Modal, transitional: RE: [Rasch] Interpretation of Item Plot

Mike Linacre (RMT) rmt at rasch.org
Tue Mar 27 20:32:54 EST 2007


>Andrew, Greg, et al.

Andrew asked:
Could you please explain what you mean by transitional and modal 
categories?  Does the Rasch model make such a distinction?
>
Reply:
Transitional vs. modal is a substantive distinction in the practical use of 
the rating scale. In rating scales, modal categories correspond to 
longer-lasting or wider stages, likely to be observed by most observers of 
a developmental process. Transitional categories correspond to 
shorter-lasting or narrower stages which may be missed by most observers of 
a developmental process. For instance, in baby-development, most parents 
notice "immobile, crawling, walking". Crawling is a modal category. But in 
the mosquito life-cycle, larva - pupa - adult, it is easy to for the 
periodic observer to miss the pupa stage, which may only last a day or two. 
Pupa is a transitional category.

In Rasch terms, a modal category has ordered Rasch-Andrich thresholds, a 
transitional category does not. But this distinction does not affect the 
algebra of the Rasch model, nor the crucial requirement that category 
counts be sufficient statistics for the parameter estimates.

Hope this helps,
Mike L.
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