[Rasch] Rasch-Poisson

Mike Linacre (RMT) rmt at rasch.org
Fri May 22 10:23:55 EST 2009


Thank you for asking us about Poisson, Anthony.

The Rasch-dichomous model can be written:

Log (Pni1 / Pni0) = Bn - Di

A Rasch-Poisson model is:

Log (Pnix / Pni(x-1)) = Bn - Di - log(x)

where x is a category on a rating scale from 0 to infinity. Since log(x) 
linearizes the x's, the x's must be on a ratio scale.

You can see that the Rasch-Poisson model becomes the Rasch-dichotomous 
model if the rating scale is limited to 0,1.

For more technical material, see Georg Rasch at www.rasch.org/memo18.htm

Mike L.

At 5/21/2009, you wrote:
>A while ago I heard about Rasch-Poisson model which supports a ratio scale.

Mike Linacre
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT:  22:4 Spring 2009

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