I am not an expert in SS at all, but from what I could get (mainly from this forum) I understand that 6 sigma level assumes a process shift (in the long term) of 1.5 sigma from the target (center of spec.). This fact of the process shift was largely discussed in previous threads, and I won’t add nothing about it.
But what I can say is that:
– The name “six sigma” comes from assuming that the process variation contained within the average value +/- 6S is fully within specification. At least, this is Cp=2 and Cpk=2. This leads to much less than 3.4 PPM (I dont remember now but think it was less that 1 part per BILLON)
– Then, somehow we get to the 1.5 sigma shift. So we have Cp=2 (like before) but now Cpk=1.67. This leads to the famous 3.4 PPM. I am not saying that I agree with the 1.5 sigma shift, but this is how it is axplained that a process with a Cpk=1.67 (average 4.5 sigma away from the closest limit) is said to be a 6S process (still, because Cp=2, the width of the spec is equal to the width of the process +/-6S, but shifted).
– Cpk=1.67 is not equal to 3.4 PPM, unless you have a Cp=2. So it is not correct (or not complete) to say, as in the forst response, that 6 sigma is Cpk=1.67 and that this is 3.4 PPM. In fact, a process with a Cpk=1.67 and a Cp=1.67 will double the PPM to 6.8, and will not be considered a 6S process. The spec limits would be cincident to the process average +/- 5S.
.
