Competently Use Capability Analysis

The capability of a process, statistically speaking, is the ability of that process – based on historical performance – to achieve measurable results that satisfy established specifications and statistical limits. In a nutshell, process capability indicates how well a process is able to perform its specified purpose.

Why is process capability important? When the capability of a process is understood and documented, it can be used for measuring continual improvement using trends over time, prioritizing the order of process improvements to be made, and determining whether or not a process is capable of meeting customer requirements. Capability analysis is considered part of the Black Belt Body of Knowledge.

Getting the Most out of a Capability Analysis

The process capability indices Pp and Cp describe how closely a process can operate within its specification limits. Many articles describe the difference between Pp and Cp simply: one is short term, one is long term. Moving beyond such a description, this article focuses on the untapped power of capability analysis and shows you how to use Pp and Cp to your advantage.

Overview of Pp Versus Cp

The difference between Pp and Cp comes from the standard deviation that is used in the calculation.

When calculating Pp, the overall standard deviation is used. This is the standard deviation of all of the data combined, regardless of any subgroups in the data.

Pp= (USL – LSL) / (6*σoverall)

Where σ = standard deviation
USL = upper specification limit
LSL = lower specification limit

To calculate Cp, the within standard deviation is used.

Cp= (USL – LSL) / (6*σwithin)

Subgroup Size Matters

Many training instructors and materials default to using 1 as the subgroup size when teaching capability analysis. There is nothing wrong with using a subgroup size of 1 (Figure 2); however, it limits what you can learn about within variation and often overestimates the true potential capability.

Benchmark and Goal-setting Analysis

Capability analysis can be particularly useful for comparing the current process to another process, benchmark or project goal.

The Power of Capability Analysis

Capability analysis is an extremely powerful analytical tool and can be used for many applications. It is important to understand the within standard deviation and to know how a benchmark capability analysis can be used to determine a future state of the process.

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