[R] PCM() non-convergence problem: how to get diagnostics?

Rob Forsyth rob.forsyth at newcastle.ac.uk
Thu Jul 26 14:22:39 CEST 2012

I am attempting to fit a partial credit model using the PCM() function in eRm. My data set comprises 88 items in 43 individuals with possible scores 0,1,2,3. 

Attempting either a PCM or RSM model fit simply hangs - no error messages but no return to command prompt and a continuously spinning "work in progress" indicator. 

The scores are from an instrument whose relevant psychometrics (e.g unidimensionality) have been established in other clinical populations: I'm studying a new disease group. I strongly suspect the problem relates to undesirable properties of the dataset: a significant missing data rate (about 20%) and a rather uneven distribution amongst the possible scores with relative paucity of 1 and 2 responses. I have in fact already deleted one item because of an "The following items do not have responses on each category" warning.

I was trying to generate some diagnostics to get a better idea of what the problem(s) are, and/or adjust default parameters used in the nlm call to see if this helped achieve convergence. Setting options(warn=1) doesn't appear to help and am unclear how set flags such as print.level for the nlm call?

Thanks for any help

Rob Forsyth

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