For an MCQ, the proportion correct is 0.70 in the upper group and 0.85 in the lower group. Using D = p_U – p_L, what is the discrimination index?

Correct answer: -0.15 D = p_U − p_L = 0.70 − 0.85 = −0.15, a negative discrimination (more lower-group than upper-group learners got it right).

• 0.15 +0.15 has the wrong sign.
• 0.85 0.85 is the lower-group proportion, not the index.
• -0.85 −0.85 is not the difference between 0.70 and 0.85.