Highly specifiable np control chart

An algorithm is presented in this article, which gives the sample size and control limits for an np chart (3-triplet np chart with fractional limits) based on the specified values of (a) the in-control fraction nonconforming value p/sub 0/, and the downward and upward shifted values p/sub -/ and p.; (b) the design run lengths (RL); and (c) the corresponding confidence levels. Specifically, whenever the nonconforming process fraction is equal to one of the three specified values, the np chart will produce a signal in a predetermined run length with a predetermined confidence level. This feature makes the operating characteristics of the np chart highly specifiable and provides the quality assurance (QA) engineers with more control on the design and operation of the np charts. The new np chart is designed by seeking an optimal combination of the sample size and the fractional control limits.