A PSYCHOPHYSICAL INTERPRETATION OF RASCH’S PSYCHOMETRIC PRINCIPLE OF SPECIFIC OBJECTIVITY

  • John R. Irwin

Abstract

Rasch’s principle of specific objectivity asserts that the comparison of two objects should be independent of everything but the two objects and their observed reactions. A consequence of the principle for psychometrics is that, for homogeneous tests conforming to the Rasch model, the estimated difference in ability between two people is independent of the difficulty of any particular test items used to compare them. The principle is exemplified by the Receiver Operating Characteristic of Luce’s psychophysical choice theory. In this application of choice theory, the operating characteristic depicts how the success-rates of two people who attempt the same task co-vary with the difficulty of the task. Because this operating characteristic depends only on the difference in ability between two people and is independent of task difficulty, it satisfies the principle of specific objectivity.

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