Improving the Zone Control Chart

Article

Fang, Jengjung; Case, Kenneth E.   (1990, ASQC)   Oklahoma State University, Stillwater, OK

Annual Quality Congress, San Francisco, CA    Vol. 44    No. 0
QICID: 9499    May 1990    pp. 494-500
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Article Abstract

The Zone Control Chart looks like a regular Shewhart chart, modified such that either side of the center line is divided into three zones. It also looks much like a Shewhart chart that has been modified for use with the AT&T Runs Rules to enhance its sensitivity and ability to detect process shifts. The Zone Control Chart uses a zone weighting scheme of 1, 2, 4 (for the three zones), and 8 (outside control limits), in accordance with a set of rules, to determine when a special cause (out-of-control condition) has occurred.Published Zone Control Chart descriptions indicate that the chart performs better than a Shewhart chart using the AT&T Runs Rules. The performance measures quoted, however, focus only on the ability of the chart to detect special causes. It is equally important to deal with not "detecting" a special cause when one does not exist.

This paper presents the Zone Control Chart and models it mathematically to determine analytically how it performs when subjected to various process centering shifts, includinq no process shift. Its performance is discussed, and the paper concludes by suggesting changes that can adjust Zone Control Chart performance to fit the intended use.

Keywords

Chemical and process industries


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