mirt (version 0.8.0)

itemplot: Displays item surface and information plots

Description

itemplot displays various item based IRT plots, with special options for plotting items that contain several 0 slope parameters.

Usage

itemplot(object, item, type = "trace", degrees = 45,
    CE = FALSE, CEalpha = 0.05, CEdraws = 1000,
    drop.zeros = FALSE, ...)

Arguments

object

a computed model object of class ExploratoryClass, ConfirmatoryClass, or MultipleGroupClass. Input may also be a list for comparing similar item types (e.g., 1PL vs 2PL)

item

a single numeric value, or the item name, indicating which item to plot

type

plot type to use, information ('info'), item trace lines ('trace'), relative efficiency lines ('RE'), expected score 'score', or information contours ('infocontour') (not for

degrees

the degrees argument to be used if there are exactly two factors. See iteminfo for more detail

CE

logical; plot confidence envelope?

CEalpha

area remaining in the tail for confidence envelope. Default gives 95% confidence region

CEdraws

draws number of draws to use for confidence envelope

drop.zeros

logical; drop slope values that are numerically close to zero to reduce dimensionality? Useful in objects returned from bfactor or other confirmatory models that contain several zero slopes

...

additional arguments to be passed to lattice

Examples

data(LSAT7)
fulldata <- expand.table(LSAT7)
mod1 <- mirt(fulldata,1,SE=TRUE)
mod2 <- mirt(fulldata,1, itemtype = '1PL')
mod3 <- mirt(fulldata,2)

itemplot(mod1, 2)
itemplot(mod1, 2, CE = TRUE)
itemplot(mod1, 2, type = 'info')
itemplot(mod1, 2, type = 'info', CE = TRUE)

mods <- list(twoPL = mod1, onePL = mod2)
itemplot(mods, 1, type = 'RE')

#multidimensional info
itemplot(mod3, 3, type = 'info')

#polytomous items
pmod <- mirt(Science, 1, SE=TRUE, SE.type = 'MHRM')
itemplot(pmod, 3)
itemplot(pmod, 3, CE = TRUE)