ExamopotamiaHow it works
KR-21 = (k ÷ (k − 1)) × (1 − M × (k − M) ÷ (k × s²)), where k is the number of questions, M the average number correct and s the standard deviation of total scores. It is a quick estimate; Examopotamia calculates the more exact KR-20 automatically for every test. Read test reliability explained for what the number means.
Frequently asked questions
What is KR-21?
KR-21 is a quick estimate of test reliability for right-or-wrong questions, using only the number of questions, the average score and the standard deviation. It assumes questions are of similar difficulty, so it slightly underestimates KR-20.
What is a good reliability score?
Around 0.80 or higher is good for graded exams and hiring decisions, 0.70 is acceptable for screening and quizzes, and 0.90+ is expected for certification.
How can I raise reliability?
Add good questions on the same skill, remove confusing ones, replace questions that everyone gets right or wrong, and improve wrong answer options nobody picks.