Discussion
Item discrimination index is based on :
Answer: B
The item discrimination index is based on the passing percentage of respondents on an item from the upper group and lower group on a criterion.
Test-takers are ranked on the total score (or another criterion), and the top and bottom groups, often 27% each, are compared. The index is the proportion passing the item in the upper group minus the proportion passing in the lower group, \(D = p_{U} - p_{L}\). A good item is passed by many more high scorers than low scorers.
The overall difficulty of the test is a different statistic, and using only the upper or only the lower group cannot show discrimination, which is by definition a comparison between the two.
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