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Measuring Process Capability
The primary measures of process capability used are the process
capability ratio, the process capability index, and the Cpk.
Some
organizations also use Process Performance Indicators (Pp and Ppk),
which are related to Cp and Cpk.
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Process
Capability Ratio (Cr)—Describes what portion of the
specification the process is taking up. The Cr should be no
greater than 75% in order for the process to be considered
capable.
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Process
Capability Index (Cp)—The inverse of the capability
ratio. The Cp should be at least 1.33 in order for the process
to be considered capable.
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Cpk—The capability measure preferred in most industries because it
indicates whether it is capable and how well-centered the process
is. The Cpk is the smaller of the Cpu (capability of the upper half
of the process) and the Cpl (capability of the lower half of the
process). Cpk = smaller of {Cpu, Cpl}
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