Sampling plans are used where the number of units presented for inspection is considerably large. When a supplier supplies his products in various lots consisting of a large number of units, the number of defective parts cannot be known. Usually in the lot-by-lot inspection method, there is a mutual agreement between the supplier and receiver that lots acceptance by sampling plans will be accepted except that the defective units obtained will be separated and returned to the supplier either for rectification or replacement.
The sampling plan lays down the procedure to be followed to understand quantitative estimates of the risks to be taken both by the supplier and the receiver. A good sampling plan is that which ensures acceptance of all good lots. The method of inspection should be less time taking and it should also be practicable.
