4.13.Performance Index (Ppk) - is a

4.13.Performance Index (Ppk) - is an indicator of whether a process is meeting customer requirements, and computed using the total process variation and therefore captures both common and special sources of variation.4.14.Capability Index (Cpk) - is an indicator of whether a process is capable of meeting customer requirements. It does not show whether the process is meeting customer requirement because it is calculated using within group variation. Therefore, it only captures common sources of variation and does not capture special causes of variation.4.15.Common causes of variation – these are causes of variation that act consistently on the process. They produce a stable and repeatable distribution over time that is usually distributed normally. An example is the play within mechanical parts in a turning machine.4.16.Special causes of variation – also called assignable causes, are any cause of variation that affects only part of the output, and may be intermittent and unpredictable. Statistical Control – when a process is stable, it is also termed “under statistical control”. The output of a process that is stable is repeatable and usually normally distributed.