[Rasch] Fit & Dimensionality

Mike Linacre (RMT) rmt at rasch.org
Mon Jul 14 16:03:23 EST 2008

Anthony quotes:
"Rasch model misfit does not necessarily contradict unidimensionality. The 
appropriateness of the scoring rule has merely been invalidated in the 
latter case" (p.381). 

Yes, this would happen when an MCQ item has been keyed incorrectly, or a 
survey item including the word "not" has not been reversed scored. 
Including different response formats (of different discrimination) in the 
same instrument can also produce misfit.

And we could imagine other situations in which items operationalizing a 
unidimensional latent trait would cause the data to misfit the Rasch model. 
For instance, on an MCQ test, items which cue other items. Or on a survey, 
items which summarize other items.

When misfit is seriously distorting the measures, the "appropriate scoring 
rule" could become "Don't score this item!"

How does this sound?

Mike L.
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