Skip to content
Glossary

What Is Item Discrimination? A Quiz Quality Metric

May 27, 20264 minSarah Mitchell
Share:XLinkedIn
In this article
  1. 1.How it's calculated (simplified)
  2. 2.Why it matters
  3. 3.How to find low-discrimination questions
  4. 4.Related concepts
  5. 5.Related reading

Short answer. Item discrimination is a statistical measure of how well a quiz question distinguishes between students who know the material and students who don't. A good question is one that strong students get right and weak students get wrong; a bad question is one where the pattern is reversed or random.

How it's calculated (simplified)

For each question, you compute:

  • The proportion of **high-scoring students** (typically top 27% on the overall quiz) who got the question right
  • The proportion of **low-scoring students** (bottom 27%) who got it right
  • The difference is the **discrimination index** (D)

    • D ranges from -1 to +1:

  • +1: All strong students right, all weak students wrong — ideal
  • +0.3 to +0.5: Good question, keep
  • 0 to +0.2: Weak question, revise
  • Negative: Weak students outperformed strong students — broken question, throw out

    • Why it matters

    A high-difficulty question can still be useful if it has good discrimination. A low-difficulty question with poor discrimination is worse than useless — it makes the quiz feel rigorous without testing anything.

    The classic "broken question" pattern: question has poor discrimination because one of the distractors is *actually defensible*. Strong students see the ambiguity and pick the "wrong" answer; weak students don't notice and pick the keyed answer. Fix: rewrite the distractor.

    How to find low-discrimination questions

    Most LMSs and quiz tools surface this:

  • Canvas, Blackboard, Moodle: quiz analytics often show discrimination index per item
  • SimpleQuizMaker: per-question analytics show miss rate; high-scorer vs low-scorer split available
  • Manual: For small classes, sort students by total score; check each question's distribution

    • After a quiz, glance at the discrimination indices. The 3-5 lowest-scoring items are candidates for rewriting before the next administration.

    Related concepts

  • Difficulty — proportion who got the item right (separate from discrimination)
  • Point-biserial correlation — a more rigorous version of discrimination
  • Cronbach's alpha — reliability of the quiz overall

    • Related reading

  • [How to Write Good Quiz Questions](/blog/how-to-write-good-quiz-questions)
  • [Multiple Choice Distractor Design](/blog/multiple-choice-distractor-design)
  • [Quiz Analytics — Teacher Guide](/blog/quiz-analytics-teacher-guide)
  • [How to Write Hard Quiz Questions](/blog/how-to-write-hard-quiz-questions)

    • See per-question analytics on your next quiz.

    Get weekly study & quiz tips

    Join teachers and students who get practical tips on quizzing, active recall, and AI-powered learning.

    Share:XLinkedIn

    Sarah Mitchell

    Curriculum Designer & Former High School Teacher

    Ready to create your first quiz?

    Use AI to generate quizzes from your own study materials in seconds.

    Try SimpleQuizMaker Free

    Related Articles

    Glossary

    What Is Item Difficulty? The Quiz Quality Metric Most Teachers Ignore

    4 min

    Glossary

    What Is FSRS? The Modern Spaced Repetition Algorithm Explained

    4 min

    Glossary

    What Is the Testing Effect? The Research Behind Active Recall

    4 min

    Back to Blog