his type of chart is used for plotting the number
of defects (not defectives) in each subgroup, when the subgroup size is the same
for each run. It also detects the presence of special causes of variation.
Just to remind you: a complex product is not necessarily a defective one,
even if some defects still exists.
An example where C chart is used is the following:
a refurbishing computer monitor company controls the polishing of monitor
screens, in order to detect the surface of scratches in the entire visible
surface of the monitor. An important assumption is that all monitors are
the same size, let's say 17". What the quality control is doing is to
determine surface scratched after refurbishment. The result is reported
to entire surface of monitor. There is a certain limit of permitted scratches;
if that limit is exceeded, the monitor is declared low grade and sold out
to a lower price. An example of collected data is the following:
Supposing that monitor number 4 shows a scratched surface of
7 percent, the resulting chart shows the observation number 4 outside the control limits,
meaning a lack of control in polishing process (see the diagram on the right).
Further investigations have to be carried
out, in order to determine the cause of that problem.