• After control charts (X and R) have been developed, and the process is in control, the process capability is 6σ


• The capability index can be calculated for a design



• The capacity Ratio can also be calculated



• The Capability Index is another useful measure



• Some general notes about these indices

- Cp is independent of process centre

- Cp = Cpk when the process is centered

- Cpk <= Cp

- Cpk of 1 means the process is OK

- Cpk < 1 means product out of spec

- Cpk = 0 process centre at a tolerance limit

- Cpk < 0 process centre is outside the tolerance range





• If Cp or Cpk are both too large (>>2) the process may be too good, suggesting less expensive, easier processes could be considered.


• A good rule of thumb for using Cp and Cpk is that if new equipment is being purchased, try to get values above 1.5, if the equipment is already available, 1.33 is acceptable.



• Try Probs #6 pg124, #32,33 pg128



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