Capability Analysis in Excel
Capability analysis determines if a process will deliver a product that meets the customers specifications (USL/LSL) and how well it is centered between the specification limits.
Not Capable to Centered & Capable
Cp < 1, Cpk <1
Cp >= 1, Cpk >= 1
Cp and Cpk
Process Capability Metrics
- Cp measures how well the data fits within the spec limits (USL, LSL)
- Cpk measures how centered the data is between the spec limits.
Use Cp Cpk when you have a sample, not the population, and are testing the potential capability of a process to meet customer needs.
Cp and Cpk use Sigma estimator.
Pp and Ppk
Process Performance Metrics
Pp measures how well the data fits within the spec limits (USL, LSL)
Ppk measures how centered the data is between the spec limits.
Use Pp Ppk when you have the total population and are testing the performance of a system to meet customer needs.
Pp, Ppk use standard deviation.
Cp Cpk Calculation Mistakes to Avoid
You can calculate Cp, Cpk and Pp Ppk manually or you can use the 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. If you're providing these results to your customers shouldn't you use a tool that's proven and affordable?
QI Macros Tools that Perform Capability Analysis
- Histogram - calculates Cp, Cpk, Pp, Ppk and twenty 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.
Learn More ...
- Process Capability Analysis Quick Reference Card (PDF, 172 KB)
- Histogram formulas and sample calculations