Kuder and Richardson (1937) developed several reliability coefficients that could overcome the problem of formula_68. They did not give the reliability coefficients particular names. Equation 20 in their article is formula_33. This formula is often referred to as Kuder-Richardson Formula 20, or KR-20. They dealt with cases where the observed scores were dichotomous (e.g., correct or incorrect), so the expression of KR-20 is slightly different from the conventional formula of formula_33. A review of this paper reveals that they did not present a general formula because they did not need to, not because they were not able to. Let formula_72 denote the correct answer ratio of item formula_73, and formula_74 denote the incorrect answer ratio of item formula_3 (formula_76). The formula of KR-20 is as follows.