Table 13.2.4 gives the number of units produced by a
company each week during part of 1994 which require rework at certain stages
of production because of faults. Rework is additional work required to bring
a substandard unit up to standard and is an additional cost one wishes to
avoid. We omit further details for reasons of confidentiality. The weeks are
consecutive except for the last one (in 1995) when the factory closed down over
the Christmas holiday period.17 We want to chart the percentages requiring
rework over time to see whether the process is in control. Such a chart is given
in Fig. 13.2.4.