Control charts are much used in quality control to track statistics such and the mean and standard deviation in a production process.
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They consist of points representing a statistic being tracked plotted against time.
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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
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The standard error
of the statistic in question using all the data is found. -
Upper and lower control limits,
and and
that 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:
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Upper and lower warning lines typically two standard errors above and below the centre line.
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Division into zones, with the addition of rules governing frequencies of observations in each zone.
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Annotation with events of interest, as determined by the Quality Engineer in charge of the process's quality.
A control chart is shown below.
