Acceptance Sampling Plan
An inspection of a product or service that determines whether or not the product will be accepted. For example, a furniture manufacturer would use an acceptance sampling plan to make acceptance decisions related to the type and quality of wood and similar raw materials they purchase for inclusion into their finished products.
Meaning of Acceptance Sampling or Sampling Inspection
One method of controlling the quality of a product is 100% inspection which requires huge expenditure in terms of time, money and labour. Moreover due to boredom and fatigue involved in repetitive inspection process, there exists a possibility to overlook and some defective products may pass the inspection point.
Also when the quality of a product is tested by destructive testing (e.g., life of a candle or testing of electrical fuses) then 100% inspection shall destroy all the products.
The alternative is statistical sampling inspection methods. Here from the whole lot of products/items to be inspected, some items are selected for inspection.
If that sample of items conforms to be desired quality requirements then the whole lot is accepted, if it does not, the whole lot is rejected. Thus the sample items are considered to be the representative of the whole lot. This method of acceptance or rejection of a sample is called Acceptance Sampling.
In general acceptance sampling method proves to be economical and is used under the assumption when the quality characteristics of the item are under control and relatively homogeneous.
Classification of Acceptance Sampling Plan
Depending upon the type of inspection acceptance sampling may be classified in two ways:
(i) Acceptance sampling on the basis of attributes i.e. GO and NOT GO gauges, and
(ii) Acceptance sampling on the basis of variables.
In acceptance sampling by attributes, no actual measurement is done and the inspection is done by way of GO & NOT GO gauges. If the product conforms to the given specifications it is accepted, otherwise rejected. The magnitude of error is not important in this case.
For example if cracks is the criteria of inspection/the products with cracks will be rejected and without cracks accepted the shape and size of the cracks shall not be measured and considered.
In acceptance sampling by variables, the actual measurements of dimensions are taken or physical and chemical testing of the characteristics of sample of materials/products is done. If the results are as per specifications the lot is accepted otherwise rejected.
Advantages of Acceptance Sampling Plan
(i) The method is applicable in those industries where there is mass production and the industries follow a set production procedure.
(ii) The method is economical and easy to understand.
(iii) Causes less fatigue boredom.
(iv) Computation work involved is comparatively very small.
(v) The people involved in inspection can be easily imparted training.
(vi) Products of destructive nature during inspection can be easily inspected by sampling.
(vii) Due to quick inspection process, scheduling and delivery times are improved.
Limitations of Acceptance Sampling Plan
(i) It does not give 100% assurance for the confirmation of specifications so there is always some likelihood/risk of drawing wrong inference about the quality of the batch/lot.
(ii) Success of the system is dependent on, sampling randomness, quality characteristics to be tested, batch size and criteria of acceptance of lot.
Terms Used in Acceptance Sampling
Following terms are generally used in acceptance sampling:
- Acceptable Quality Level (AQL)
It is the desired quality level at which probability of a acceptance is high. It represents maximum proportion of defectives which the consumer finds acceptable or it is the maximum percent defectives that for the purpose of sampling inspection can be considered satisfactory.
- Lot Tolerance Percent Defective (LTPD) or Reject able Quality Level (RQL)
It is the quality level at which the probability of acceptance is low and below this level the lots are rejected. This prescribes the dividing line between good and bad lots. Lots at this quality level are considered to be poor.
- Average outgoing Quality (A.O.Q)
Acceptance sampling plans provides the assurance that the average quality level or percent defectives actually going to consumers will not exceed certain limit. Fig demonstrates the concept of average outgoing quality related with actual percent defectives being produced.
The AOQ curve indicates that as the actual percent defectives in a production process increases, initially the effect is for the lots to be passed for acceptance even though the number of defectives has gone up and the percent defectives going to the consumer increases.
If this upward trend continues, the acceptance plan beings to reject lots and when lots are rejected, 100% inspection is followed and defective units are replaced by good ones. The net effect is to improve the average quality of the outgoing products since the rejected lots which to be ultimately accepted contain all non-defective items (because of 100% inspection).
- Operating Characteristic Curve or O.C. Curve
Operating characteristic curve for a sampling plan is a graph between fraction defective in a lot and the probability of acceptance. In practice the performance of acceptance sampling for distinguishing defectives and acceptable or good and bad lots mainly depends upon the sample size (n) and the number of defectives permissible in the sample.
The O.C. curve shown in Fig. is the curve of a 100 percent inspection plan is said to be an ideal curve, because it is generated by and acceptance plan which creates no risk either for producer or the consumer. Fig. Shows the O.C. curve that passes through two stipulated points i.e. two pre-agreed points AQL and LTPD by the producer and the consumer.
Usually the producer’s and consumer’s risks are agreed upon Fig. and explicitly recorded in quantitative terms.
This leads to following two types of risks:
The merit of any sampling plan depends on the relationship of sampling cost to risk. As the cost of inspection go down the cost of accepting defectives increases.
Characteristics of O.C. Curve
(i) The larger the sample size and acceptance number steeper will be the slope of O.C. curve.
(ii) The O.C. curve of the sampling plans with acceptance number greater than zero are superior to those with acceptance number as zero.
(iii) Fixed sample size tends towards constant quality production.