Page 261 - Lean six sigma demystified

Chapter 7 Redu C ing Va R iation with Six Sigm a 239

1. Cp is the capability index. It measures how well your data might fit
between the upper and lower specification limits. It doesn’t really care if
the process is centered within the limits, only if the data would fit if the
data were centered.
2. Cpk is the centering capability index. It measures how well your data are
centered between the upper and lower specification limits. Cp and Cpk
use an estimation of the standard deviation to calculate the spread of your
data. If the variation between samples is small, Cp and Cpk are better
predictors of capability.
3. Pp is the performance index. Like Cp, it measures how well your data fit
within the USL and LSL. Unlike Cp, Pp uses the actual standard deviation
of your data, not the estimate.
4. Ppk is the performance centering index. Like Cpk, it measures how well your
data are centered between the USL and LSL. Again, Ppk uses the standard
deviation to determine the spread of your data.
If you want to dig into the formulas for these indicators, go to my website
www.qimacros.com//formulas/cp-cpk-formula.html or Google “Cp Cpk.”

note These indicators are only valid when your process is stable (i.e., in sta-
tistical process control). We’ll look at stability and SPC in Chap. 7.

Cp and Cpk should be used together to get a sense of process capability.
Using Pp and Ppk will help confirm process capability. Ideally, all four indica-
tors should be greater than 1.33 (all data fit within specification limits and are
centered and at least four sigma—6,210 PPM). From a Six Sigma perspective,

Cp and Cpk directly correlate with Six Sigma targets.

Cp and Cpk and Pp and Ppk Cr Sigma Level

1.0 1.0 3
1.33 0.75 4
1.66 0.6 5
2.0 0.5 6

Cr and Z Target or ∆Z

The indicators, Cr and Z Target, provide another way to measure spread and
centering (Fig. 7.8).