Calculates the conventional item discrimination index, based on complete test results only. Follows the formulation described in Lopez (1998), where item discrimination \(d = p(UG) - p(LG)\). UG and LG are the upper and lower groups -- the upper and lower 27% of students as ranked by their overall test scores -- and \(p(UG)\) and \(p(LG)\) are the proportion of correct answers in each group.

addDiscriminationIndex(mctd, percentile = 0.27)



Existing mcTestAnalysis data object


Percentile defining upper and lower groups (standard is 0.27)


The Item Discrimination Index: Does it Work? Tristan Lopez A. Rasch Measurement Transactions, 1998, 12:1 p. 626.