- Publisher's Description
- Cp Cpk Formulas versus Pp Ppk Formulas
- The Real Truth Behind Cpk and Ppk Capability and Potential Process Studies
- Techmentool: Quality (QA/QC)- Process Capability Study - Cpk - Ppk value - Cpk index
- Cp: A Tale of Two Tails
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- Cpk: Location, Location, Location!
- CPK/PPK formula
If you're using Minitab Statistical Software for capability analysis, t he capability indices Cp and Cpk are good examples of this.
If you focus on only one measure, and ignore the other, you might miss seeing something critical about the performance of your process. Cp is a ratio of the specification spread to the process spread.
The process spread is often defined as the 6-sigma spread of the process that is, 6 times the within-subgroup standard deviation. Higher Cp values indicate a more capable process.
Cp Cpk Formulas versus Pp Ppk Formulas
By using the 6-sigma process spread, Cp incorporates information about both tails of the process data. Obviously, Process B has a serious issue with its location in relation to the spec limits that Cp just can't "see.
Like Cp, Cpk is also a ratio of the specification spread to the process spread. But unlike Cp, Cpk compares the distance from the process mean to the closest specification limit, to about half the spread of the process often, the 3-sigma spread.
The Real Truth Behind Cpk and Ppk Capability and Potential Process Studies
When the distance from the mean to the nearest specification limit is considerably greater than the one-sided process spread, Cpk is high. When the distance from the mean to the nearest specification limit is less than the one-sided process spread, Cpk is low.
Techmentool: Quality (QA/QC)- Process Capability Study - Cpk - Ppk value - Cpk index
Notice how the location of the process does affect the Cpk value—by virtue of its being calculated using the process mean. Yet there's something important that Cpk doesn't do.
Cp: A Tale of Two Tails
Because it's a "worst-case" estimate that uses only the nearest specification limit, Cpk can't "see" how the process is performing on the other side. Notice that Process X has nonconforming parts in relation to both spec limits, while Process Y has nonconforming parts in relation to only the upper spec limit USL.
But Cpk can't "see"any difference between these two processes. To get the two-sided picture of each process, in relation to both spec limits, you can look at Cp, which would be higher for Process Y than for Process X.
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Avoid getting too fixated on any single statistic. To see a concrete example of how Cp and Cpk work together, using real data from the National Renewable Energy Laboratory, see this post by Cody Steele. By the way, the potential "blind spot" for Cp and Cpk also applies to Pp and Ppk.
The only difference is that the process spread for those indices is calculated using the overall standard deviation, instead of the within-subgroup standard deviation. For more on that distinction, see this post by Michelle Paret. Minitab is the leading provider of software and services for quality improvement and statistics education.
Cpk: Location, Location, Location!
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Search for a blog post:. When the specification spread is considerably greater than the process spread, Cp is high.
When the specification spread is less than the process spread, Cp is low. Let's Pour Over the Details. Who We Are Minitab is the leading provider of software and services for quality improvement and statistics education.