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
Capable
Capability Analysis of Histogram NOT CAPABLE
Cp < 1, Cpk <1
Capability Analysis of Histogram that is Capable
Cp >= 1, Cpk >= 1
Not Capable to Centered & Capable
Capability analysis before and after process improvement efforts

Process capability metrics are used to determine how well the output of a stable process meets these specifications:

Cp and Cpk
Process Capability Metrics

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
Process Performance Metrics

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 fits but is not centered between specification limits

Process improvement goal is to make changes that will move the output closer to the target without increasing variation.

Centered But Does Not Fit

process is centered but does not fit between spec limits

Process improvement goal is to reduce variation so that the process fits between the spec limits.

One-sided Spec Limits

One-Sided Spec Limit

process fits but is not centered between specification limits

Sometimes, people think a "hard limit," such as zero, is a "spec limit".

It is not

In this case, Cpk uses CpkU which means Cpk > 1.

One-Sided Spec Limit

process fits but is not centered between specification limits

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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