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SPC 101: Differentiating between Cpk and Ppk values

DataNet's Beena Anand, Six Sigma Blackbelt, service delivery consultant, and WinSPC expert, takes the confusion out of Cpk and Ppk values.


Just like control charts tell us about the stability of the process, capability analyses tell us about process capability. Capability Indices help us judge whether the process is capable of producing parts that are within specifications. Among the various capability indices, Cpk and Ppk are popular in the manufacturing world. However, many users report they are also the most confusing.


These two indices consider process centering to calculate the process capability.

The formula for the Cpk and Ppk are:

Where:
= Average of all readings
USL= Upper Specified Limit
LSL = Lower Specification Limit

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(Process sigma) = Rbar / d2
(where Rbar = Average Range and d2 = statistical constant)

S (Sample Sigma) =

So which capability index should you use, Cpk or Ppk? Although they show similar information, they have slightly different uses.

Cpk uses an "estimated sigma" value in its formula. It is commonly used to measure whether the process is capable of meeting a customer's requirements.

Ppk uses the "actual sigma" value in its formula so it is commonly used to measure whether the process is actually meeting a customer's requirements.