Displays item surface and information plots

itemplot displays various item based IRT plots, with special options for plotting items that contain several 0 slope parameters. Supports up to three dimensional models.

Usage

itemplot(
  object,
  item,
  type = "trace",
  degrees = 45,
  CE = FALSE,
  CEalpha = 0.05,
  CEdraws = 1000,
  drop.zeros = FALSE,
  theta_lim = c(-6, 6),
  shiny = FALSE,
  rot = list(xaxis = -70, yaxis = 30, zaxis = 10),
  par.strip.text = list(cex = 0.7),
  npts = 200,
  par.settings = list(strip.background = list(col = "#9ECAE1"), strip.border = list(col =
    "black")),
  auto.key = list(space = "right", points = FALSE, lines = TRUE),
  ...
)

Arguments

object: a computed model object of class SingleGroupClass 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'), standard errors ('SE'), item trace lines ('trace'), cumulative probability plots to indicate thresholds ('threshold'), information and standard errors ('infoSE') or information and trace lines ('infotrace'), category and total information ('infocat'), relative efficiency lines ('RE'), expected score 'score', or information and trace line contours ('infocontour' and 'tracecontour'; not supported for MultipleGroupClass objects)
degrees: the degrees argument to be used if there are two or three factors. See iteminfo for more detail. A new vector will be required for three dimensional models to override the default
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
theta_lim: lower and upper limits of the latent trait (theta) to be evaluated, and is used in conjunction with npts. Default uses c(-6,6)
shiny: logical; run interactive display for item plots using the shiny interface. This primarily is an instructive tool for demonstrating how item response curves behave when adjusting their parameters
rot: a list of rotation coordinates to be used for 3 dimensional plots
par.strip.text: plotting argument passed to lattice
npts: number of quadrature points to be used for plotting features. Larger values make plots look smoother
par.settings: plotting argument passed to lattice
auto.key: plotting argument passed to lattice
...: additional arguments to be passed to lattice and coef()

References

Chalmers, R., P. (2012). mirt: A Multidimensional Item Response Theory Package for the R Environment. Journal of Statistical Software, 48(6), 1-29. doi:10.18637/jss.v048.i06

Author

Phil Chalmers rphilip.chalmers@gmail.com

Examples

if (FALSE) { # \dontrun{

data(LSAT7)
fulldata <- expand.table(LSAT7)
mod1 <- mirt(fulldata,1,SE=TRUE)
mod2 <- mirt(fulldata,1, itemtype = 'Rasch')
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
itemplot(mod3, 4, type = 'info')
itemplot(mod3, 4, type = 'info',
  col.regions = colorRampPalette(c("white", "red"))(100))
itemplot(mod3, 4, type = 'infocontour')
itemplot(mod3, 4, type = 'tracecontour')

# polytomous items
pmod <- mirt(Science, 1, SE=TRUE)
itemplot(pmod, 3)
itemplot(pmod, 3, type = 'threshold')
itemplot(pmod, 3, CE = TRUE)
itemplot(pmod, 3, type = 'score')
itemplot(pmod, 3, type = 'score', CE = TRUE)
itemplot(pmod, 3, type = 'infotrace')
itemplot(pmod, 3, type = 'infocat')


# use the directlabels package to put labels on tracelines
library(directlabels)
plt <- itemplot(pmod, 3)
direct.label(plt, 'top.points')

# change colour theme of plots
bwtheme <- standard.theme("pdf", color=FALSE)
plot(pmod, type='trace', par.settings=bwtheme)
itemplot(pmod, 1, type = 'trace', par.settings=bwtheme)

# additional modifications can be made via update().
# See ?update.trellis for further documentation
(plt <- itemplot(pmod, 1))
update(plt, ylab = expression(Prob(theta))) # ylab changed

# infoSE plot
itemplot(pmod, 1, type = 'infoSE')

# uncomment to run interactive shiny applet
# itemplot(shiny = TRUE)
    } # }