## 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.