[Rasch] Negative pt-bis and fit of 1.0? How can this be?

Stuart Luppescu slu at ccsr.uchicago.edu
Wed Mar 7 07:25:48 EST 2012

On Tue, 2012-03-06 at 11:28 -0800, Mark Moulton wrote:
> I've kind of come around to the same point of view as you.  Fit
> statistics are much more complex than point-biserials (or their Rasch
> equivalent) and are driven by lots of factors, making them harder to
> interpret.  Does an item fit the construct of a test?  A fit statistic
> may or may not give a clear answer to that question.  Point-biserials
> almost always do. 

In this case, I plotted the point-biserials against the fit statistics
and came out with a (nearly) perfect straight line. It doesn't look like
either is any better than the other. (I also did it for the standardized
fit statistic and that plot looks similar, but slightly curved.) The
plot can be viewed here:

Stuart Luppescu -=- slu .at. ccsr.uchicago.edu        
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