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Pp and Ppk: Process Performance Indices

Capability indices: Ppk

Ppk is an index of process performance which tells how well a system is meeting specifications. Ppk calculations use actual sigma (sigma of the individuals), and shows how the system is actually running when compared to the specifications. This index also takes into account how well the process  is centered within the specification limits.

If Ppk is 1.0, the system is producing 99.73% of its output within specifications. The larger the Ppk, the less the variation between process output and specifications.

If Ppk is between 0 and 1.0, not all process output meets specifications.

If the system is centered on its target value, Ppk should be used in conjunction with the Pp index. If the system is centered on its target value, Ppk and Pp will be equal. If they are not equal, the smaller the difference between these indices, the more centered the process is.

See also:
>> How do I compare the Cp/Pp and Cpk/Ppk?
>> Cp
>> Cr
>> Cpm
>> Pp
>> Cpk
>> Pr
>> Capability indices

Additional reference material

Additional sections from legacy pp:

Capability indices: Pp

The Pp index is used to summarize a system’s performance in meeting two-sided specification limits  (upper and lower). Like Ppk, it uses actual sigma (sigma of the individuals ), and shows how the system is actually running when compared to the specifications. However, it ignores the process average and focuses on the spread. If the system is not centered within the specifications, Pp alone may be misleading.

The higher the Pp value, the smaller the spread of the system’s output. Pp is a measure of spread only. A process with a narrow spread (a high Pp) may not meet customer needs if it is not centered within the specifications.

If the system is centered on its target value, Pp should be used in conjunction with Ppk to account for both spread and centering. Pp and Ppk will be equal when the process is centered on its target value. If they are not equal, the smaller the difference between these indices, the more centered the process is.

See also:
>> How do I compare the Cp/Pp and Cpk/Ppk?
>> Cp
>> Cr
>> Cpm
>> Ppk
>> Cpk
>> Pr
>> Capability indices

Additional sections from legacy pr:

Capability indices: Pr

The calculations for capability analysis are based on the following assumptions:

  1. The data is normally distributed. In other words, the shape shown by the histogram follows the “normal” bell curve.
  2. The system being studied is stable and no assignable causes for variation are present. A control chart of the system has been made to determine stability before a capability analysis is done.
  3. The mean of the system being studied falls between the upper and lower specification limits defined for the process.

If these assumptions are not met, the results of a capability analysis will be misleading.

See also:
>> How do I compare the Cp/Pp and Cpk/Ppk?
>> Cp
>> Cr
>> Cpm
>> Pp
>> Cpk
>> Ppk
>> Capability indices

Additional sections from legacy process-performance-indices:

Process performance indices

Process performance indices use sigma of the individuals.

Pp

Pp for one-sided specifications

If you are using one sided specifications, use the following formulas to determine the Cp:

Upper specification

Lower specification

Ppk

Where:

Zmin is the smaller of Zupper and Zlower.

Using sigma of the individuals:

Pr

Pp

Pp for one-sided specifications

If you are using one sided specifications, use the following formulas to determine the Cp:

Upper specification

Lower specification

Ppk

Ppk

Where:

Zmin is the smaller of Zupper and Zlower.

Using sigma of the individuals:

Pr

Pr

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