X-Bar Charts With Variable Sampling Intervals


Reynolds Jr., Marion R.; Amin, Raid W.; Arnold, Jesse C.; Nachlas, Joel A.   (1988, ASQC and the American Statistical Association)   Virginia Polytechnic Institute and State University, Blacksburg, VA; University of West Florida, Pensacola, FL; Virginia Polytechnic Institute and State University, Blacksburg, VA

Technometrics    Vol. 30    No. 2
QICID: 9349    May 1988    pp. 181-192
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Article Abstract

The usual practice in using a control chart to monitor a process is to take samples from the process with fixed sampling intervals. This article considers the properties of the X-bar chart when the sampling interval between each pair of samples is not fixed but rather depends on what is observed in the first sample. The idea is that that time interval until the next sample should be short if a sample shows some indication of a change in the process and long if there is no indication of a change. The proposed variable sampling interval (VSI) X-bar chart uses a short sampling interval if X-bar is close to but not actually outside the control limits and a long sampling interval if X-bar is close to target. If X-bar is actually outside the control limits, then the chart signals in the same way as the standard fixed sampling interval (FSI) X-bar chart. Properties such as the average time to signal and the average number of samples to signal are evaluated. Comparisons between the FSI and the VSI X-bar charts indicate that the VSI chart is substantially more efficient.


Control charts,Process control,Quality control (QC),Variable time delay

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