This paper presents a review of the exact formulae for control limits for P charts and C charts. Also presented are appropriate graphical methods so that these limits may ho easily evaluated. The use of the exact methods, given appropriate tables and/or curves, is no more difficult than the approximate methods which are prevalent in the literature. This note is not intended to criticize the approximate techniques which have been used ; rather it is intended to illustrate the use of existing theory whereby the errors of approximation may be eliminated.
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Wortham et al. (1972) studied this question.