I'm using the fwdmsa package to identify deviant cases in a Mokken scale
analysis. I've run into a problem. When I use scales comprising a few items,
iI tend to get an error:
Error in y[order(res[-msamp])][1:(length(samp) + 1 - length(msamp))] :
only 0's may be mixed with negative subscripts
I understand that the error is triggered when the algorithm is fetching
cases to enter into the next step of the forward search. I don't understand
what I can do to remedy this error.
The data are dichotomized (1,0) respsonses from a multiple-choice exam that
150 students have completed. If I run the entire test (37 items) , the
fwd.msa algorithm works fine. However, the entire test is not
unidimensional, so I want to perform separate analyses with the several
unidimensional scales comprised by the entire test. Yet when I select those
4-5 item scales, I run into this error.
Any ideas how to proceed?
The data are 150 responses to a 37 item test. For the first 15 items, they
look like this (the variable names are cumbersome and so have been removed)
:> head(by364.data)
1 1 1 1 0 1 0 1 1 0 1 0 1 1 1
1
2 1 1 1 0 1 1 1 1 1 1 0 0 1 1
1
3 0 1 1 1 0 0 1 0 1 1 1 0 1 1
0
4 1 1 1 0 0 1 1 1 1 1 1 1 0 1
1
5 1 1 1 1 0 1 0 1 1 1 1 0 1 1
0
6 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1
When I run the fwd.MSA(by364.data) on the full data set "works" fine.
However, with shorter (for reasons of unidimensionality) scales, I
continuously run into the error above. Any suggestions?
Robert
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