Exploring Pp: A Journey into Process Performance

Last Updated on 11 September 2023

Process Performance (Pp) is a statistical tool used to evaluate the performance of a production process. Unlike Cp and Cpk that estimate the ‘capability’ of a process, Pp examines the ‘performance’ or long-term ability of the process to produce output within specification limits.

How is Pp Calculated?

The Pp formula is similar to that of Cp with one significant difference. While Cp uses short-term sigma (σ), Pp utilizes the long-term sigma (σLT) for calculation:

Pp = (USL - LSL) / 6σLT

To precisely estimate Pp, a larger sample size is required, often collected over a longer span of time to account for systemic variations and shifts in the process.

Why is Pp Important?

Understanding Pp illuminates the production process’s efficacy under natural, uncontrolled conditions over a longer time frame. It also discloses any latent issues that short-term evaluations may overlook. Like Cp, a Pp value of 1.0 is deemed satisfactory, yet a Pp value of 1.5 or more is considered excellent in Six Sigma operations.

Pp: A Practical Example

Going back to our faithful bakery example, we continue to target loaves weighing 950g ± 50g, the USL set at 1000g, and the LSL at 900g. Assuming we have the long-term standard deviation (σLT) of 15g:

Calculating Pp:

Pp = (1000 - 900) / (6 * 15) = 1.11

With a Pp greater than 1, our bakery’s process performance over a longer timeframe holds up quite well.

Though Pp and Cp look similar, they serve different purposes. Including Pp in your Six Sigma toolkit makes you more proficient in assessing the long-term performance of your processes.

Remember, the leaner our methods, the closer we get to perfection. Use Pp to edge closer to your ultimate Six Sigma Black Belt aspiration.