[Rasch] Use of e in the Rasch model

Anthony James luckyantonio2003 at yahoo.com
Fri Sep 9 18:57:09 EST 2011


Can't we write the model as P=a-d/1+(a-d). This is additive. Why do we need

I know it will be awkward when a=d but the multiplicative form has this problem
too when d=0.


--- On Wed, 9/7/11, Mike Linacre <rmt at rasch.org> wrote:

From: Mike Linacre <rmt at rasch.org>
Subject: Re: [Rasch] Use of e in the Rasch model
To: rasch at acer.edu.au
Date: Wednesday, September 7, 2011, 7:22 AM

Anthony writes: p = (a/d)/(1+(a/d)) is within 0 to 1.

Comment: Yes, that is the multiplicative form of the Rasch model. Georg
Rasch uses it in his book, "Probabilistic Models".

If we want to transform multiplicative numbers, like a and d, into
additive numbers, then we take logarithms .... as Georg Rasch himself
does in his book.

Mike L. 

Mike Linacre

Editor, Rasch Measurement Transactions

rmt at rasch.org
Latest RMT: 25:1 Summer 2011
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