[R] Problem retrieving data from R2InBUGS

[Barth B. Riley]

Dear list

I am calling the functiton bugs() provided by R2WinBugs to performs an IRT analysis. The function returns a set of estimated parameters over n replications/iterations. For each replication, two sets of person measures (theta1 and theta2) and two sets of item difficulty parameters (diff1 and diff2) are returned. The code used to obtain these estimates is as follows:

sim <- bugs(data,init,model.file="drift.bugs",parameters=c("theta1","theta2","diff1","diff2"),n.chains=n.chains,n.iter=n.reps,n.burnin=1000,bin=1,n.thin=1,digits=3, codaPkg=T)
sim.list <- read.bugs(sim)

However, when I read the coda files using read.bugs the order of the columns in the parameter  matrix are as follows:

  [1] "deviance"    "diff1[1]"    "diff1[10]"   "diff1[100]"  "diff1[11]"   "diff1[12]"   "diff1[13]"
   [8] "diff1[14]"   "diff1[15]"   "diff1[16]"   "diff1[17]"   "diff1[18]"   "diff1[19]"   "diff1[2]"
...
...
.............................................................. "diff2[10]"   "diff2[100]"  "diff2[11]"
 [106] "diff2[12]"   "diff2[13]"   "diff2[14]"   "diff2[15]"   "diff2[16]"   "diff2[17]"   "diff2[18]"
 [113] "diff2[19]"   "diff2[2]"    "diff2[20]"   "diff2[21]"   "diff2[22]"   "diff2[23]"   "diff2[24]"
...
...
............................................................................. "theta1[1]"   "theta1[10]"
 [204] "theta1[100]" "theta1[101]" "theta1[102]" "theta1[103]" "theta1[104]" "theta1[105]" "theta1[106]"
 [211] "theta1[107]" "theta1[108]" "theta1[109]" "theta1[11]"  "theta1[110]" "theta1[111]" "theta1[112]"

The columns, in other words, are not in numerical order according to the index of the item difficulty or person measure parameter. Rather they appear to be in text-sorted order (e.g., 1, 10, 100...). Any ideas as to what might be going on?


Thanks

Barth

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