Should Observations Be Grouped for Effective Process Monitoring?

Journal of Quality Technology vol. 36 issue 4 - October 2004

Abstract: [This abstract is based on the authors' abstract.] During process monitoring, it is assumed that a special cause will result in a sustained shift in a process parameter that will continue until the shift is detected and the cause is removed. In some cases special, however, causes may produce a transient shift that lasts only a short time. Control charts used to detect these shifts are usually based on samples taken at the end of the sampling interval d, but another option is to disperse the sample over the interval. For this purpose, combinations of two Shewhart or two cumulative sum (CUSUM) charts are considered. Results demonstrate that the statistical performance of the Shewhart chart combination is inferior compared with the CUSUM chart combination.

Keywords: Average time to signal - Cumulative sum control chart (CUSUM) - Exponentially weighted moving average control charts (EWMA) - Statistical process control (SPC) - Shewhart control chart - Shifts and drifts in tolerances - Sampling interval

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