Use of Boxplots for Process Evaluation

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Abstract
The use of boxplots in place of single points in a quality control chart can provide an effective display of the information usually given in X̄ and R charts, show the degree of compliance with specifications and identify outliers. An example from a process evaluation illustrates the use of the proposed boxplot chart and compares it to the customary X̄ and R charts. To achieve a degree of robustness, the present paper compares several resistant measures of location and dispersion in terms of their efficiency for data from the normal, double exponential and “one-out” distributional situations. The results indicate that the mean and range of the middle half of the sample (suitably defined) combine to yield a boxplot chart with good statistical properties. For each pair of location and dispersion measures studied, a table gives the necessary factors to construct control limits.

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