eRm (version 1.0-4)

plotPImap: Person-Item Map

Description

A person-item map displays the location of item (and threshold) parameters as well as the distribution of person parameters.along the latent dimension. Person-item maps are useful to compare the range and position of the item measure distribution (lower panel) to the range and position of the person measure distribution (upper panel). Items should ideally be located along the whole scale to meaningfully measure the `ability' of all persons.

Usage

plotPImap(object, item.subset = "all", sorted = FALSE,
   main = "Person-Item Map", latdim = "Latent Dimension",
   pplabel = "Person\nParameter\nDistribution", cex.gen = 0.7,
   xrange = NULL, warn.ord = TRUE, warn.ord.colour = "black",
   irug = TRUE, pp = NULL)

Arguments

object

Object of class Rm or dRm

item.subset

Subset of items to be plotted. Either a numeric vector indicating the column in X or a character vector indicating the column name. If "all", all items are plotted. The number of items to be plotted must be > 1.

sorted

If TRUE, the items are sorted in increasing order according to their location on the latent dimension.

main

Main title of the plot.

latdim

Label of the x-axis, i.e., the latent dimension.

pplabel

Title for the upper panel displaying the person parameter distribution

cex.gen

cex as a graphical parameter specifies a numerical value giving the amount by which plotting text and symbols should be magnified relative to the default. Here cex.gen applies to all text labels. The default is 0.7.

xrange

Range for the x-axis

warn.ord

If TRUE (the default) asterisks are displayed in the right margin of the lower panel to indicate nonordinal threshold locations for polytomous items.

warn.ord.colour

Nonordinal threshold locations for polytomous items are coloured with this colour to make them more visible. This is especially useful when there are many items so that the plot is quite dense. The default is "black", so that there is no distinction made.

irug

If TRUE (the default), all thresholds are plotted below the person distribution to indicate where the included items are most informative.

pp

If non-NULL, this contains the person.parameter data of the data object, avoiding the need to recalculate it.

Author

Patrick Mair, Reinhold Hatzinger, patches from Julian Gilbey and Marco J. Maier

Details

Item locations are displayed with bullets, threshold locations with circles.

References

Bond, T.G., and Fox Ch.M. (2007) Applying the Rasch Model. Fundamental Measurement in the Human Sciences. 2nd Edition. Lawrence Erlbaum Associates.

Examples

res <- PCM(pcmdat)
plotPImap(res, sorted=TRUE)