GainSeeker SPC provides calculations for process capability and process performance.
Process capability
Process Capability (PC) measures the effect of variation within a subgroup. For this reason, PC must use the Factors Method for calculating standard deviation. The procedures for estimating the standard deviation using the Factors Method are documented in Estimating standard deviation using the Factors Method. If the data is from a non-normal distribution, they should not be taken as absolute values. They do not relate to the probability of making parts out-of-specification.
Process Capability is the ratio of the natural process spread to the specification limit spread, calculated at six sigma control limits. Process Capability is used with small sample sizes.
PC% = Process Capability
Process performance
Process Performance (PP) measures the effect of variation within and between subgroups. For this reason, PP uses the Sample Standard Deviation (SSD) Method for calculating standard deviation. The procedures for calculating the standard deviation using the SSD Method are documented in Estimating standard deviation using the Sample Standard Deviation Method.
Process Performance is the ratio of the natural process spread to the specification limit spread, calculated at six sigma control limits. Process Performance is used with large sample sizes and is not calculated for non-normal distributions.
PP% = Process Performance
Assumptions while calculating process capability and process performance:
References
General Motors Corporation. General Motors Statistical Process Control Manual. August, 1984. Pages 4-8, 4-12.