[R] small scales in fwdmsa

[Rob Cassidy]

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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