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Why are there three curves shown in the Ppk plot?

April 26, 2017Curves plot ppk

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Why are there three curves shown in the Ppk plot?

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The middle curve in the Ppk plot shows the running Ppk value versus observation number. The other two curves indicate the running 95% confidence limits for the true but unknown population Ppk value determined from the sample data. If the confidence interval falls completely below the target Ppk, the process is not capable. If the confidence interval falls completely above the target Ppk and if all other conditions are satisfied, the process is capable. If the confidence interval contains the target Ppk, then the data are insufficient to make a decision.