eRm (version 1.0-6)

plotDIF: Confidence intervals plot of item parameter estimates.

Description

Performs an plot of item parameter conficence intervals based on LRtest subgroup splitting.

Usage

plotDIF(object, item.subset = NULL, gamma = 0.95, main = NULL,
          xlim = NULL,  xlab = " ", ylab=" ", col = NULL,
          distance, splitnames=NULL, leg = FALSE, legpos="bottomleft", ...)

Value

plotCI returns a list containing the confidence limits of each group in each LRtest object.

Arguments

object

An object of class LR (if more objects should be plotted, the argument has to be defined as a list).

item.subset

Subset of items to be plotted. Either a numeric vector indicating the items or a character vector indicating the itemnames. If nothing is defined (default), all items are plotted.

gamma

The level for the item parameter's confidence limits (default is gamma = 0.95).

main

Main title for the plot.

xlim

Numeric vector of length 2, giving the x coordinates ranges of the plot (the y coordinates depend on the number of depicted items).

xlab

Label for the x axis.

ylab

Label for the y axis.

col

By default the color for the drawn confidence lines is determined automatically whereas every group (split criterion) is depicted in the same color.

distance

Distance between each item's confidence lines -- if omitted, the distance shrinks with increasing numbers of split criteria. Can be overriden using values in (0, 0.5).

splitnames

For labeling the splitobjects in the legend (returns a nicer output).

leg

If TRUE a legend is provided by default.

legpos

Position of the legend with possible values "bottomright", "bottom", "bottomleft", "left", "topleft", "top", "topright", "right" and "center". The default value for the legend is "bottomleft".

...

Further options to be passed to plot.

Author

Kathrin Gruber, Reinhold Hatzinger

Details

If there are items that cannot be estimated for some reasons, certainly these ones are not plotted. For plotting several objects of class LR, the subgroup splitting by LRtest has to be carried out for the same data set (or at least item subsets of it).

Plotting a certain subset of items could be useful if the objects of class LR contain a huge number of estimated items.

The default level for the conficence limits is gamma = 0.95. (If the conficence limits should be corrected it is useful to use a correction, e.g., Bonferroni: 1 - (1 - gamma) / number of estimated items.)

See Also

LRtest, confint.threshold, thresholds

Examples

# the object used is the result of running   RM(raschdat1)
res <- raschdat1_RM_fitted     # see ? raschdat1_RM_fitted

if (FALSE) {
# LR-test on dichotomous Rasch model with user-defined split
splitvec <- rep(1:2, each = 50)
lrres <- LRtest(res, splitcr = splitvec)

# LR-test with mean split
lrres2 <- LRtest(res, split = "mean")

# combination of LRtest-objects in a list
RMplotCI <- list(lrres, lrres2)}

# the object raschdat1_RM_plotDIF is the result of the computations outlined
# above and is loaded to save computation time. see ?raschdat1_RM_plotDIF
RMplotCI <- raschdat1_RM_plotDIF

# Confidence intervals plot with default assumptions
plotDIF(RMplotCI)

# Confidence intervals plot with Bonferroni correction
plotDIF(RMplotCI, gamma = (1 - (0.05/10)))

# Confidence intervals plot for an item subset
plotDIF(RMplotCI, item.subset = 1:6)

# with user defined group color and legend
plotDIF(RMplotCI, col = c("red", "blue"), leg = TRUE, legpos = "bottomright")

# with names for the splitobjects
plotDIF(RMplotCI, col = c("red", "blue"), leg = TRUE, legpos = "bottomright",
        splitnames = c(paste("User", 1:2), paste(rep("Mean", 2), 1:2)))