Why same trace lines?

[Todd McKay]

Hi all,

I'm trying to analyze the SAT12 data using a scoring key (similar to the example from the mirt package .PDF). I'm applying the same model, just changing the last score from 5 to 3 in the scoring key. For the life of me, though, I cannot figure out why my item trace lines for item 32 look the same! Any help? I feel I'm making a small slip-up somewhere, but I can't find it!

Here is my code:

  #--- THE PRELIMINARIES

library(mirt)

library(ggplot2)

SAT12Miss <- SAT12

SAT12Miss[SAT12Miss == 8] <- NA

head(SAT12Miss)

  #--- FIT A 2PL AND NOMINAL MIX >>> FINAL SCORE = 5 <<<

SAT12Miss.Mix5 <- mirt(SAT12Miss, 1, itemtype = c( rep("2PLNRM", 31), rep("nominal", 1)), 

key = c(1,4,5,2,3,1,2,1,3,1,2,4,2,1,5,3,4,4,1,4,3,3,4,1,3,5,1,3,1,5,4,5))

coef(SAT12Miss.Mix5, printSE = T)

plot(SAT12Miss.Mix5, type = "trace")

  #--- FIT A 2PL AND NOMINAL MIX >>> FINAL SCORE = 3 <<<

SAT12Miss.Mix3 <- mirt(SAT12Miss, 1, itemtype = c( rep("2PLNRM", 31), rep("nominal", 1)), 

key = c(1,4,5,2,3,1,2,1,3,1,2,4,2,1,5,3,4,4,1,4,3,3,4,1,3,5,1,3,1,5,4,3))

coef(SAT12Miss.Mix3, printSE = T)

plot(SAT12Miss.Mix3, type = "trace")

[Phil Chalmers]

It seems to be one rare occasion was the scoring keys are virtually identical. This is something I noted in a workshop I gave a while back with this scoring key. It seems that 3/5 provides basically the same fit to the data. If you modified any of the other key elements then you'll likely notice a substantially worse fit to the data when using anova(mod, mod2). 

Phil

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