Explanation of Discrimination Index

The formula being used for the Discrimination Index figure on the Test Item Analysis report is:

DI = (a – b) / n

Where:

DI Discrimination Index
a Response frequency of the upper 27% of the scorers
b Response frequency of the lower 27% of the scorers
n Number of respondents in the upper 27% of the scorers

The results typically are a range of values (-1 to 1) and the significance of those values may be read as the more positive, the better the item distinguishes between those who have mastered the subject and those who have not. This also means that a negative result for Discrimination Index means that those who scored lower on the exam did better on this question than those who did well on the exam, which an instructor may find interesting.