The Rasch Model and 1PL Item Response Theory: Item Difficulty, Person Ability, and Invariant Measurement
Year: 2026
Type: article
Abstract: The Rasch model occupies a distinctive position in modern psychometric theory. Although it is frequently described as a one parameter logistic (1PL) Item Response Theory (IRT) model, treating the two as fully interchangeable obscures an important conceptual distinction. Both frameworks employ a logistic function relating the probability of a response to the difference between a person's location and an item's location on a latent continuum, and under common parameterisations the equations are mathematically equivalent. Rasch measurement, however, is distinguished by a stronger theoretical commitment: invariant comparison, specific objectivity, unidimensional measurement, and the requirement that observed data demonstrate adequate conformity to the model rather than the model being adjusted to fit the data. Published in the PsychtrixWeb Research Note series by Psychtrix Initiative Limited. Read online: https://www.psychtrixweb.online/research-notes/013-abstract
Author: Oladunmoye, Enoch O.
Institution: Kampala International University
Language: English
Field: Decision Sciences
Domain: Social Sciences
Open Access status: green
OpenAlex ID: W7214926497