Control Charts

Control charts are much used in quality control to track statistics such and the mean and standard deviation in a production process.

  • They consist of points representing a statistic being tracked plotted against time.

  • The mean of this statistic – the mean, the mean variance typically - using all the samples is calculated.

  • A centre line is drawn at the value of the mean of the statistic

  • The standard errorof the statistic in question using all the data is found.

  • Upper and lower control limits,and andthat indicate the threshold at which the process output is considered statistically 'unlikely' are drawn typically at 3 standard errors from the mean line(above).

The chart may have other optional features, including:

  • Upper and lower warning lines typically two standard errors above and below the centre line.

  • Division into zones, with the addition of rules governing frequencies of observations in each zone.

  • Annotation with events of interest, as determined by the Quality Engineer in charge of the process's quality.

A control chart is shown below.

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