University
Maths Notes: Probability and Statistics – 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 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:
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.
