Item Facility vs. Item Discrimination

Item facility describes group success

For a dichotomously scored item, facility is commonly the proportion of examinees who answered correctly. A value near 1 indicates that most of the group answered correctly; a value near 0 indicates that few did.

It is sometimes called a difficulty index, though the direction can be confusing: higher facility means an easier item for the observed group.

Discrimination describes differentiation

Discrimination indicators estimate whether success on an item is associated with stronger overall test performance. A weak or negative value can signal a wrong key, ambiguous wording, misalignment, unusual content, or sampling noise.

It can also reflect an item measuring something intentionally different, so the content must be reviewed before a decision is made.

Interpret the pattern, not one number

A relatively easy item can still discriminate, and a difficult item can be useful when it targets advanced learning. Use facility, discrimination, point-biserial evidence, distractor behavior, reliability, and curriculum alignment together.

Documenting the review creates an evidence trail for improving both the test and the question bank.

Frequently asked questions

Is a high facility value good?

It means many examinees answered correctly. Whether that is desirable depends on the item purpose, expected mastery, and assessment design.

What can negative discrimination indicate?

Possible causes include an incorrect key, ambiguity, misalignment, guessing, unusual group behavior, or a small unstable sample. The item and its administration should be investigated.