Improving QC Efficiency with Zone Control Charts

Article

Jaehn, Alfred H.   (1987, ASQC)   Consolidated Papers, Inc., Wisconsin Rapids, WI

41st Annual Quality Congress, May 1987, Minneapolis, MN    Vol. 41    No. 0
QICID: 3348    May 1987    pp. 558-563
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Article Abstract

This paper introduces a unique control chart method that can be employed in the chemical and process industries to make quality control on the operating floor fast, simple, and efficient. Seven straight lines divide the chart into six equal zones, corresponding to one, two, and three sigma limits. Weighted scores are assigned to each zone. Appropriately termed "zone control chart," it combines the "run sum" concept developed by early SQC practitioners with standard control chart tests for instability.The zone control chart is an all-purpose quality control tool that can be universally applied to any process where data are normally distributed. The need for designing and constructing a separate control chart for each individual process and measurement is eliminated.Zone control charts make the operator's job easier. Tediously locating exactly where to plot points is no longer necessary. Instead, circles containing a cumulative zone score are placed in the corresponding zone. Corrective action is signaled when the cumulative score reaches a critical value. Knowledge of various control chart rules for identifying a shift in-process level is no longer a requirement. Statistically sound tests for this purpose are automatically incorporated into the procedure.This paper includes an illustration of the efficiency of zone control charts in detecting changes in the process mean, in terms of average run length. Comparisons with conventional control chart procedures are made. In addition, experiences with the application of zone control chart procedures in a paper mill are cited.

Keywords

Chemical and process industries


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