test battery was analyzed using the monotone homogeneity model and the double monotonicity model of Mokken. Unidimensionality was evaluated using the automatic item selection procedure. Findings showed that the C-Test passages form a strong unidimensional scale, fit well to the monotone homogeneity model, and, after deleting one item, have a sufficient fit to the double monotonicity model. The findings also indicated that the items form a hierarchy, and persons can be located on an ordinal scale using their C-Test sum scores. Implications of the study for C-Test validity and application are discussed.