Table 1.

Selected Item Response Theory (IRT) Models Classified by Item Parameter and Item Response Format

	Item Difficulty Modeled (Rasch Models)	Item Difficulty and Discrimination Modeled	Item Difficulty, Discrimination, and Guessing Modeled
Dichotomous responses (e.g., yes/no)	One parameter-logistic model (Rasch G)	Two-parameter-logistic model (Birnbaum; Hambleton and Swaminathan)	Three-parameter logistic (Hambleton and Swaminathan)
Polytomous responses (e.g., never/sometimes/always)	Andrich's rating scale model (Andrich)	Graded response model (Samejima)	None
	Partial credit model (Masters GN)	Generalized partial credit model (Muraki)