Compare methods

	Testlet Response Theory	Item Response Theory
Field	Education	Psychometrics
Family	Latent structure	Latent structure
Year of origin	2007	1952–1968
Originator	Howard Wainer, Eric Bradlow & Xiaohui Wang	Frederic M. Lord (and Allan Birnbaum for the 2PL/3PL models)
Type	Item response model accommodating local dependence within item bundles (testlets)	Probabilistic measurement model
Seminal source	Wainer, H., Bradlow, E. T., & Wang, X. (2007). Testlet Response Theory and Its Applications. Cambridge University Press. ISBN: 9780521681261	Lord, F. M. & Novick, M. R. (1968). Statistical Theories of Mental Test Scores. Addison-Wesley.
Aliases	TRT, Testlet Models, Random-Effects Testlet Model, Item-Bundle IRT	IRT, latent trait theory, item characteristic curve theory, modern test theory
Related	4	5
Summary	Testlet response theory (TRT) extends item response theory to tests built from testlets — bundles of items sharing a common stimulus, such as several questions about one reading passage. Standard IRT assumes items are conditionally independent given ability, but items within a testlet violate this because they draw on the same passage. TRT adds a testlet-specific random effect that absorbs this local dependence, preventing the overstated precision and biased parameters that result from ignoring it. Developed by Wainer, Bradlow, and Wang, it is widely used wherever passage-based or scenario-based items appear.	Item response theory models the probability that a respondent answers an item correctly (or endorses it) as a function of the respondent's latent trait level and the item's own statistical properties — difficulty, discrimination, and guessing. Unlike classical test theory, IRT places persons and items on the same scale, yielding measurement that is sample-independent for items and test-independent for persons.