From: Stan Hilliard
Date: 08 Jun 2000
Time: 01:52:18
Greetings Larry,
You said: "Can "N" be variable?"
Yes, the size of N has very little effect on the oc curve as long as it meets the criterion of infinite population. (n < 10*N)
You also said: "If so, than what is to keep from using lots based on purchase order alone?"
The larger the lot, the greater the chance of it containing more that one natural population. This can happen if a lot contains multiple production runs, multiple raw material batches, changes in environmental conditions, different operators, etc.
You said: "My question is why is this so vague?"
Because the root cause of variability is specific to each process. The answer requires knowledge of specifics and can't be generalized.
For purchased material, the vendor owns the process and is the one who has or can gain the necessary knowledge.
Sincerely, Stan Hilliard