Process capability

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The Process Capability Study answers the question, "is my process good enough?" This is quite different from the question answered by a Control chart, which is, "has my process changed?"

Properly, use of a Control Chart to establish that a process is stable and predictable precedes the use of a Capability Study to see if items produced by the process are good enough.

Four indices are produced by a Capability Study, the Cp index, Pp index, Cpk index, and Ppk index. The most optimistic of these is Cp, which disregards centering, and is insensitive to “shifts and drifts” (special cause) in the data. It indicates what the capability of the process would be if no such problems existed. The most realistic estimate is Ppk, which indicates how the process really is. Because of their different properties, experienced users can compare these indices, and know what type remedial action a process needs: removal of special cause, centering, or reduction of natural variation.

In the common case, the same formula that produces Cp also produces Pp. The difference is in how standard deviation is estimated. The same is true for Ppk and the Cpk formula.

Generally, Ppk numbers below 1.0 indicate a process in need of work, while numbers as high as 1.5 indicate an excellent process.

If a process is stable and predictable, and has consistently provided high Ppk numbers for a long period, the process is a candidate for removing final inspection, which is likely to produce more defects than it finds. The process should then have the input variables controlled, and undergo periodic audits.

Perhaps the most common Capability Study error is requesting suppliers to provide just Cpk, along with shipped goods. Cpk can easily be manipulated by selectively changing the order of the data. To get spectacularly inflated values, simply sort the data. If you are to rely on a single index, then Ppk is the appropriate choice. Authoritative texts allow the substitution of Cpk for Ppk if the process is stable and predictable. This is allowed because in that case, they are equal.