Quality News

Five Ways to Use Shewhart’s Charts
Donald J. Wheeler

The many different ways of using control charts in both service and manufacturing applications may be summarized under five major headings. These five categories are arranged in order of increasing sophistication below.



The first of these is that of Report Card Charts. Report Card Charts are charts kept for the files. They may be occasionally used for information about how things are going, or for verification that something has or has not occurred, but they are not used in real time for operating or improving the processes and systems present. This is a valid but weak usage of control charts.

The next category consists of Process Adjustment Charts. Some product characteristic may be plotted on a control chart and used in a feedback loop for making process adjustments, or some input characteristic may be tracked and used in a feed-forward loop for the same purpose. In many cases these Process Adjustment Charts will result in substantially more consistent operations than was the case prior to the use of control charts. (This assumes that one will know how to properly adjust the process. In some cases such knowledge can only be gained by some of the following uses of control charts.) However, once this initial improvement has been achieved, Process Adjustment Charts simply strive to preserve the new status quo. The potential for dynamic and continual improvement is missing from this usage of the charts. Unfortunately, this seems to be the only usage considered in most of the articles recently published in the journals.



The third category is Process Trial Charts. These are charts used to analyze the data from simple experiments performed upon the process. This short-term usage of control charts is a simple and easy-to- understand alternative to the use of ANOVA and other statistical techniques. This usage is often found in
conjunction with the next category.

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