Statistical Process Control (SPC) Thought ” Without data you are just another person with an opinion” .W.Edwards Deming.
You Don’t Have to be a Six Sigma Black Belt to Conduct Process Capability Analysis
Now it is possible to reach the level six sigma 3.4 ppm (3.4 per million opportunities defect)
Why Not Now?
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.
If you will need to conduct Process Capability Metrics …You should use Cp & Cpk
Cp is the capability index.
It measures how well the data might fit between the spec limits (USL, LSL). It doesn’t care if the process is centered within the limits only if it would fit if it was centered.
Cpk is the centering capability index.
It measures how well the data is centered 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.
Important Quote: Cp and Cpk use Sigma estimator.
Cp, Cpk Formula & Calculations:
(Cpk > 1.33 is desirable)
Sigma Estimator Formula & Calculations:
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
One-Sided or Unilateral Specification Limits
Use CpU (USL) or CpL (LSL) for Cpk
But if you will need to conduct Process Performance Metrics… you should use Pp and Ppk
Pp is the performance index.
It measures how well the data might fit between the spec limits (USL, LSL). It doesn’t care if the process is centered within the limits only if it would fit if it was centered.
Ppk is the performance centering index.
It measures how well the data is centered 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.
Important Quote: Pp, Ppk use standard deviation.
Pp, Ppk Formula & Calculations (Ppk > 1.33 is desirable)
Now The Key Question…!
How do Cp and Cpk Relate to Sigma?
Cp Cpk 1.0 1.33 1.66 2.0
Sigma 3 Sigma 4 Sigma 5 Sigma 6 Sigma
% of Spec Tolerance Used 100% 75% 60% 50%
Here we go ..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 off-center.
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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