[Rasch] Comparison Rasch 3-parm IRT

Ricardo Primi rprimi at mac.com
Sat Sep 14 22:20:45 EST 2013

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.)


Em 14/09/2013, às 06:55, Mike Linacre escreveu:

> Thank you for your email, Ricardo.
> You wrote: In general high ability are estimated with higher S. E. 
> because items are relatively easy for the sample.
> Reply: Yes. We expect a U-shaped graph when S.E. is plotted against theta.
> You wrote:  Suppose two datasets A and B with everything equal but the 
> discrimination in A is generally lower (low correlation theta-score) 
> than dataset B. Does WINSTEPS will have different discrimination in each 
> dataset maintaining it constant within datasets? or will it calibrate as 
> 1 in both ? Does this differences will impact measures variance ?
> Reply: In Winsteps, and in Rasch, differences in test discrimination 
> appear as differences in logit range. In general, the higher the test 
> discrimination, the higher the range. When the test discrimination 
> becomes perfect (a Guttman pattern), the range of the measures in logits 
> becomes infinite.
> You wrote: And finally if the lower asymptote is really greater than 
> zero for most of the items what estimate of reliability should we trust 
> more ? Rasch or 3PL ?
> Reply: I always tend to trust the lower estimate of reliability more.  
> You can estimate a Rasch reliability that approximates the 3-PL 
> reliability by using CUTLO= in Winsteps. If c=.2, then CUTLO=1.2
> OK?
> Mike L.
> Mike Linacre, mike at winsteps.com
> Winsteps: www.winsteps.com/winsteps.htm - current version 3.80.1 - 
> September 2013
> Facets: www.winsteps.com/facets.htm - current version 3.71.2 - May 2013
> Winsteps and Facets Tutorial PDFs at www.winsteps.com/tutorials.htm
> Your comments and questions are welcomed.
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