Which on is "most important" ... Ppk, Cpk, or Cpm? In other words, which one is the most practical to use for an operation that has yet to use any of these? Or are all three equally important? Please advise.
Thanks,
CUFF
You can have a long discusion about this question,
, any indicator is a simplification of the real thing, there are some things that you can't see on any of them, as trends, outliers, cyclical trends, etc.
IMHO, any of them could be a good thing to follow, if there is no outliers on the data. Most of the people could say that Cpk is better than ppk because, ppk is for non-stable process and being non-stable, you can't trust on it to predict the process capability (as Don Wheeler, Advanced topics on SPC), even they say that you shouldn't calculate it if the stability has not been proved.
Some say that Cpmk, is good if your process is not centered, but the good point is that it take in account the centering to the target,
with cpk or ppk you can have the same or better index if you are farther from the target but with less variation, and that is wrong, cpmk is more robust to this kind of event. The problem with Cpmk/Cpm is that just a few use it.
The question of ppk or cpk is ... when is stable?