My Reading on ASQ CQA HB Part V Part 2

Charlie Chong/ Fion Zhang
Part VD2
VD2. Process Performance Metrics
Process capability is the range within which a process is normally able to operate given the inherent variation
due to design and selection of materials, equipment, people, and process steps. Knowing the capability of a
process means knowing whether a particular specification can be held if the process is in control. If a process is
in control, one can then calculate the process capability index. Several formulae are used to describe the
capability of a process, comparing it to the specification limits; the two most popular indexes are Cp and Cpk.
• C p indicates how the width of the process compares to the width of the specification range, while
• C pk looks at whether the process is sufficiently centered in order to keep both tails from falling outside
specifications.
Following are the formulae:
C p =
Specification range
Process range
=
Upper Spec ;Lower Spec
6σ
C pk =
Upper Spec ;Lower Spec
3σ
Upper Spec ;Averafge
3σ
whichever smaller ?
,
or
C pk =
Upper Spec ;Lower Spec
3σ
or
Upper Spec ;Averafge
3σ
, whichever smaller
Process Capability
Note that the σ used for this calculation is not the standard deviation of a sample.
It is the process sigma based on time- ordered data, such as given by the formula R-bar/d 2 .
Following are the rules often used to determine whether a process is considered capable:
• Cpk > 1.33 (capable)
• Cpk = 1.00 – 1.33 (capable with tight control)
• Cpk < 1.00 (not capable)