Q 321. What is Acceptance Sampling (or Lot Quality Assurance Sampling)? Provide examples of business areas where this can be used.

 

 

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Benchmark Six Sigma Expert View by Venugopal R

“Acceptance sampling” refers to sampling methods used to take a decision on accepting or rejecting a lot. This method has been widely used as part of the ‘Incoming goods acceptance’ procedures for organizations that buy materials from suppliers or sub-contractors. However, the method is applicable to other areas as well, viz. Finished goods clearance, In-process acceptance.

 

Some of the business examples where Acceptance Sampling may be applied:

1. To evaluate the ‘Lots’ or ‘Batches’ of incoming components for a manufacturing organization
2. To assess the quality of transactions executed during a period of time in a BPO industry
3. A bank who process a large batch of cheques are by automatic optical character reading, may use Acceptance sampling for verifying whether the output meets zero defect acceptance criteria.
4. A department store can verify the weight of pre-packed goods in a consignment on samples to decide on accepting / rejecting the consignment.
5. Pharma approval authorities may use Acceptance Sampling as one amongst the many procedures adopted to take decisions on certifying the release of a batch of medicines.

The main advantage of using Acceptance Sampling is to save the cost, efforts, handling and time involved in inspecting the entire lot. By taking a decision to accept or reject based on the Acceptance Sampling procedure, the rejected lots are usually expected to be reworked, replaced or segregated by the concerned supplier.

 

However, there are certain disadvantages as well. One of the disadvantages in using Acceptance Sampling is the presence of ‘sampling risks’. There are two types of sampling risk:

1.     Good lots can be rejected (Producer’s risk or alpha risk)

2.     Bad lots can be accepted (Consumer’s risk or Beta risk)

‘Good lots’ mean lots containing defect levels that exceed acceptable levels. Assume that we fix the Acceptable Quality Level (AQL) as 1.0%. Then, ideally the sampling plan should accept all lots that have defective level less than 1.0%, and reject all lots that have defective level higher than 1.0%. The ideal operating characteristic curve (OC curve) would then look as shown below:

The above situation is only ideal, and in reality, the OC curve for an sampling plan would look like something as shown below:

 

The AQL is defined as the maximum percentage non conforming that for purpose of sampling inspection is considered satisfactory as a process average. The OC curve gives the probability of acceptance of the AQL. Similarly another point towards the lower end of the Y axis will represent what is knows as the LQL (Limiting Quality Level). This determines the limiting quality for which we expect a very low probability of acceptance.

 

These two points on the OC curve define a sampling plan. Any sampling plan will have its OC curve. 

 

In the modern world with the focus moving towards Lean, JIT, Process Capabilities and Supplier certifications, the importance of Acceptance Sampling Plan as a long term control measure is reducing. However, it is important to understand the principles behind it and it still continues to have application in many situations

Acceptance Sampling also known as Sampling Inspection or Lot Quality Assurance Sampling is a statistical tool or technique for quality control, where a whole lot of products or items are to be inspected, some items from the lot are selected for inspection. If the sample of items meets the desired quality requirements, then the whole lot is accepted, else, the same lot is rejected. Thus, the basic concept of sampling here is testing the samples for acceptance or rejection.

 

Acceptance Sampling was first developed in World War II as a quick fix for manufacturing that was originally applied by the US Military to the testing of bullets during World War II. Harold Dodge who was a veteran of the Bell Laboratories quality assurance department, developed the methodology. He was acting as a consultant to the Secretary of War.

 

Advantages:

• Less expensive due to less inspection.
• Less handling of the product hence reduced damage.
• Applicable to destructive testing.
• Less personnel required for inspection of activities.
• Reduces the amount of inspection errors.
• As it is an economical method, it is possible to carry out sample inspection at various stages.
• A good sampling plan can give better results than that achieved by 100 percent inspection, as there exits inspection fatigue in 100% inspection.

Disadvantages:

• There exists a risk of accepting bad lots and rejecting good lots.
• This method requires proper planning and documentation of the procedure whereas 100% inspection does not.

Examples:

1. A production line that produces helmets rarely produces defective items, however when a defect occurs the entire lot is impacted as the machine is out of calibration. Hence Quality consultant decides testing 1 helmet in every lot of 10,000 is sufficient quality control. When a helmet is defective, the entire lot is tested and the problem is immediately escalated to quality consultant.
2. A chain of restaurants controls quality by paying secret or surprise visits and evaluations.
3. In a food manufacturing company, basic inspection of ingredients are done before producing a batch. However, as an additional control, the company samples for detailed testing. When a test fails, production is stopped and the issue is investigated.

Glory has given the best description and uses for Acceptance Sampling and hence her answer has been selected as the best answer.

 

Also review the answer provided by Mr Venugopal R, Benchmark Six Sigma's in-house expert.