Process Capability in Excel with Cp and Cpk

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

Process capability is measured by two primary capability indices:

Cp is the capability index. It measures how well the data fits between the upper and lower specification limits. The higher the value, the better the fit.

Cpk is the centering capability index. It measures how well the data is centered between the specification limits. The higher the value, the more centered the data.

In the process capability example below, the process fits between the spec limits but is not centered.

In the process capability example below, the process is centered but does not fit between the spec limits (see the flatter curve). The goal is to reduce variation so that the process fits between the spec limits (see the taller more narrow curve).

Process Capability 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.

Pp is the performance index. Like Cp it measures how well your data fits within the USL and LSL. It uses standard deviation in the calculation instead of sigma estimator.

Ppk is the performance centering index. Like Cpk it measures how well your data is centered between the USL and LSL. It uses standard deviation in the calculation instead of sigma estimator.

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.