cpk和PPK 在应用上有什么差别？

For Pp and Ppk calculations, the standard deviation used in the denominator is based on all of the data evaluated as one sample, without regard to any subgrouping. This is sometimes referred to as the overall standard deviation, stotal.
For Cp and Cpk calculations, the standard deviation is based on subgroups of the data using subgroups ranges, standard deviations or moving ranges. This “within-subgroup” process variation can be considerably smaller than the overall standard deviation estimate, especially when there are long-term trends in the data.