Q
We manufacture product on an automatic line with a predetermined lot size. QA selects samples approximately every 30 minutes of production. Often times, they have met their sample size prior to the completion of the full production run. Rather then stop sampling, they continue to test samples on 30 minute time intervals until the production run is complete, exceeding the required sample size.
example: Lot size 4400, sample size 80, A=2, 2 rejects found in SS=80 therefore lot is acceptable. Continue to sample, since production run is not yet complete, and find a reject at sample 90. 3 rejects have now been found in sample size of 90 even though lot passed with only 2 rejects in required sample size of 80.
My questions are:
1) Should they stop sampling once the sample size has been met?
2) If the accept level is exceeded with samples selected AFTER the full sample size has been evaluated, strictly speaking, is this cause for failure of the lot?
Thanks in advance for your inputs.
example: Lot size 4400, sample size 80, A=2, 2 rejects found in SS=80 therefore lot is acceptable. Continue to sample, since production run is not yet complete, and find a reject at sample 90. 3 rejects have now been found in sample size of 90 even though lot passed with only 2 rejects in required sample size of 80.
My questions are:
1) Should they stop sampling once the sample size has been met?
2) If the accept level is exceeded with samples selected AFTER the full sample size has been evaluated, strictly speaking, is this cause for failure of the lot?
Thanks in advance for your inputs.
statistically-based answer, but you focused in on the two key issues. 