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PROCESS CAPABILITY
Once you have brought a process under statistical
control (no assignable/special causes or other non-normal distributions of subgroup
average plot points; i.e., there are only common causes of variation remaining), you can
calculate a process capability index. In addition to the process being under statistical
control (so that it is repeatable, or predictable), the raw data must be normally
distributed. This is because you will be using an estimate of the process standard
deviation to calculate the process capability. Standard deviation does not apply to
non-normal distributions (more advanced techniques provide data transformation to help in
these situations). It is not necessary, however, for the raw data to be normally
distributed in order to use SPC charts. This is due to the Central Limit Theorem.
A Cp Index is a ratio of the process specification range (Upper Spec Limit - Lower Spec
Limit) divided by 6 standard deviations. A Cpk index will be the same as a Cp if a process
mean is centered on the spec target, and lower if the mean is not centered on target. |
