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The process potential index, or Cp, measures a process's potential
capability, which is defined as the allowable spread over the actual spread. The
allowable spread is the difference between the upper specification limit and the
lower specification limit. The actual spread is determined from the process data
collected and is calculated by multiplying six times the standard deviation, s.
The standard deviation quantifies a process's variability. As the standard
deviation increases in a process, the Cp decreases in value. As the standard
deviation decreases (i.e., as the process becomes less variable), the Cp
increases in value.
By convention, when a process has a Cp value less than 1.0, it is considered
potentially incapable of meeting specification requirements. Conversely, when a
process Cp is greater than or equal to 1.0, the process has the potential of
being capable.
Ideally, the Cp should be as high as possible. The higher the Cp, the lower
the variability with respect to the specification limits. In a process qualified
as a Six Sigma process (i.e., one that allows plus or minus six standard
deviations within the specifications limits), the Cp is greater than or equal to
2.0.