Three suggestions for improving control charting procedures

ISSN： 0265-671X

Source： International Journal of Quality & Reliability Management, Vol.12, Iss.5, 1995-05, pp. : 61-74

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Abstract

Discusses some of the practical difficulties commonly experienced with Shewhart control charts, and makes three suggestions for overcoming these difficulties. The first suggestion is to distinguish the "out-of-control" points by some means other than control lines - perhaps by ringing them on the graph - and to avoid the phrases "in control" and "out of control". Argues that this would avoid some common misconceptions. The second suggestion is a procedure, method A, for calculating action and control levels based on the median and the quartiles of the points plotted. This is a much simpler and more general procedure than the usual ones. Argues that method A has a number of advantages over the standard procedures (for mean charts, range charts, P charts, C charts and so on) provided that a long sequence of sample data is available. The third suggestion, method B, is an alternative to the second: this is a bootstrapping procedure which is conceptually simple but requires considerable computer power for implementation. Method B also has advantages over the standard procedures whenever the points plotted are calculated from a random sample of data. In addition, unlike method A, it is feasible if there is only a short sequence of samples of data available. All three suggestions are designed to simplify the idea of control charts, and so are likely to facilitate the learning process.