Cp index

A Process Capability Index. The ratio of the tolerance (specification, or permitted amount of variation) to the process variation.

A value of 1 indicates the process variation exactly equals the tolerance.

Values of less than 1 indicate the allowable variation (the tolerance) is less than the process variation.

Values of more than 1 indicate that the process variation is less than the tolerance.

Values greater than 1 are most desirable, and many customers recommend values of 1.33 or greater. This allows a margin of error in case a process shift occurs that is not immediately detected.

Normal distributions:

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Johnson distributions:

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where:

σx is Process Sigma, calculated using the Moving Range (when subgroup size = 1) or Subgroup Sigma (subgroup size >1) statistic.

Ordinate0.99865 and ordinate0.00135 are the z-values of the non-normal cumulative distribution curve at the 99.865 percentage point and the 0.135 percentage points, respectively.