Suppose the high-achieving group contains 20 students, of whom 18 answer an item correctly, while 8 students in the low-achieving group answer it correctly. Using D = (NH − NL) / n, where n = 20, the discrimination index is:

A. 0.20
B. 0.40
C. 0.50
D. 0.90
Correct Answer: C. 0.50

D = (NH − NL) / n = (18 − 8) / 20 = 10/20 = 0.50.

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