Process capability is a process’s ability to consistently produce product within the specified tolerance limits. It is quantified with indices such as Cp and Cpk.
Three conditions before calculating
1. The process must be in statistical control. An index calculated before special causes have been eliminated is meaningless; the number changes completely in the next period.
2. The measurement system must be verified. If MSA has not been done, it is not known how much of the calculated variation comes from the process and how much from the measurement.
3. The data distribution must be checked. The classic calculation rests on the assumption of a normal distribution. If the distribution is not normal, a different approach is required.
Common mistake
Capability values calculated and reported to the customer without these conditions being met are among the data most frequently questioned in audits.
For the difference between short-term and long-term studies, see the Pp and Ppk indices.
Planning, conducting and reporting capability studies can be worked through on your own process data in the SPC training. For the measurement side, a combined programme with the MSA training is recommended.