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Should we calculate Cronbach's alpha instead of KR20/KR21 for measuring the reliability of an achievement test, or a test with dichotomous data?

chl
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abhijit
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    Of possible interest: [Assessing reliability of a questionnaire: dimensionality, problematic items, and whether to use alpha, lambda6 or some other index?](http://stats.stackexchange.com/q/11628/930), [Omega vs. alpha reliability](http://stats.stackexchange.com/q/39182/930) – chl Oct 04 '13 at 14:32

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KR 20 is an algebraic simplification of alpha for dichotomous data. If your data are dichotomous then you can use either one -- they will give the same result.

KR 21 is an approximation that is of historical interest only. Don't use it.

Ray Koopman
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