We
soon learned that control charts are a wonderful means of displaying data to use to
identify information, when people understand what the data mean and how to interpret
the charts. So while we have titled this section ???The Tyranny of Control Charts,???
that is really a misnomer. We will discuss control charts at length.
Control charts are used to identify process variation over time. All processes
vary. The degree of variance and the causes of the variance can be determined using
control charting techniques. While there are many types of control charts, the ones
we have seen the most often are the:
c-chart. This chart uses a constant sample size of attribute data, where the
average sample size is greater than 5. It is used to chart the number of defects
(like 12 or 15 defects per thousand lines of code). c stands for the number of
nonconformities within a constant sample size.
u-chart. This chart uses a variable sample size of attribute data. This chart
is used to chart the number of defects in a sample or set of samples (like 20
out of 50 design flaws were a result of requirements errors).
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