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Acceptance Sampling: What It Is, How It Works

What is Acceptance Sampling?

Acceptance sampling serves as a vital statistical tool in quality control for companies. It involves selecting a specific number of items from a batch for testing, providing insight into the overall quality of the entire lot.

Testing every single product is often impractical due to cost and time constraints, and extensive testing may damage or render items unsellable. By examining a representative sample, companies can gauge quality without compromising the majority of their production.

Key points:

  • Acceptance sampling entails randomly testing samples from a batch to assess overall quality, employing statistical analysis.
  • When properly implemented, acceptance sampling is an effective quality control method.
  • While initially developed during World War II to address manufacturing needs, acceptance sampling should complement, not replace, more comprehensive quality control approaches over the long term.

How Acceptance Sampling Works

Various methods are employed, with the most basic one involving random testing of a single unit for every x units manufactured (referred to as an (n, c) plan). Acceptance criteria are determined by the number of defective units (c) discovered within the sample size (n). Alternate approaches include employing multiple sampling, where several (n, c) evaluations are conducted. While employing multiple samples incurs higher costs, it potentially offers greater accuracy.

A History of Acceptance Sampling

Acceptance sampling has a rich history that dates back to the early 20th century. It was primarily developed during World War II to address the need for efficiently assessing the quality of munitions production. Designed by Bell Labs employees Harold Dodge and Harry Romig, the method was a response to the impracticality of inspecting every bullet, shell, or component produced.

American physicist/statistician Walter A Shewhart is often credited with laying the foundations for acceptance sampling. He introduced the concept of a sampling plan, which involved assessing a random sample of items to make inferences about the quality of an entire lot. Shewhart’s work paved the way for the development of various sampling techniques and standards that are widely used in quality control and manufacturing processes today.

However, Dodge stated in 1969 it’s crucial to note that acceptance sampling differs from controlling acceptable quality levels.

Based on particular sampling plans, acceptance sampling is directed at individual lots, serving as an immediate, short-term evaluation—a kind of on-the-spot inspection. In contrast, the acceptable quality level (AQL) operates on a wider, longer-term scale, encompassing the entire product line. It operates as a fundamental component within a well-structured manufacturing process and system.

Types of Acceptance Sampling

The acceptance sampling plan encompasses three primary types.

Single Sampling Plan

This strategy involves extracting a sample from the batch and assessing it against specific quality standards. Essentially, it aims to verify whether the defective items fall within acceptable limits. If the batch fails to meet these criteria, the entire lot is rejected. Single sampling plans are typically used for assessing products manufactured in smaller batches.

Double Sampling Plan

In a double sampling approach, two samples are drawn from the lot and compared against predetermined quality criteria. This method establishes two acceptance thresholds: the smallest acceptance number and the largest acceptance number. The lot is accepted if the defective items are fewer than the smallest acceptance number but rejected if they exceed the largest acceptance number. When the defective items fall between these numbers, a second sample is taken. If the total defective count from both samples surpasses the second acceptance number, the entire batch is rejected.

Multiple or Sequential Sampling Plan

Multiple sampling involves employing more than two samples to make a decision. Sequential sampling, for instance, utilises several samples. Post each group’s sampling, a quality criterion assessment is performed. If the batch does not surpass the specified limit, the process continues.

Advantages and Disadvantages of Acceptance Sampling

Advantages 

Initially, this method proves cost-effective for inspection due to the limited number of items tested, minimising expenses. Reduced product handling contributes to lower potential for damage. The probability of inspection errors remains minimal. It serves as a straightforward means of assessing quality and adherence to standards. In cases where the defective items exceed the acceptable limit, it prompts manufacturers to enhance quality as the entire lot faces rejection. Moreover, this sampling approach demands limited inspection time, a smaller workforce, and fewer specialists for execution.

Disadvantages 

However, one drawback is the smaller sample size compared to the overall lot, which can make it challenging to assess suitability for specific situations. Moreover, there’s no assurance that every batch will meet specifications, leading to the possibility of accepting subpar batches.

Acceptance Sampling Example

Floral company Fresh Scents Pty Ltd ships a batch of 20,000 artificial flower bouquets to event management outfit Platinum Happenings Pty Ltd. Upon receipt, the Platinum staff intends to perform a quality check on the bouquets. However, due to the time and cost involved in individually inspecting and assessing each bouquet, they opt to employ the acceptance sampling by attributes method, picking out 60 flower bouquets from the total lot of 20,000.

Under the acceptance criteria the Platinum team develops, the shipment will be approved if the defective bouquets number less than five. However, if there are at least five defectives, the entire shipment will be rejected and sent back to Fresh Scents.

FAQs

When should acceptance sampling be used? 

Due to its reliance on statistical inference drawn from a limited sample size, it lacks the precision of more comprehensive quality control methods. Therefore, it’s best suited for situations where producing a large number of items makes extensive testing impractical or unfeasible. Alternatively, it’s useful when examining a unit would render it unusable again, such as in the case of testing a fire extinguisher.

How does acceptance sampling differ from process control?

Sampling serves as a means to assess product quality before or after production, whereas process control, a prevalent statistical technique, actively monitors and manages the production process to achieve desired outputs consistently. Thus, it remains an ongoing aspect throughout production.

Conclusion

Acceptance sampling is a powerful quality control technique used in a variety of industries, each with its own specific standards and guidelines. By understanding its principles and applying them in various scenarios, businesses can maintain and improve the quality of their products, ultimately ensuring customer satisfaction, regulatory compliance, and safety.

DISCLAIMER: This article is for informational purposes only. BARTERCARD has no corporate interests in the development of any acceptance sampling techniques or standards.

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