Hello all,

I have a brief IRT related question regarding the ltm package.

I am am using tpm which is part of the ltm package. I am constraining the
item parameters for this package since they have been estimated earlier. I
notice that the difficulty parameter changes, despite the fact that it is
supposed to be constrained. The a and c parameters do not change.

Here is what I am doing (in principle):

#example values for the constraints
difficulties <- c(-0.5, 1, 0.1, 1.25)
discrimination <-c(0.6, 1.1, .5, .3)
items <- rep(1:4, 3)
parms2 <- rep(1:3, each = 4)
vals <- c(rep(.25, 4),difficulties, discrimination)
constr2 <- cbind(items, parms2, vals)
constr2


#note that dataframe LSAT comes with the ltm package
tpm(LSAT, type = "latent.trait", constraint = constr2)


Note that the difficulty parameter changes. Do any of you have any
suggestions why that is the case? Does this influence the estimate of the
other item parameter (the ones I am trying to estimate?)

Your thoughts & help would be greatly appreciated,
Alexander

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