Control Chart - Chart Details

Chart Details

A control chart consists of:

• Points representing a statistic (e.g., a mean, range, proportion) of measurements of a quality characteristic in samples taken from the process at different times
• The mean of this statistic using all the samples is calculated (e.g., the mean of the means, mean of the ranges, mean of the proportions)
• A centre line is drawn at the value of the mean of the statistic
• The standard error (e.g., standard deviation/sqrt(n) for the mean) of the statistic is also calculated using all the samples
• Upper and lower control limits (sometimes called "natural process limits") that indicate the threshold at which the process output is considered statistically 'unlikely' and are drawn typically at 3 standard errors from the centre line

The chart may have other optional features, including:

• Upper and lower warning limits, drawn as separate 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