[Rasch] Person Misfit & Item Fit

Mike Linacre (RMT) rmt at rasch.org
Tue Aug 7 08:58:48 EST 2007


Dear Greg:

Yes, your experience with trimming misfitting persons matches mine.

You are removing the noisiest part of the data, so the remaining data must 
have a slightly closer-to-Guttman pattern. Consequently we expect to see a 
slightly increased measure dispersion in the estimates from the trimmed data.

However, I have yet to see this change of measure dispersion have any 
substantive implications except where person measures are adjacent to 
measure-defined cut-points.

Mike Linacre

At 8/6/2007, Petroski, Greg wrote:
>I am fitting a PCM and have a fairly large sample (5k).  As an experiment 
>I ran the calibration with all subjects and then again with the 500 worst 
>fitting (OUTFIT range from 2 to 9.9) individuals excluded. There was 
>change in parameter estimates and item fit, but not huge, not what I 
>expected. This is comforting, but has this been the experience of others 
>or is it likely just a quirk of my data and the fact that the sample size 
>is large to begin with?

Mike Linacre
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT:  20:4 Spring 2007
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