The ability of a process to meet specifications (customer expectations) is defined as Process Capability, which is measured by indexes that compare the spread (variability) and cantering of the process to the upper and lower specifications. The difference between the upper and lower specification is known as tolerance.
This analysis requires that the process be normally distributed. Distributions with other shapes are beyond the scope of this material. Specifications are not related to control limits – they are completely separate. Specifications reflect “what the customer wants”, while control limits tell us “WHAT A PROCESS CAN DELIVER”.
The first step is to compare the natural six-sigma spread of the process to the tolerance. This index is known as Cp and Cpk. These are the formulas use sigma calculators
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 centred).
- Cpk > 1 Process is capable and centred between the LSL and USL.
- Cp = Cpk, the process is centred at the midpoint of the specification limits.
- Cp > Cpk, the process is off-centre.
Pp and Ppk
(Formulae use standard deviation)
- 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.
