Process Capability (Cp) and Performance (Cpk)
What does CP and Cpk tell you?
The Cp and Cpk indices are the primary capability indices. Cp shows whether the distribution can potentially fit inside the specification, while Cpk shows whether the overall average is centrally located. If the overall average is in the center of the specification, the Cp and Cpk values will be the same.
What does it mean when CP is higher than Cpk?
When Cp is greater than Cpk, the mean is nearer to one specification limit or the other. Once you understand your process, you can make a good decision about how to prioritize your process improvement efforts.
What does a Cpk of 1.33 mean?
A CpK of 1.33 means that the difference between the mean and specification limit is 4σ (since 1.33 is 4/3). With a CpK of 1.33, 99.994% of the product is within specification. Similarly a CpK of 2.0 is 6σ between the mean and specification limit (since 2.0 is 6/3).
What does a Cpk of 1.67 mean?
Excellent, Capable
The Capability Index, Cpk.
The outcome of a Process Capability study is a single metric, which provides an indication of the ability of a process to consistently provide output which is within required specifications. CPK <1.00 (Poor, incapable) 1.00< CPK <1.67 (Fair) CPK >1.67 (Excellent, Capable)
Is a higher or lower Cpk better?
The higher the Cpk, the better is the capability of the process to meet its requirements. In the industry, a Cpk of less than 1.66 needs a closer look. A Cpk that’s less than 1.33 needs some action to make it higher, and a Cpk of less than 1.0 means that the process is not capable of meeting its requirements.
What if Cpk is lower than CP?
If Cpk is less than Cp and Cp is greater than one, center the process in the specification. This should make Cpk comparable to Cp.
What if CP is greater than 1?
A Cp of greater than one indicates that the process spread is less than the width of the specification. Potentially this means that the process can fit inside the specification limits.
What does a Cpk of 1.4 mean?
A Cpk of 1.4 equates to a sigma level of 4.2 and a defect rate of less than 0.0027%. If there is significant bias then only one tail of the distribution will causing a significant chance of defects and so the defect rate may be only half this value.
What does a high CP value mean?
What is Cpk (Simple Explanation of Cp and Cpk)
What can be concluded when CP equals Cpk?
If Cp and Cpk are approximately equal, then the process is centered between the specification limits. If Cp and Cpk differ, then the process is not centered.
What does a Cpk of 1.4 mean?
A Cpk of 1.4 equates to a sigma level of 4.2 and a defect rate of less than 0.0027%. If there is significant bias then only one tail of the distribution will causing a significant chance of defects and so the defect rate may be only half this value.
What should be the CP and Cpk values for Six Sigma?
According to Six Sigma philosophy, Cp or Pp and Cpk or Ppk should be greater than 1.50. From a technical standpoint, Six Sigma deems a process being acceptable only after achieving a maximum defect rate of 3.4 parts per million opportunities.