Cp

potential process capability

Cp = (USL − LSL) / (6 · σ_within)

Cp compares the width of the tolerance with the natural spread of the process (six standard deviations within subgroups). It ignores where the process is centred, so it shows what the process could achieve if it were perfectly centred.

Cp needs both specification limits. A process with Cp 2.0 but Cpk 0.8 has a narrow spread but sits far off centre.

Calculate it

Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.

Related terms

• Cpk: process capability index
• Pp: process performance
• Sigma within: short-term standard deviation