Page 344 - Six Sigma Demystified

324 Six SigMa DemystifieD

Process Capability index

Process capability indices attempt to indicate, in a single number, whether a
process can consistently meet the requirements imposed on the process by in-
ternal or external customers. Much has been written about the dangers of these
estimates, and users should interpret capability only after understanding the
inherent limitations of the specific index being used.

When to Use

Process capability attempts to answer the question, Can we consistently meet
customer requirements? Once process capability has been calculated, it can be
converted into a corresponding defects per million opportunities (DPMO) or
sigma level. It is important to remember that process capability indices are
meaningless if the data are 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 defini-
tion, changing in an out-of-control process. Therefore, only use process capabil-
ity indices (and resulting DPMO and sigma-level estimates) if the process is in
control for an extended period.
The statistic C can be used when there are both and upper and lower speci-
p
fications for the process. C can be used when only one or the other or both
pk
specifications are provided.

Measure Stage

• To provide a process baseline estimate for a controlled process

Improve Stage
• To provide an estimate of the improved process and to verify that the
improved process is in a state of statistical control

Control Stage
• To continuously monitor the process to verify that it remains in a state of
statistical control at the desired capability level