[R] ltm package error for grm (IRT)

[Tom Fletcher]

Ben,


This is because you do not have all the possible response options
represented for each item. For example, in your data below, item 2 has
no '1's. 

I don't know if there is a workaround for this (other than deleting an
item or faking a response), or if this is a function of IRT itself? 

Tom Fletcher



-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of ben kelcey
Sent: Wednesday, October 14, 2009 4:32 PM
To: r-help at r-project.org
Subject: [R] ltm package error for grm (IRT)

Using the grm function (graded response IRT model) in the ltm package I
receive the following error:

Error: subscript out of bounds

for several scales I'd like to examine. Here's a small example that if
run a few times will likley produce the error at least once

 ch<-array(round(runif(50,1,5)),c(10,5))
grm(ch,start.val="random")
## or
grm(ch,constrained=F,IRT.param=T,start.val="random")

I have cannot figure out where I have gone wrong and was unable to find
mention of similar errors in the archives. Any help would be much
appreciated. A subset of my actual data that produces the same error is
pasted below

Thank you,
ben

  t2_sr1 t2_sr2 t2_sr3 t2_sr4 t2_sr5
      1      5      1      5      5
      1      4      1      4      4
      4      2      4      2      1
      4      5      4      1      4
      1      5      5      1      5
      1      4      1      5      5
      3      2      4      2      4
      5      3      5      1      1
      4      2      4      4      2
      1      4      1      4      5
      2      4      4      4      2
      1      4      2      3      4
      2      2      4      3      2
      2      4      2      3      3
      4      3      2      3      3
      2      5      2      5      5
      1      4      1      4      4
      3      3      2      3      4
      2      4      2      3      4
      3      3      3      2      2


> sessionInfo()
R version 2.9.2 (2009-08-24)
i386-pc-mingw32
locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
States.1252;LC_MONETARY=English_United
States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252 attached
base packages:
[1] splines   stats     graphics  grDevices utils     datasets  methods
base
other attached packages:
 [1] car_1.2-16      epicalc_2.9.2.7 survival_2.35-4 foreign_0.8-37
faraway_1.0.4
 [6] ltm_0.9-1       polycor_0.7-7   sfsmisc_1.0-8   mvtnorm_0.9-7
msm_0.9.3
[11] MASS_7.2-48
loaded via a namespace (and not attached):
[1] tools_2.9.2

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