In: Math
Imagine that you are purchasing small lots of manufactured product. If it is very costly to test a single item, it may be desirable to test a sample of items from the lot instead of testing every item in the lot. Suppose each lot contains 10 items. You decide to sample 3 items per lot and reject the lot if you observe 1 or more defectives.
a) If the lot contains 1 defective item, what is the probability that you will accept the lot?
b) What is the probability that you will accept the lot if it contains 2 defective items?
Number of items in the lot = 10
Let X : number of defective items in the sample of 3 items
Reject the lot if X
Accept the lot if X =0; No defective items in the sample of 3 items from the lot i.e all 3 items to be non defective.
a) If the lot contains 1 defective item.
Probability of an item being defective = 1/10 = 0.1
Probability of an item being non-defective = 1-Probability of an item being defective = 1-0.1 = 0.9
Probability that you will accept the lot = Probability that all the 3 items to be non-defective = Probability of item 1 being non-defective x Probability of item 2 being non-defective x Probability of item 3 being non-defective = 0.9 x 0.9 x 0.9 = 0.729
If the lot contains 1 defective item, Probability that you will accept the lot = 0.729
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b) if a lot contains 2 defective items
Probability of an item being defective = 2/10 = 0.2
Probability of an item being non-defective = 1-Probability of an item being defective = 1-0.2 = 0.8
Probability that you will accept the lot = Probability that all the 3 items to be non-defective = Probability of item 1 being non-defective x Probability of item 2 being non-defective x Probability of item 3 being non-defective = 0.8 x 0.8 x 0.8 = 0.512
Probability that you will accept the lot, if it contains 2 defective items = 0.512