Process Performance Index (Ppk)

What is this formula?

The Process Performance Index (Ppk) measures the actual long-term ability of a process to produce within specification limits.

Unlike Pp, Ppk also considers how well the process is centered relative to the specification limits.

When to use it

Use this formula when:

- Evaluating long-term process performance.

- Assessing customer specification compliance.

- Monitoring manufacturing quality.

- Supporting Six Sigma programs.

- Comparing actual process capability.

Example

A process has:

USL = 50.5 mm

LSL = 49.5 mm

μ = 49.9 mm

s = 0.15 mm

Formula:

Ppu = (USL − μ)/(3s)

Ppl = (μ − LSL)/(3s)

Substitution:

Ppu = (50.5 − 49.9)/(3×0.15)

Ppu = 1.33

Ppl = (49.9 − 49.5)/(3×0.15)

Ppl = 0.89

Result:

Ppk = min(1.33, 0.89)

Ppk = 0.89

Applications

- Statistical Process Control

- Six Sigma

- Manufacturing quality

- Process auditing

- Industrial quality engineering

Note

Ppk is the long-term counterpart of Cpk. While Cpk is based on short-term process variation, Ppk uses overall process variation and therefore better reflects the actual performance experienced by customers over time.