I have started exploring potential of R in applying IRT to a dataset. I have
a data of about 20k students who took a Maths test, a diagnostic in nature.
I find that I don't get stable solution while using tpm() even after passing
an argument "start.val = RANDOM". What should be done in this case to
achieve stable solution?
The item parameters estimated would not be sensible when the stable solution
is not arrived at. However I find that discrimination parameter of one of
the item estimated is negative in that case.
(I used tpm() from ltm package.)
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