Question
- The customer expects certain levels of inspection: pull 157 bottles for visual testing, but then they also want 20 pulled for dimensional testing. Can’t the 20 additional bottles be a subset of the original testing sample?
- When calculating the lot, do you pull the samples before or after your calculations? Do the samples get included in the produced quantity or not? For example: If the customer orders 10,000 bottles and the level 2 inspection pulls 200 bottles that drops the total shipped to the customer to 9,800 pieces. If 10,200 bottles are produced then the inspection level increases so that 315 bottles need to be pulled for testing. What is the correct sample size and production number?
Answer
Here are the responses to your questions:
- Yes since the first inspection is visual, you can use a subset for the additional testing.
- The lot size is 10,000. You should be putting the samples back into the lot if they are not destroyed by the testing. You send what is contracted for. You are sampling with replacement.
Jim Bossert
SVP Process Design Manger, Process Optimization
Bank of America
ASQ Fellow, CQE, CQA, CMQ/OE, CSSBB, CSSMBB
Fort Worth, TX
For more on this topic, please visit ASQ’s website.
In some industries, pharmaceuticals for example, sampled product is not returned to industry. When sampling with replacment is not possible, calculating sample size by the hypergeometric distribution is appropriate.