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.
Not Capable
Cp < 1, Cpk <1
Cp < 1, Cpk <1
Centered - Does not Fit
Cp < 1, Cpk >= 1
Cp < 1, Cpk >= 1
Fits Not Centered
Cp >= 1, Cpk <1
Cp >= 1, Cpk <1
Capable
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 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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