Stepwise SPC Chart and Its Application


Wong, Henry H.   (1988, ASQC)   Polysar Limited, Sarnia, Ontario, Canada

Annual Quality Congress, Dallas TX    Vol. 42    No. 0
QICID: 3454    May 1988    pp. 403-408
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Article Abstract

The study of many of our chemical processes indicates that process variation has two distinct components: the gradual drifting of the process mean and short-term fluctuation around the process mean. The process drift has made direct application of the traditional Shewhart control chart difficult and, in many cases, inappropriate. This very nature of our processes is a new quality challenge because all our training programs have been based on traditional SPC methods and all our process control actions are based on the Shewhart control chart. In addition, all our quality measurements are also based on the assumption that there is no drift in our testing processes.

It is important for us to continue to apply the training we have already provided for our people, and it is equally important for us to make sure that the interpretation of the quality data is correct. The way to achieve both of these goals is the introduction of the Stepwise SPC Chart.

In applying this method, we first approximate the gradual drift of the process with a step function and then construct a standard Shewhart control chart around each segment of the step function. Since the development of the step function is based on sequential analysis, a method similar to the CUSUM chart, this method combines the benefits of the two commonly used control charts: - the Shewhart control chart and the CUSUM chart.

The Stepwise SPC Chart serves two separate purposes:

  1. To clearly indicate any significant drift of the process and trigger the prescribed corrective actions;
  2. To signal that the fluctuation of the process may have been caused by an assignable cause.
With this method, we can now understand our processes better and we also have a better appreciation of the true quality of our products. As a result, we can design and implement quality improvement projects more effectively.


Chemical and process industries

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