difTID(Data, group, focal.name, props = NULL,
thrTID = 1.5, purify = FALSE, nrIter = 10,
save.output=FALSE, output=c("out","default"))
## S3 method for class 'TID':
print(x, ...)
## S3 method for class 'TID':
plot(x, plot="dist", pch=8, number=TRUE, col="red",
save.plot=FALSE, save.options=c("plot","default","pdf"), ...)data) of group membership. See Details.group which corresponds to the focal group.NULL (default) or a two-column matrix with proportions of success in the reference group and the focal group. See Details.FALSE)."default" (default value). See Details.TID class object."dist" (default) to display the perpendicular distances, or "delta" for the Delta plot. See Details.pch and col graphical options.TRUE).FALSE)."default" (default value), and the third component is the file extension, eithplot or the print functions.prop matrix.purify option.purify is TRUE.purify is TRUE.nrIter of allowed iterations.
Returned only if purify is TRUE.save.output argument.output argument.Data is a matrix whose rows correspond to the subjects and columns to the items.
In addition, Data can hold the vector of group membership. If so, group indicates the column of Data which corresponds to the group
membership, either by specifying its name or by giving the column number. Otherwise, group must be a vector of same length as nrow(Data).
Missing values are allowed for item responses (not for group membership) but must be coded as NA values. They are discarded from the computation of
proportions of success.
The vector of group membership must hold only two different values, either as numeric or character. The focal group is defined by
the value of the argument focal.name.
Alternatively, one can provide the matrix of proportions of success in for each item in each group. This matrix must have the same format as that provided to
the trItemDiff function; see the corresponding help file for further details.
The threshold (or cut-score) for classifying items as DIF must be supplied through the thrTID argument. The default value is 1.5, as being one of the most
commonbly used values (e.g. Facon and Nuchadee, 2010; Muniz, Hambleton, and Xing, 2001; Robin, Sirecci, and Hambleton, 2003). Other values can be specified instead.
Item purification can be performed by setting purify to TRUE. Purification works as follows: if at least one item was detected as functioning
differently at some step of the process, then the intercept and slope parameters of the major axis are re-computed by discarding all items previously flagged as
DIF. All perpendicular distances are then re-computed for all items. The process stops when either two successive applications of the method yield the same
classifications of the items, or when nrIter iterations are run without obtaining two successive identical classifications. In the latter case a warning
message is printed.
The output of the difTID, as displayed by the print.TID function, can be stored in a text file provided that save.output is set to TRUE
(the default value FALSE does not execute the storage). In this case, the name of the text file must be given as a character string into the first component
of the output argument (default name is "out"), and the path for saving the text file can be given through the second component of output. The
default value is "default", meaning that the file will be saved in the current working directory. Any other path can be specified as a character string:
see the Examples section for an illustration.
Two types of plots are available through the plot.TID function. If the argument plot is set to "dist" (the default value), then the
perpendicular distances are represented on the Y axis of a scatter plot, with each item on the X axis. If plot is set to "delta", the Delta plot is
returned, that is, the scatter plot of pairs of Delta scores for each item, with the reference group on the X axis and the focal group on the Y axis. The type of
point and the color are fixed by the usual pch and col arguments. Option number permits to display the item numbers instead. Detection
thresholds are also printed. Also, the plot can be stored in a figure file, either in PDF or JPEG format. Fixing save.plot to TRUE allows this
process. The figure is defined through the components of save.options. The first two components perform similarly as those of the output argument.
The third component is the figure format, with allowed values "pdf" (default) for PDF file and "jpeg" for JPEG file.trItemDiff, dichoDif# Loading of the verbal data
data(verbal)
# Excluding the "Anger" variable
verbal <- verbal[colnames(verbal)!="Anger"]
# Three equivalent settings of the data matrix and the group membership
r <- difTID(verbal, group=25, focal.name=1)
difTID(verbal, group="Gender", focal.name=1)
difTID(verbal[,1:24], group=verbal[,25], focal.name=1)
# With item purification and threshold 1
r2 <- difTID(verbal, group="Gender", focal.name=1, purify=TRUE, thrTID=1)
# Saving the output into the "TIDresults.txt" file (and default path)
difTID(verbal, group=25, focal.name=1, save.output = TRUE,
output = c("TIDresults","default"))
# Providing the proportions of success only
props <- cbind(colMeans(verbal[verbal[,25]==0,1:24]),
colMeans(verbal[verbal[,25]==1,1:24]) )
difTID(prop=props)
# Graphical devices
plot(r2)
plot(r2, plot="delta")
# Plotting results and saving it in a PDF figure
plot(r2, save.plot = TRUE, save.options = c("plot", "default", "pdf"))
# Changing the path, JPEG figure
path <- "c:/Program Files/"
plot(r2, save.plot = TRUE, save.options = c("plot", path, "jpeg"))Run the code above in your browser using DataLab