The Impact of Multidirectional Item Parameter Drift on IRT Scaling Coefficients and Proficiency Estimates

Abstract Item parameter drift (IPD) occurs when item parameter values change from their original value over time. IPD may pose a serious threat to the fairness and validity of test score interpretations, especially when the goal of the assessment is to measure growth or improvement. In this study, we examined the effect of multidirectional IPD (i.e., some items become harder while other items become easier) on the linking procedure and rescaled proficiency estimates. The impact of different combinations of linking items with various multidirectional IPD on the test equating procedure was investigated for three scaling methods (mean-mean, mean-sigma, and TCC method) via a series of simulation studies. It was observed that multidirectional IPD had a substantive effect on examinees' scores and achievement level classifications under some of the studied conditions. Choice of linking method had a direct effect on the results, as did the pattern of IPD. Notes 1The EAP method was chosen because it is often used in large-scale assessments and to avoid the problems associated with the maximum likelihood estimation (MLE) method (e.g., lack of estimates with certain response patterns (all items correct/incorrect)). The EAP method tends to result in reduced SEs and the proficiency estimates shrunk slightly toward the mean value. However, a choice of estimation method will not affect the study since the study consistently uses the EAP throughout the study.