Cp Cpk Formulas versus Pp Ppk Formulas
Formulas for Cp Cpk and Pp Ppk are Similar
Cp, Cpk Formula & Calculations
(Cpk > 1.33 is desirable)
Cp and Cpk use Sigma estimator.
Pp, Ppk Formula & Calculations
(Ppk > 1.33 is desirable)
Pp, Ppk use standard deviation.
One-Sided or Unilateral Specification Limits
Use CpU (USL) or CpL (LSL) for Cpk.
Mistakes to Avoid When Performing Manual Calculations
You can calculate Cp, Cpk and Pp Ppk manually or you can use proven software like QI Macros for Excel. Based on calls to our tech support line, most people who try to perform manual calculations or build their own Excel formulas end up with incorrect results.
For example, they use standard deviation instead of sigma estimator for Cp and Cpk. Or they get the constant wrong based on their sample size. Or they don't understand there are several methods of calculating sigma estimator.
If you're providing these results to your customers shouldn't you use a tool that's proven and affordable?
Sigma Estimator Formulas and Calculations Used in QI Macros
Pooled Standard Deviation is the default estimator for QI Macros Cp and Cpk calculations since May 2015.
Between/Within Deviation is an option added to the histogram macro and template in December 2015.
Sbar/c4 and Rbar/d2 can still be used by changing the estimator field on the histogram data sheet.
d2 is a constant based on subgroup size
c4 is a constant based on subgroup size
Rbar = Average(Ri) (Average of the Ranges in samples)
sbar = Σ(σi)/n
Pp and Ppk Calculations in Weibull Histogram
Unlike the Pp and Ppk calculations which rely on a Normal distribution, the Weibull histogram uses the WEIBULL (or WEIBULL.DIST) function to calculate the Z-scores for the USL/LSL and from these it cacluates Pp and Ppk. There is no Cp or Cpk calculation possible in the Weibull Histogram.
If there is only a USL or LSL, the Weibull histogram Ppk is either PpU or PpL.
QI Macros Tools that Calculate Cp, Cpk and Pp, Ppk
- Histogram - calculates Cp, Cpk, Pp, Ppk and 20 other metrics using your data and spec limits.
- Weibull Histogram - calculates Pp, Ppk and other metrics using your data and spec limits.
- Capability Suite - creates six charts including histogram, control charts, probability plot, values plot and capability plot. Also calculates Cp, Cpk and Pp, Ppk.
- Cp Cpk Template - calculates Cp, Cpk and Pp, Ppk on multiple sets of data.