Monitoring variability is a vital part of modern statistical process control. The conventional Shewhart R and S charts address the setting where the in-control process readings have a constant variance. In some settings, however, it is the coefficient of variation, rather than the variance, that should be constant. For example, this setting is common in clinical chemistry, and then conventional R and S charts cannot be used. This paper develops a chart, equivalent to the S chart, for monitoring the coefficient of variation using rational groups of observations.
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