[Rasch] Validity in Rasch

Mike Linacre rmt at rasch.org
Thu Mar 24 16:02:46 EST 2011

Svend writes: What we may disagree about is what to do when items do not 
fit the model.

Here, an extra category of fit of the data to the model is helpful:

1. Statistically-acceptable fit
2. Substantively-good-enough fit
3. Unacceptable fit

The criteria for good-enough fit depend on the situation in the same way as 
industrial-quality-control tolerance limits depend on their situation. So, 
before rejecting an item (or a person, a rater, ...), we need to decide 
"what amount of misfit is acceptable?" For instance, in rated performances, 
we may decide that observations that overfit the Rasch model (too 
predictable) are always acceptable.

Mike L.

Mike Linacre
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT:  24:4 Spring 2011
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