From: Mark Bannister
Date: 25 Aug 1999
Time: 11:16:32
This always confuses me, and it sounds like a silly question, but: In SPC what do you mean by sample size? Or rather what time frame does sample size apply to?
For example: I am quoting a new job of 200K parts per order. Customer wants to insure that 99.99% of parts are in tolerance. part dimension is 5.5 mm with +/- .025 mm tolerance I have estimated Std at .0112. (big time guess) Tolerable error: .008 mm (measurement accuracy only .0078mm)
If I plug that in to equation sample size = sqr((Z * s) /E) sample size = sqr((3.291 * .0112) / .008) = 19.76
That gives me a sample size of 20. So process is in control do I look at 20 parts per day? per 200K order (can't be!) per shift? per what!?!?! Why does not this formula not take into account the lot size? Am I asking 2 different questions?
Also, does the sample size refer to actually how many parts I look at or how many sub-groups of n number parts.
Thanks for any insight. Mark Bannister