The performance of acceptance sampling depends on the method of selecting the sample and the acceptance level of defectives. There are a number of sampling schemes known as sampling plans. Their choice depends upon the nature of the manufacturing system and the degree of consumer and producer risks which one wants to cover. The sampling schemes are:
(i) Single sampling plan
(ii) Double sampling plan
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(iii) Sequential sampling plan.
(i) Single Sampling Plan:
Here, a single sample of size n is drawn from a lot of size N, and the lot is accepted if the number of defectives in the lot (d) is less than the specification (c) i.e.
A sample of size n is drawn and the items of the sample are inspected. Let the number of defectives in the sample be d. If.
(a) d < c, the lot is accepted and the defective items i.e. d are replaced.
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(b) d> c, the lot is rejected and then the entire lot is inspected and the defective items are replaced.
It is evident that single sampling plan is simple to understand and easy to execute. Moreover, the information from each sample can be represented by a control chart.
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(ii) Double Sampling Plan:
This type of scheme is more economical than single sampling scheme when the quality of incoming goods is high. Here sample is drawn in two stages. The second sample is drawn only when clear cut decision cannot be drawn from the first sample. Let c1 and c2 be the specified defectives in first and second samples respectively. The following are the steps in the method:
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Draw a sample of size n1 from the lot. Inspect the items of the sample and let d\ be the number of defective items. If d1 < c1, the lot is accepted after replacing the d1 defective items. d1 < C2, the lot is rejected and all the items of the lot are inspected and defectives replaced. c\<d\< go to step (b).
Take another sample of size n<± and let the number of defective items in the second sample be d2. Thus, the total number of defective items in a sample of size + n2 from the lot will be d = d1 + If d < c1, accept the lot after replacing d defective items.
d > C2, reject the lot. All the items of the lot are inspected and defectives replaced.
Double sampling plan provides another opportunity for the rejected lot of the first sample, reducing the possibility of 100% inspection of the lot. It is experienced that lots with border line defective items have a better opportunity of being accepted in double sampling scheme. But the per unit inspection cost in double sampling is found to be higher than that in single sampling scheme.
(iii) Sequential Sampling Plan:
This plan is simply an extension of double sampling plan. At each stage of sampling, the cumulated results are analysed to take a decision of accepting or rejecting a lot. If at any stage no final decision can be taken, then another sample is drawn to take further decision. The scheme helps in reducing the size of inspection to maintain a given level of protection.
The preference of any sampling scheme mainly depends on the degree of accuracy desired by the organisation as well as the resources available for inspection.
It is evident that acceptance sampling provides economy in inspection with fairly good chance of maintaining the desired quality standards.