Figure 2.5
A typical pair of Shewhart charts. (a) Averages chart and
(b) ranges chart. Point A shows a lack of control of averages only,
point B of ranges only and point C of both together.warning limits (inner control limits) and action limits (outer control limits). These represent the values
at which there is respectively 95% (1.96σ) or 99.9% (3.09σ) confidence that the process is going out of
control. The use of the averages chart is thus based on an estimate of the variability and an assumption
that this remains constant. It is hence invalidated if a significant change in variability occurs. Therefore
the ranges chart fulfils a second important function in providing a check on such changes in variability.
Figure 2.5 shows a pair of Shewhart charts. Point A illustrates the pattern which occurs when the
averages are going out of control but the ranges remain in control. This would suggest perhaps that the
supply of one of the feedstocks for the process is being interrupted. Point B shows the ranges going out
of control with the averages remaining under control. One explanation of this would be that the mixing
had become inefficient. Finally, point C shows both averages and ranges going out of control which
implies a serious fault in the process.