Measure long-term process performance using overall standard deviation — and compare it to short-term capability to reveal instability, drift, and variation that only appears over time.
Use the overall (long-term) standard deviation — calculated from all measurements, not within subgroups.
Enter your specification limits, process mean, and overall standard deviation. Optionally add within-subgroup standard deviation to compare long-term vs short-term performance.
Pp uses the overall standard deviation — all data over time, including variation from shifts, batches, operators, and environment. It tells you how the process actually behaves in the long run, not just under ideal short-term conditions.
Ppk adds centering to Pp — it takes account of where the mean sits relative to the spec limits. A large gap between Pp and Ppk means the process is off-centre. A large gap between Cpk and Ppk means the process is unstable over time.
Dividing Pp by Cp (or Ppk by Cpk) gives the stability ratio. A ratio close to 1.0 means the process is stable — long-term and short-term variation are similar. A ratio significantly below 1.0 indicates drift, shift, or instability between subgroups.