Assessment of differential item functioning in testlet-based items using the Rasch testlet model

Abstract

This study presents a procedure for detecting differential item functioning (DIF) for dichotomous and polytomous items in testlet-based tests, whereby DIF is taken into account by adding DIP parameters into the Rasch testlet model. Simulations were conducted to assess recovery of the DIP and other parameters. Two independent variables, test type and anchor item method, were manipulated. The proportion of DIP items in the tests was set at approximately 40%. All parameters could be accurately recovered whenever the model was the generating model or only one DIF-free item was set as the anchor and all other items were treated as presumed DIP items. The bootstrap method is recommended to approxi..