Process Capability Analysis in Excel
You Don't Have to be a Black Belt to Perform Capability Analysis
A capable process meets customer requirements 100% of the time. Customer requirements are defined using an upper specification limit (USL) and a lower specification limit (LSL). Think of these specification limits as goal posts.
Not Capable 
Capable 

Cp < 1, Cpk <1 
Cp >= 1, Cpk >= 1 
Not Capable to Centered & Capable 

Process capability metrics are used to determine how well the output of a stable process meets these specifications:
Cp and Cpk 


Cp  How well the data would fit within the spec limits (USL, LSL) 
Cpk  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 Cpk  Formulas use Sigma estimator. 
Pp and Ppk 


Pp  How well the data would fit within the spec limits (USL, LSL) 
Ppk  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  Formulas use standard deviation. 
Cp vs Cpk Examples
Fits But Is Not Centered 
Process improvement goal is to make changes that will move the output closer to the target without increasing variation. 
Centered But Does Not Fit 
Process improvement goal is to reduce variation so that the process fits between the spec limits. 
Onesided Spec Limits
OneSided Spec Limit 
People confuse a "hard limit," such as zero, for a "spec limit". It is not In this case, Cpk uses CpkU which means Cpk > 1. 
OneSided Spec Limit 
If you enter an LSL of zero, Cpk uses CpkL which means Cpk < 1 because the mean is closer to zero. 
Capability Analysis Metrics Rules of Thumb
 Cp > 1 Process is capable (product will fit between the customer's upper and lower specification limits if the process is centered).
 Cpk > 1 Process is capable and centered between the LSL and USL.
 If Cp = Cpk the process is centered at the midpoint of the specification limits.
 If Cp > Cpk the process is offcenter.
 Cp and Cpk should be close in value to Pp and Ppk.
 If Cp and Cpk are much greater than Pp or Ppk (33% greater), your process may not be stable enough to conduct a capability analysis. Use control charts to evaluate the stability of your process.
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