Smoothing of bivariate test score distributions - Model selection targeting test score equating

Wallin, Gabriel and Wiberg, Marie (2025) Smoothing of bivariate test score distributions - Model selection targeting test score equating. Journal of Educational and Behavioral Statistics. ISSN 1076-9986

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Abstract

Observed-score test equating is a vital part of every testing program, aiming to make test scores across test administrations comparable. Central to this process is the equating function, typically estimated by composing distribution functions of the scores to be equated. An integral part of this estimation is presmoothing, where statistical models are fit to observed score frequencies to mitigate sampling variability. This study evaluates the impact of commonly used model fit indices on bivariate presmoothing model-selection accuracy in both item response theory (IRT) and non-IRT settings. It also introduces a new model-selection criterion that directly targets the equating function in contrast to existing methods. The study focuses on the framework of non-equivalent groups with anchor test design, estimating bivariate score distributions based on real and simulated data. Results show that the choice of presmoothing model and model fit criterion influences the equated scores. In non-IRT contexts, a combination of the proposed model-selection criterion and the Bayesian information criterion exhibited superior performance, balancing bias, and variance of the equated scores. For IRT models, high selection accuracy and minimal equating error were achieved across all scenarios.