[Rasch] Comparison Rasch 3-parm IRT

Svend Kreiner svkr at sund.ku.dk
Sun Sep 15 17:43:05 EST 2013

It goes witout saying (almost) that 3pl estimates and Rasch estimates are "equivalent" if items actually fit the Rasch model except that we expect 3pl estmates to have a little larger standard errors than Rasch estmate, because th 3pl estmates use some of the available information in data to estimate two extra parameter per item.

If items do not fit the Rasch model, the two sets of estimates can not be expected to be equivalent. We may be able to find examples where they are similar, but also examples where they are not. But, given that items do not fit the Rasch model, you should not use Rasch estimates anyway, so why bother about the difference anyway.

Cheers Svend
From: rasch-bounces at acer.edu.au [rasch-bounces at acer.edu.au] on behalf of Mike Linacre [mike at winsteps.com]
Sent: Sunday, September 15, 2013 12:32 AM
To: rasch at acer.edu.au
Subject: Re: [Rasch] Comparison Rasch 3-parm IRT

Yes, Ricardo.

Ben Wright reached the same conclusion in his reanalysis of a major
dataset: "3PL ability estimates and item difficulties are statistically
equivalent to Rasch measures." - http://www.rasch.org/rmt/rmt91b.htm

Mike L.

On 9/14/2013 5:20 AM, Ricardo Primi wrote:
> Thanks a lot Mike
> It is interesting that then both models will tell the same history about the subjects (practically the same theta estimates and same S.E.)
> Thanks
> Ricardo

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