What is the difference between Cpk vs Ppk or Cp vs Pp?

As visible in the formulas the calculations are almost the same only for the Cpk and Cp the is used and for the Pp and the Ppk the s is used.

is the an estimate of the variation within the subgroups. So with the you filter out the:
• Drift of the process in time
• Tool wear
• Drift due different operators
• Drift due material batches

What is a subgroup?

If in a production process after every hour 5 measurements are done, the subgroup size is 5. When calculating the STDEV within data is ordered in groups of 5 to filter out the drift between the different groups.

But it includes the short therm variation of the process.

Cpk Cp	Ppk Pp
Variation Within the subgroups	Overall variation of all measurements

When use Cpk Cp or Ppk Pp

Cpk Cp	Ppk Pp
Once the process is in a state of statistical control	When you are initially setting up your process
The potential of the process capability	The actual process capability status over the time period
Short term performance	Long term performance

A process is in control when

1. Most points are near the average
2. A few points are near the control limits
3. No points are beyond the control limits

What if the process is not in control

1. Do a Gauge R&R and try to improve the measurement.
2. Label the data in subgroups to identify the factors that can cause that the process is not in control.