Process Capability Analysis in Excel

Process Capability Indices Cp Cpk

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. Process capability analysis determines how well the output of a stable process meets these specifications.

Cp and Cpk
Process Capability Metrics

  • Cp measures how well the data fits within the spec limits (USL, LSL)
  • Cpk measures 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 and Cpk use Sigma estimator

Pp and Ppk
Process Performance Metrics

  • Pp measures how well the data fits within the spec limits (USL, LSL)
  • Ppk measures 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 use standard deviation.


 

Cp vs Cpk Examples

Fits But Is Not Centered

process capability analysis example fits but is not centered

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 capability analysis example centered not fit

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


Process Capability Analysis 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, your process may not be stable enough to conduct a capability analysis. Use control charts to evaluate the stability of your process.

QI Macros Tools that Perform Process Capability Analysis

  • Histogram - calculates Cp, Cpk, Pp, Ppk and twenty other metrics using your data and spec limits.
  • Capability Suite - creates six charts including histogram, control charts, probability plot, values plot and capability plot. Also calculates Cp, Cpk and Pp, Ppk.
  • Cp Cpk Template - calculates Cp, Cpk and Pp, Ppk on multiple sets of data.

Learn More ...


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