plotDIFLogistic function

Function for characteristic curve of 2PL logistic DIF model

Plots characteristic curve of 2PL logistic DIF model

plotDIFLogistic(x, item = 1, item.name, group.names = c("Reference",
  "Focal"), Data, group, match, draw.empirical = TRUE)

Arguments

• x: an object of "Logistic" class. See Details .
• item: numeric: number of item to be plotted
• item.name: character: the name of item to be used as title of plot.
• group.names: character: names of reference and focal group.
• Data: numeric: the data matrix. See Details .
• group: numeric: the vector of group membership. See Details .
• match: character or numeric: specifies observed score used for matching. Can be either "score", or numeric vector of the same length as number of observations in Data. See Details .
• draw.empirical: logical: whether empirical probabilities should be calculated and plotted. Default value is TRUE.

Details

This function plots characteristic curves of 2PL logistic DIF model fitted by difLogistic() function from difR package using ggplot2.

Data and group are used to calculate empirical probabilities for reference and focal group. match should be the same as in x$match. In case that an observed score is used as a matching variable instead of the total score or the standardized score, match needs to be a numeric vector of the same the same length as the number of observations in Data.

Examples

# loading libraries
library(difR)

# loading data based on GMAT
data(GMAT, package = "difNLR")
Data <- GMAT[, 1:20]
group <- GMAT[, 21]

# DIF detection using difLogistic() function
x <- difLogistic(Data, group, focal.name = 1)
# Characteristic curve by logistic regression model
plotDIFLogistic(x, item = 1, Data = Data, group = group)

# Using name of column as item identifier
plotDIFLogistic(x, item = "Item1", Data = Data, group = group)

# Renaming reference and focal group
plotDIFLogistic(x, item = 1, group.names = c("Group 1", "Group 2"), Data = Data, group = group)

# Not plotting empirical probabilities
plotDIFLogistic(x, item = 1, draw.empirical = FALSE)

See Also

difR::difLogistic(), ggplot2::ggplot()

Author(s)

Adela Hladka

Institute of Computer Science of the Czech Academy of Sciences

hladka@cs.cas.cz

Patricia Martinkova

Institute of Computer Science of the Czech Academy of Sciences

martinkova@cs.cas.cz