Methods of Sampling inspection

To assure better quality on one hand and to come out of the difficulties acceptance sampling method i.e. method of sampling inspection is used. When the decision regarding acceptance or rejection of an entire lot of products or materials or parts or components is taken on the basis of number of defectives in a sample, it is called acceptance sampling. Acceptance sampling is a process of evaluating a portion of production or material from the whole lot for either accepting or rejecting the material on the basis of whether it conforms or does not conform to the set quality standard. A sample is drawn from each lot for inspection. If the amount of defective is less than the prescribed minimum, the lot is accepted. If not, the lot is rejected as being below the standard. Here, decision is taken without going through 100% inspection of the entire lot. Acceptance sampling reduces the work of inspection and the quality standards. The maximum limit of the number of defectives in a sample, to consider it acceptable, is determined first, and on the basis of this limit, decision regarding acceptance or rejection is taken. If the number of defects in a sample is less than the prescribed number, the lot is accepted and if the number of defects is more than the prescribed number, the lot is rejected. Acceptance sampling is used to determine whether (1) to accept or reject a particular lot of materials etc. purchased or (2) to pass or not to pass a particular batch of finished products for dispatch to the customers or (3) to pass or not to pass a particular lot of materials in process. Most often it is used in receiving inspection department where material, parts, components etc. bought are inspected. It is used less at the plant level because control charts are used right at the operations in so many places. Large lots of products are never produced and made complete first and then inspected. A few at a time are inspected as they are being made. Sampling inspection can be carried out either as sampling by attributes or sampling by variables.

Acceptance sampling is mostly attributed to inspection rather than variable inspection. And even more than in the case of operation control charts, acceptance sampling is a matter of calculated risks, because it deals with large quantities of already finished products. There is always a slight chance that bad lots will be passed or that good lots will be rejected. Always, when there are large quantities of products, there are going to be at least a few defective pieces in every lot. Thus acceptance sampling does not eliminate the risks of getting the good lost rejected and bad lots accepted or passed. Both buyer and seller understand that there will be some defective products and contracts are drawn accordingly. In fact, the number of expected defective products will be reflected in the price, If the buyer wants the products he buys to have a very low percentage of defective, he pays more than if he is less demanding.

When the products arrive at the buyer’s plant, he inspects them and accepts or rejects tem depending on whether the rejects in the sample he looks at are above or below the ratio allowed. Both buyer and seller take some risks. The consumer runs the risk that now and then he will accept a lot with too many defectives. The sample might not have its share of defectives. The producer (seller) runs the risk that now and then a good lot will not pass inspection. (if the sample happens to contain more than its share of defectives). Acceptance sampling does not get rid of these risks, but it allows us to decide how much risk to run and to inspect accordingly. The more determinate you want to be, the bigger sample you must inspect (and pay more for inspection costs).

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