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The Partial Credit Model and Null Categories

Published online by Cambridge University Press:  01 January 2025

Mark Wilson  and
Geofferey N. Masters
Mark Wilson*
Affiliation:
University of California, Berkeley
Geofferey N. Masters
Affiliation:
Australian Council for Educational Research
*
Requests for reprints should be sent to Mark Wilson, Graduate School of Education, University of California, Berkeley, CA 94720.
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Abstract

A category where the frequency of responses is zero, either for sampling or structural reasons, will be called a null category. One approach for ordered polytomous item response models is to downcode the categories (i.e., reduce the score of each category above the null category by one), thus altering the relationship between the substantive framework and the scoring scheme for items with null categories. It is discussed why this is often not a good idea, and a method for avoiding the problem is described for the partial credit model while maintaining the integrity of the original response framework. This solution is based on a simple reexpression of the basic parameters of the model.

Keywords

sampling zerostructural zeropartial credit model
Type
Original Paper
Information
Psychometrika , Volume 58 , Issue 1 , March 1993 , pp. 87 - 99
Copyright
Copyright © 1993 The Psychometric Society

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Footnotes

We are indebted to the editor, associate editor, and three anonymous reviewers for their insightful comments and thorough review of the manuscript. The first author's work was supported by a National Academy of Education Spencer Fellowship.

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