Abstract
This study investigated the effect of testlets on regularization-based differential item functioning (DIF) detection in polytomous items, focusing on the generalized partial credit model with lasso penalization (GPCMlasso) DIF method. Five factors were manipulated: sample size, magnitude of testlet effect, magnitude of DIF, number of DIF items, and type of DIF-inducing covariates. Model performance was evaluated using false-positive rate (FPR) and true-positive rate (TPR). Results showed that the simulation had effective control of FPR across conditions, while the TPR was differentially influenced by the manipulated factors. Generally, the small testlet effect did not noticeably affect the GPCMlasso model's performance regarding FPR and TPR. The findings provide evidence of the effectiveness of the GPCMlasso method for DIF detection in polytomous items when testlets were present. The implications for future research and limitations were also discussed.