Statistical quality control is a method which makes it possible to estimate the likely quality of a large lot of products or components on the basis of inspection of just a small sample. Mathematical techniques of statistical analysis are used to to determine the sampling plans that result in assuring that the conclusions drawn about the quality of the whole lot based on the quality of the sample. The sample plans are designed to achieve a specified permissible level of maximum defective percentage, and a level of confidence about the conclusion based on samples being correct. The sample plans specifies the size of the sample to be drawn, how the sample is to be drawn, the maximum level of defective that a sample may have without the whole lot being rejected. The sampling plan also specifies the action being taken in case quality based on sample is found to be unacceptable. These may include alternatives such as rejecting the whole lot, or inspecting the whole. The design of such sampling plans is best done by a qualified statistician. Actually, sampling plans to suit a variety of different requirements fave been worked out by experts and are available for use by others in form of standard tables of sampling plans.
I do not intend to provide a sampling plan for the situation described in question. However, I would like to point out that a sample of 20 is too small to make meaningful estimate about quality of lot for which maximum permissible defective probability is 0.01. With this sample size even if the the actual probability 2.5 times the maximum permissible, a sample of 20 will show no defective in about 50 percents of such samples drawn.
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