Interpretation & Calculations
Process Performance attempts to answer the question: does this sample from the process meet customer requirements? It differs from Process Capability in that Process Performance only applies to a specific batch of material. Samples from the batch may need to be quite large to be representative of the variation in the batch.
Process Performance is only used when process control cannot be evaluated. An example of this is for a short pre-production run.
The calculated Process Performance and Process Capability indices will likely be quite similar when the process is in statistical control.
Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).