Kuder–Richardson formulas
two methods, Kuder–Richardson formula 20 (K-R 20) and Kuder–Richardson formula 21 (K-R 21), for assessing the internal consistency reliability of a test or subtest made up of dichotomous variables. The K-R 21 requires that all of the dichotomous variables be equally difficult, whereas the K-R 20 does not. For example, a teacher could use the K-R 21 formula to calculate internal consistency reliability for a test of 20 equally difficult items but would use the K-R 20 formula for test items differing in difficulty level. [Frederic Kuder (1903–2000) and Marion Webster Richardson (1896–1965), U.S. psychologists]