[Rasch] Rasch-Poisson

Anthony James luckyantonio2003 at yahoo.com
Fri May 22 02:24:47 EST 2009

Dear folks,
A while ago I heard about Rasch-Poisson model which supports a ratio scale. What is that?
I know that there is an alternative parameterization for the Rasch model which is:
P (X=1| b) = e^b/ (e^b+e^d)
Where b is ability and d is difficulty. This supports a ratio scale too. 
1. Is this the Poisson model?
2. How can one convert interval Rasch measures into ratio measures with this equation ?
3. Why is it called Poisson anyway?


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