createItem Function

[Seamus Donnelly]

Hi Everyone,

I'm trying to fit a complementary log log version of the 2PL model (not sure if this has a name in the IRT literature). I've been using the createItem function to do so like so:

name <- 'cloglog'

par <- c(a = .5, b = 2)

est <- c(TRUE, TRUE)

P.cloglog <- function(par, Theta, ncat){

  a <- par[1]

  b <- par[2]

  P1 <- 1 - exp(-exp(a*(Theta - b)))

  cbind(1 - P1, P1)

}

clog <- createItem(name, par=par, est=est, P=P.cloglog)

m2 <- mirt(grammar_short, 1, "cloglog", customItems=list(cloglog=clog))

With some sets of starting values, the model converges immediately and I get a warning. With other starting values, the model converges after 2 or 3 iterations, but the fit is really poor. I've tried reducing the tolerance but that doesn't change anything. I don't have any problems when I use MIRT's in-built model types with this data set. I'm assuming there's an error with the code above somewhere, but can't find it. Does anybody see anything?

thanks!

Seamus

[Phil Chalmers]

Interesting, I don't see any obvious reason why this would be occurring. Could you provide a reproducible example of this behaviour (you don't have to post it publicly; a personal email for this type of report is fine). Cheers.

Phil

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