Process Capability Analysis in Excel
You Don't Have to be a Black Belt to Perform Capability Analysis
A capable process meets customer requirements 100% of the time. Customer requirements are defined using an upper specification limit (USL) and a lower specification limit (LSL). Think of these specification limits as goal posts.
Cp < 1, Cpk <1
Cp < 1, Cpk >= 1
Cp >= 1, Cpk <1
Cp >= 1, Cpk >= 1
Process capability metrics are used to determine how well the output of a stable process meets these specifications:
Cp and Cpk
|Cp||How well the data would fit within the spec limits (USL, LSL)|
|Cpk||How centered the data is between the spec limits.|
|Use Cp Cpk||When you have a sample, not the population, and are testing the potential capability of a process to meet customer needs.|
|Cp Cpk||Formulas use Sigma estimator.|
Pp and Ppk
|Pp||How well the data would fit within the spec limits (USL, LSL)|
|Ppk||How centered the data is between the spec limits.|
|Use Pp Ppk||When you have the total population and are testing the performance of a system to meet customer needs.|
|Pp Ppk||Formulas use standard deviation.|
Cp vs Cpk Examples
Fits But Is Not Centered
|Process improvement goal is to move the output closer to the target without increasing variation.|
Centered But Does Not Fit
Process improvement goal is to reduce variation so that the process fits between the spec limits.
One-Sided Spec Limit
Don't confuse a "hard limit," (i.e. zero) for a "spec limit".
One-Sided Spec Limit
If you enter an LSL of zero, Cpk uses CpkL which means
Cpk < 1 because the mean is closer to zero.
Capability Analysis Metrics Rules of Thumb
- Cp > 1 Process is capable (product will fit between the customer's upper and lower specification limits if the process is centered).
- Cpk > 1 Process is capable and centered between the LSL and USL.
- If Cp = Cpk the process is centered at the midpoint of the specification limits.
- If Cp > Cpk the process is off-center.
- Cp and Cpk should be close in value to Pp and Ppk.
- If Cp and Cpk are much greater than Pp or Ppk (33% greater), your process may not be stable enough to conduct a capability analysis. Use control charts to evaluate the stability of your process.
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