# ISS (Statistical Services) Statistics Paper II (Old Subjective Pattern): Questions 1 of 39

» Statistical Quality Control » Sequential Sampling Plans

Appeared in Year: 2015

### Describe in Detail

Distinguish between the single sampling plan and double sampling plan. Discuss how the O. C curves can be used for comparing two sampling plans.

## Explanation

A single sampling plan in which a decision about the acceptance or rejection of a lot is based on one sample that has been inspected where double sampling plan when a decision about the acceptance or rejection of a lot has not been reached after single sample inspection from a submitted lot, a decision will always be reached when the second sample from the same lot has been inspected.

Two numbers specify a single sampling plan. They are the number of items to be sampled (n) and a pre-specified acceptable number of defects (c). If there are fewer or equal defects in the lot than the acceptance number, c, and then the whole batch will be accepted. If there are more than c defects, the whole lot will be rejected or subjected to 100 % screening. If the number of defects in n _{1} smaller sample is less than or equal to some lower limit c _{1}, the lot can be accepted. If the number of defects exceeds an upper limit c _{2}, the whole lot can be rejected. But if the number of defects in the n _{1} sample is between c _{1} and c _{2}, a second sample n _{2} is drawn and the cumulative results determine whether to accept or reject the lot. The concept is called double sampling.

The operating characteristic (OC) curve describes how well an acceptance plan discriminates between good and bad lots. A curve pertains to a specific plan, that is, a combination of n (sample size) and c (acceptance level). It is intended to show the probability that the plan will accept lots of various quality levels. O. C curves can be used to compare these two plans. Choose between the plans by studying their relative ability to detect rejectable lots. The steeper curve, the large the sample size. Complete this picture by comparing the costs of the sampling with the resulting performance. It’s also evaluate and develop the sampling plan.