Roles & Benefits  |  Process Control  |  Process Capability  |  Course Outline


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.  The Cp index uses a standard deviation estimated from R-bar.  The Ppk index uses a standard deviation that is calculated directly from the data (and usually longer-term).  The Cpm and Ppm indices use the mean squares deviation from the target rather than the process average.

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