Explanatory item response models for instrument validation: A tutorial based on an elicited imitation test

This tutorial introduces explanatory item response models for the purpose of validating instruments used in applied linguistics research. These models can be used to investigate how theoretically important, construct-relevant characteristics of items influence the difficulty associated with successful responses, which constitutes valuable validity evidence. We focus on two item-explanatory models, the linear logistic test model for dichotomous responses and the linear rating scale model for ordinal responses, in the context of elicited imitation tests commonly used to measure oral proficiency in L2 research. Examples are provided using open data and the open-source statistical software R.