[R] ability estimate with GRM of IRT

SHEN,FENG fshen at ufl.edu
Fri Jan 19 05:30:06 CET 2007

Hi my friends,

I have an issue with ability estimates when running GRM of IRT.  I 
have responses from 242 subjects but got 183 ability estimates.  
Below is what I did to get the estimates.

1) I have a csv file "P1.csv" and I imported it into R and loaded 
the "ltm" package by doing:

2) I created a subset that included columns 2 to 9 for the 
analysis by doing:

3) I converted the subset into data.frame format by doing:

4) I checked the descriptive stats for the s1df by doing:

And it was confirmed that 242 subjects' responses were imported.

5)I ran GRM on the s1df dataset by doing:
grm<-grm(s1df, Hessian=T)

6)Finally, I ran the ability estimated by doing:

And I got 183 factor-scores for observed response patterns.  
Besides, the 183 estimates are not for the first 183 subjects of 
the 242 because the response patterns of No 1 on the factor-scores 
list do not match those of No 1 in the s1 dataset and the same 
thing holds true for the rest of the response patterns.

Could you help me find out what the problem is?

Many thanks in advance!


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