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When
to Use Process Capability Charts
Process capability attempts
to answer the question: can we consistently meet customer requirements?
The number one limitation of process capability indices is that they are
meaningless if the data is not from a controlled process. The reason is
simple: process capability is a prediction, and you can only predict something
that is stable. In order to estimate process capability, you must know the
location, spread, and shape of the process distribution. These parameters
are, by definition, changing in an out of control process. Therefore, only
use Process Capability indices if the process is in control for an extended
period.
The same argument holds
for a Histogram. If the purpose of creating a histogram is to see the "shape"
of the process, it will be very misleading if the process is not stable.
For example, a process that is stable could very well have the same histogram
as a process undergoing a trend, since the only difference in the data would
be the order of the points. Since the histogram does not consider the sequence
of the points, you would see no difference between histograms.
See also:
Distributions
Rational
subgroups
Process
Performance
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