Fraction Defective Chart for High Quality Processes with Adjusted Control Limits to Improve In-control Performance

The np-chart is used to monitor the number of nonconforming items in a sample. The performance of the np-chart in Phase II depends on the accuracy of estimated parameter in Phase I. Despite the fact that taking large sample sizes ensure the performance of estimated parameter charts, it can be impractical for attribute control charts. Recently, the performance of traditional c-chart and np-chart with some adjustments have been studied. In practice, high quality processes with very low rate of nonconformities are often observed. The traditional control charts suffer a serious inaccuracy in control limits specification in this way. Having all this in mind, this paper presents a new np-chart based on simple adjustments derived from Cornish-Fisher expansions to improve such inaccuracy with the application of bootstrap. Based on the conditional in-control average run length (ARL0), their performances are compared through a simulation study.

Keywords:

np-chart , Bootstrap , Adjusted limits , Cornish-Fisher expansions , ARL0