[Rasch] compare nested models in Facets

Mike Linacre rmt at rasch.org
Wed Dec 21 18:18:26 EST 2011

Iasonas, you are describing a conventional statistical technique for 
comparing nested models. Please Google "testing nested models" for references.

In a measurement context, it often makes more sense to investigate whether 
the elements of the 4th facet have substantively and statistically 
different measures. This is one reason that Facets reports a fixed-effect 
chi-square for each facet.


Mike L.

At 12/21/2011, you wrote:
>"The reason I need this, is to do the following: I run a model with 4 
>facets and then
>I used the same data but removed one facet from the analysis. Presumably, 
>if I deduct
>the "Data log-likelihood chi-square" of the second model from that of the
>second, then this gives me the improvement of the model fit for df1-df2 
>where the df
>is the "Approximate degrees of freedom" for each model. Then, I assume that
>this is a chi-square with df1-df2 degrees of freedom in order to test if I 
>should actually
>model the 4th facet or not. Is this procedure correct to test the two 
>models? Do we havea  reference for this?"

Mike Linacre
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT: 25:3 Winter 2011
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