Discussion
Which of the following is/are true about the item discrimination index ?
(I) It is a measure of reliability
(II) It is a measure of validity
Answer: B
The item discrimination index is a measure of validity (statement II only).
In item analysis the discrimination index shows how well an item separates people who do well on the whole test (or on an outside criterion) from those who do badly, usually as a difference in proportions passing between the top and bottom groups or as an item-total correlation. That is an item-level validity index: an item that discriminates well is measuring what the test as a whole measures.
Reliability belongs to the test's consistency of scores, and at item level it is a separate index (an item reliability index, which also depends on the item's spread). The discrimination index is not that, so statement (I) is not what the index measures. (C) and (D) follow from the two statements and are therefore wrong too.
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