Process capability

A process is a unique combination of tools, materials, methods, and people engaged in producing a measurable output; for example a manufacturing line for machine parts. All processes have inherent statistical variability which can be evaluated by statistical methods.

The Process Capability is a measureable property of a process to the specification. The output of this measurement is usually illustrated by an histogram and calculations that predict how many parts will be produced out of specification.

Two parts of process capability are: 1) Measure the variability of a process, and 2) Compare that variability with a proposed specification or product tolerance.




Contents [hide]
1 Measure the Process
2 Compare Process to a Standard
3 See also
4 External links
5 References



[edit] Measure the Process
The output of a process usually has at least one or more measureable characteristics that are used to specify outputs. These can be analyized statistically. Where the output data shows a normal distribution the process can be described by the process mean (average) and the standard deviation.

A process needs to be established with appropriate process controls in place. A control chart analysis is used to determine whether the process in "in statistical control". If the process is not in statistical control then capability has no meaning. Therefore the process capability involves only common cause variation and not special cause variation.

A batch of data needs to be obtained from the measured output of the process. The more data that is included the more precise the result, however an estimate can be achieved with as few as 17 data points. This should include the normal variety of production conditions, materials, and people in the process. With a manufactured product, it is common to include at least three different production runs, including start-ups.

The process mean (average) and standard deviation are calculated. With a normal distribution, the "tails" can extend well beyond plus and minus three standard deviations, but this interval should contain about 99.73% of production output. Therefore for a normally distribution of data the process capability is often described as the relationship between six standard deviations and the required specification.


[edit] Compare Process to a Standard
The output of a process needs to meet a customer requirement, specification, or product Tolerance (engineering). It is useful to compare the variability of the process to the intended requirement to determine its suitability.

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.



References
Pyzdek, T, "Quality Engineering Handbook", 2003, ISBN 0824746147
Bothe, D. R., "Measuring Process Capability", 2001, ISBN 0070066523
Godfrey, A. B., "Juran's Quality Handbook", 1999, ISBN 007034003
ASTM E2281 Standard Practice for Process and Measurement Capability Indices
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