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Psychometrika

, Volume 58, Issue 1, pp 87–99 | Cite as

The partial credit model and null categories

  • Mark Wilson
  • Geofferey N. Masters
Article

Abstract

A category where the frequency of responses is zero, either for sampling or structural reasons, will be called anull 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.

Key words

sampling zero structural zero partial credit model 

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Copyright information

© The Psychometric Society 1993

Authors and Affiliations

  • Mark Wilson
    • 1
  • Geofferey N. Masters
    • 2
  1. 1.Graduate School of EducationUniversity of CaliforniaBerkeley
  2. 2.Australian Council for Educational ResearchAustralia

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