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Print version ISSN 0103-6513On-line version ISSN 1980-5411


EPPRECHT, Eugenio Kahn; LEIRAS, Adriana  and  COSTA, Antonio Fernando Branco. Design of the statistical control of low fraction nonconforming processes restricted to small samples. Prod. [online]. 2007, vol.17, n.1, pp.139-150. ISSN 0103-6513.

We present the non-conventional design of np charts used for the surveillance of processes characterized by a very low fraction nonconforming, with a costly and/or time-consuming inspection procedure, which makes the inspection of large samples prohibitive. Under these circumstances, the trade-off between the false-alarm rate and the speed with which the chart signals process deteriorations is the basis to design the control chart, once (depending on the sample size) the conventional np chart with "three-sigma" limits offers little protection against false alarms or is slow in detecting process disturbances. The study was motivated by the real case of a newspapers packaging process. The nominal number of newspapers in the package depends on the newspaper's breadth, and packages with a number of newspapers different from nominal are considered nonconforming.

Keywords : np charts; statistical process control; control by attributes; design of control charts.

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