Last Updated on 10 September 2023
Process Performance Index (Ppk) is a statistical measure of the long-term capability of a process. It considers both the variability of the output and where the process average (mean) sits in relation to the specification limits. This makes Ppk an extension of the Pp index, presenting a more robust view of the overall process performance.
How is Ppk Calculated?
Like Cpk, the calculation for Ppk uses the process mean and standard deviation. However, Ppk uses long-term standard deviation (σLT) and gives an indication of how the process will perform in the future. The mathematical formula is stated as:
Ppk = min [ (USL - μ) / 3σLT , (μ - LSL) / 3σLT ]Why is Ppk Important?
Through Ppk, we can predict the capability of a process to produce output within specified limits in the future. A high Ppk value (≥ 1.5) indicates that, assuming the process remains the same, you can expect minimal defects. In contrast, a low Ppk suggests that the process requires improvements to meet customer expectations consistently.
Ppk: A Practical Example
Let’s once again visit our bakery scenario. Our target remains to bake loaves weighing 950g ± 50g, USL is 1000g and the LSL is 900g. Furthermore, let’s assume a long-term standard deviation (σLT) of 15g, and the process mean (μ) is 950g.
Calculating Ppk:
Ppk = min [ (1000 - 950) / (3 * 15) , (950 - 900) / (3 * 15) ]Ppk = min [ 1.11, 1.11 ]Ppk = 1.11Hooray! Our bakery’s process demonstrates fair long-term performance as our Ppk is greater than 1. That said, there’s room for improvement to achieve true Six Sigma excellence.
Mastering the calculations and implications of Ppk, like Cp, Cpk, and Pp, strengthens your Six Sigma toolkit, helping you enhance your organization’s processes. As we know, the application of theory is instrumental in achieving Six Sigma proficiency, just as in Karate.
Continue striving, keep learning, and persist in polishing your techniques as we progress on this rewarding journey towards becoming Six Sigma Black Belts.
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