an object of class "pair" as a result from function pair.
itemnumber
an integer, defining the number of the item to plot the respective category probability for. This is set to an arbitrary default value of itemnumber = 1 to avoid error messages when you forget to choose an item to plot the expected score curves for.
ra
an integer, defining the (logit) range for x-axis
plot
a logical (default plot = TRUE), defining wether to supress plotting an just return a matrix of category probabilities