What is Acceptance Sampling?
Acceptance sampling is the process of accepting or rejecting a lot based on a sample drawn from the lot. It is the process where you choose a sample from a given population or a lot of a finished product, evaluate the sample on predefined parameters, and then decide whether to accept or reject the lot based on the results from the sample.
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To understand the procedure, consider the following example:
- You are the owner of a bakery and have received a consignment of pastries from a local catering firm. To decide whether the pastries meet the customer’s criteria of taste and freshness, you choose a sample from the lot.
- You test the sample on the predefined criteria of taste and freshness. You can even ask some customers to test the sample.
- Based on the test results, you can accept or reject an entire lot of pastries.
Acceptance sampling thus aims to help you decide whether to accept or reject a lot. It should be noted that acceptance sampling does not include any means to improve the quality of the product. It just acts as an auditing tool in performing quality control activities.
Acceptance sampling also includes an inherent risk that you decide to accept or reject a lot based on the choice of a sample. If you do not correctly select a sample, then you risk accepting a bad lot.
Reasons for Acceptance Sampling
Acceptance sampling is a popular method. Some of the reasons for its popularity are as follows:
- It is economical in terms of time and money.
- It puts the responsibility of sampling on a specific individual or a group of individuals.
- It helps manufacturers to remove a poor lot, which results in quality improvement.
- It provides accurate results if samples are inspected carefully and systematically. This reduces the number of errors in products.
- It helps manufacturers to identify the real issue of rejection and focus on product quality.
- It ensures that there is minimum damage to the lot, as it examines only a sample of the entire lot.
- It provides a stronger motivation for vendor improvement.
Types of Sampling Plans
A sampling plan specifies the parameters of the sampling process for accepting or rejecting a sample.
Single Sampling Plan
In this sampling method, you define the maximum number of defects. Based on this number, you select a random sample from a lot. You inspect every item of the sample and label it as good or bad after inspection.
The steps involved in the single sampling plan procedure:
Double Sampling Plan
If the results of the single sampling are inconclusive, then you perform a double sampling of the lot.
- Collect a random sample from the lot for inspection.
Multiple Sampling Plan
This sampling plan is the same as for double sampling except that the number of samples necessary to make a decision may be more than two. In this plan, smaller samples are used sequentially until a decision to accept or reject the lot is made.
When units are randomly chosen from a lot and tested one by one, and the cumulative number of inspected defects is recorded, the process is called sequential sampling.
If the cumulative number of defects is more than the Upper Specification Limit (USL) specified for that sample, then reject the whole lot.
If the cumulative number of rejects is less than or equal to the Lower Specification Limit (LSL), then accept the lot.
If the number of defects falls within these two boundaries, continue to sample units from the lot.
In conclusion, you can choose a sampling plan depending on the cost of collecting a sample and testing it. If the cost is high for double or multiple sampling plans, then you can opt for the single sampling plan.
Selection of the best sampling plan-single, double, or multiple depends on the types of products being inspected and their expected quality level.
For example, a low-quality batch of goods can be identified quickly and more cheaply with sequential sampling. On the other hand, there are instances when a single sampling plan is easier and simpler for workers to conduct.
Producer’s and Consumer’s Risks
In acceptance sampling, two parties are usually involved:
- The producer of the product
- The consumer of the product
When specifying a sampling plan, each party wants to avoid expensive mistakes in accepting or rejecting a lot.
The producer wants to avoid the mistake of having a good lot rejected because he/she usually must replace the rejected lot. This is called producer’s risk. On the other hand, the consumer or customer wants to avoid the mistake of accepting a bad lot because defects found in a lot that has already been accepted are usually the responsibility of the consumer. This is called consumer’s risk.