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RE: st: RE: Flat or discontinuous region encountered! Help


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Flat or discontinuous region encountered! Help
Date   Wed, 23 Aug 2006 18:22:36 +0100

These questions are very difficult to answer. They 
are really questions for your advisor or committee
of advisors or whoever is responsible for guiding 
you on your dissertation. They do not lend themselves
to answers within a list like Statalist. Also, if 
someone else solves your problem for you, do they 
get the degree instead? 

In extremely general terms, what you report sounds 
most like a situation in which your model is not
especially appropriate for your data. There aren't 
magic tricks here. Again, depending on what your 
project is, finding a more appropriate dataset or 
changing the model appear indicated, so I 
am just describing a perfect circle here. 

There may be better advice from anyone who 
knows what a 2-PL MLIRT model is, which does
not include me. (Item response theory???) 

Nick 
n.j.cox@durham.ac.uk 

Prathiba Natesan
 
> Thank you for the suggestions Nick.
>      Could you please tell me how I can change my data in such a case?
> Is there a way I can look for anamolies/problems? I am running a 2-PL
> MLIRT model on 2 datasets, a simulated dataset and a real data (with a
> sample size of abt 19,000). Both of them seem to give me the same
> problems.
>     Also, on #2, even if I let the program run for abt a day or 2, the
> program produces about 100-200 iterations of the estimates which look
> identical. Sometimes the estimates are identical upto the 3rd or 4th
> decimal place. Do you think I should still wait longer for it to
> converge?
>   On a similar note, when the iterations fail to converge, what can be
> done? What does it say abt the data?
> I am currently working on my dissertation and these questions are
> driving me crazy. I would appreciate any suggestions you might have.

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