Is Cpk higher than PPK?
Is Cpk higher than PPK?
The automotive confusion aside, Ppk will always be greater than Cpk since Ppk is long term capabliity and Cpk is short term capability. The only way that Cpk will be greater than Ppk (other than a misuse of the terms and formulas) is under the conditions stated in a previous post.
What are acceptable Cpk and PPK values?
For stable processes and normally distributed data, a Cpk value ≥ 1.33 should be achieved. • For chronically unstable processes with output meeting specification and a predictable pattern, a Ppk value ≥ 1.67 should be achieved.”
How is Cpk and PPK calculated?
If we look at the formulas for Cpk and Ppk for normal (distribution) process capability, we can see they are nearly identical: The only difference lies in the denominator for the Upper and Lower statistics: Cpk is calculated using the WITHIN standard deviation, while Ppk uses the OVERALL standard deviation.
What is CP Cpk and PPK?
Cp, Cpk, Pp and Ppk are all parameters (indices) that can help us to understand how our process is operating relative to the specifications, or in other words, they measure how close our process is running to its specification limits. For requirements we measure the process specifications.
How PPK is calculated?
Ppk can be determined by diving the Z score by three. A z score is the same as a standard score; the number of standard deviations above the mean. Z = x – mean of the population / standard deviation.
What does PPK less than 1 mean?
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.
What does a PPK of 1.0 mean?
What is PPK value?
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.
What is PPK quality?
What is the difference between CPK and PPK in capability charts?
You note one capability chart that has a Ppk = 1.14 and a Cpk = 2.07. Why are those different? Well, it doesn’t matter. The Cpk is above 1.33, which is what you asked the supplier for.
What are the specification limits for CPK?
The specification limits are 10.0 +/- 0.05 mm. In this example, the chart shows that if you have only 28 samples, the estimate for Cpk is (Cpk +/- 0.50). Therefore, if our Cpk is 1.25, the confidence interval predicts an actual Cpk of (0.75 to 1.75).
Does the CPK of a supplier matter?
Well, it doesn’t matter. The Cpk is above 1.33, which is what you asked the supplier for. Time to work on something else. You just missed a very important piece of information about your supplier’s performance.