Shewhart Charts & Pre-Control: Rivals Or Teammates?


Martin, Tripp   (1992, ASQC)   Peterson Spring, Southfield, Michigan

Annual Quality Congress, Nashville TN    Vol. 46    No. 0
QICID: 9811    May 1992    pp. 16-22
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Article Abstract

The past few years have seen a debate between advocates of classical Shewhart control charts and advocates of Pre-Control. Control charts statistically monitor the variation in a process, while Pre-Control monitors variation in a product with respect to specifications. Both techniques were developed to aid in the economic production of parts within specification. This paper examines the claims made by proponents of each technique from the practitioner's perspective. Pre-Controllers advocate their method as a replacement for costly and time-consuming charts while charters respond with detailed statistical analyses of the insensitivity of Pre-Control. The two methods are examined with respect to the process improvement cycle, which consists of three stages: Process Analysis; Process Maintenance; and Process Improvement. It is in the second area that these methods are examined, for they are most useful in that arena. The questions of "Does it work?," "Can I use it?," and "When should I use it?" are discussed for each tool. The published supplier quality requirements of Chrysler, Ford, and General Motors are examined regarding acceptance of each of these methods. The place of both tools in variation reduction and continual improvement is considered. Control charts should be used primarily as a tool for problem solving, not process monitoring. Replacement of control charts in some areas with other monitoring methods, such as Modified Pre-Control, will result in a more cost efficient operation. This approach advocates the use of both tools in the manufacturing environment, as opposed to an outright rejection of either method.


Comparison,Control charts,Precontrol,Process control,Process improvement,Shewhart control chart,Standards and specifications,Statistical process control (SPC)

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