KR-21 Calculator
Estimate the reliability of a right/wrong test from summary statistics alone. Enter the number of items, the mean total score and the variance of scores.
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KR-21 vs KR-20
KR-21 is a quicker estimate than KR-20. It needs only the number of items, the mean score and the score variance, rather than every student’s answer to every item. In exchange it assumes all items are equally difficult, an assumption that rarely holds exactly, so KR-21 is usually a slight underestimate of the true reliability.
When it is handy
Use KR-21 when you have the summary statistics from a scoring report but not the full item-by-item response matrix. If you do have the raw right/wrong responses, KR-20 gives a more accurate figure.
Frequently asked questions
What is an acceptable KR-21 value?
As with KR-20 and Cronbach’s alpha, 0.70 and above is generally considered acceptable for a test, and 0.80 and above is good.
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