We have written about this Cpk/Ppk Ford thing before.
First the formulas are not the same* In the latest installment of definitions (a couple of decades old) Cpk uses the within subgroup standard deviation (hence why it is sometimes called short term). Ppk uses the standard deviation of all of the individual values. So Ford uses Ppk of 1.67 for the initial (short term) capability because in pre-production and early production there is not a lot of material and process variation. Which makes sense… Once a process is up and running and normal variation is now occurring they accept the ‘long term’ variation of 1.33.
*NOW for the confusion. In the very beginning There was no Ppk. There was no short term or long term. There was only Cpk and it used the standard deviation of all of the individual values. There also was no conflating it with a defect rate…but companies have to be different and put their own spin on things…so some companies hav e given up on the whole Cpk/Ppk thing and have reverted to the original definition of Cpk. (Which Ford still calls Ppk).
Of course the whole Cpk/Ppk thing is stupid and useless but what the heck if your Customer requires it then you gotat do it.
Just be sure you KNOW which formula for standard deviation is used…
First the formulas are not the same* In the latest installment of definitions (a couple of decades old) Cpk uses the within subgroup standard deviation (hence why it is sometimes called short term). Ppk uses the standard deviation of all of the individual values. So Ford uses Ppk of 1.67 for the initial (short term) capability because in pre-production and early production there is not a lot of material and process variation. Which makes sense… Once a process is up and running and normal variation is now occurring they accept the ‘long term’ variation of 1.33.
*NOW for the confusion. In the very beginning There was no Ppk. There was no short term or long term. There was only Cpk and it used the standard deviation of all of the individual values. There also was no conflating it with a defect rate…but companies have to be different and put their own spin on things…so some companies hav e given up on the whole Cpk/Ppk thing and have reverted to the original definition of Cpk. (Which Ford still calls Ppk).
Of course the whole Cpk/Ppk thing is stupid and useless but what the heck if your Customer requires it then you gotat do it.
Just be sure you KNOW which formula for standard deviation is used…
