iSixSigma

Acceptance Number

Definition of Acceptance Number:

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In life, acceptability is often a matter of personal taste and mood at any given moment. Unfortunately, this kind of subjective measurement isn’t useful in a commercial production environment. That’s why sample testing techniques use acceptance numbers as a quantifiable standard to determine if a batch of product meets basic quality control standards.

Overview: What are acceptance numbers?

You can’t discuss acceptance numbers without talking about acceptance sampling. This kind of sampling is the technique of randomly selecting items from a large batch of products to test them for defects. This quality control process is used in situations when testing every item is not possible because of logistical constraints or because testing would destroy the product in question.

This kind of quality control involves taking random samples from every batch of products or materials. These selected items are then individually tested or inspected to determine if they are defective. As long as the number of defective items is less than the acceptance number, the entire product lot is accepted as within quality standards.

4 pros and cons of acceptance numbers

In some ways, acceptance sampling and numbers are a compromise between efficiency and quality. While this kind of sampling may be the only viable way to test products in a particular manufacturing environment, it’s not without its drawbacks.

1. Pro: affordable and efficient

Testing by sampling is a relatively effective way to examine very large batches of products. Even if testing every item is technically possible, the financial and time costs can be high enough to render it untenable.

2. Pro: applicable to intermediary processes

Acceptance sampling is typically done at the end of the manufacturing process, but it doesn’t have to be limited to this step. You can actually apply it to materials or partial stages to prevent flaws from compounding through the production line.

3. Con: reliance on random sampling

The biggest weakness of acceptance samples and numbers is a reliance on randomness. There is always a chance that the samples selected don’t accurately represent the whole lot. This means there is still a small chance of shipping sub-quality products.

4. Con: possibility of false failures

Along the same line, there is also a chance that you will flag a product lot as defective when it’s not. This means you may end up discarding or rejecting some products when there’s nothing significantly wrong with them.

Why is the acceptance number important to understand?

This number is important to understand for any manufacturer who relies on acceptance sampling as part of their quality control program.

1. Planning for failure

When you establish an acceptance number, you are acknowledging that it’s at least possible that you will ship that many defective products with each lot. This means that you should prepare your clients or customers to address this possibility and encourage appropriate quality practices on their end.

2. Managing risks

There are always potential challenges when you ship defective products, which means you need to implement a basic risk management strategy accordingly. It’s important to understand the likelihood of returns, complaints or even lawsuits that may result from defective products.

3. Finding meaning behind the numbers

Changes in acceptance rate of product lots doesn’t always indicate a flaw in your production process. Sometimes you need to consider the possibility of problems in your testing technique, whether it’s operator error, faulty tools or a bad sampling process.

An industry example of an acceptance number

A chemical manufacturer produces canisters of different types and colors of paint for use in construction. Since physically applying the paint or subjecting it to chemical tests renders the canister useless for retail, the company relies on random sampling to test the quality.

The company produces large volumes of each paint all at once, resulting in batches of about 5,000 canisters per cycle. They test this batch by pulling 100 canisters at random and putting them through physical and chemical testing. Their acceptance number is 10, which means if there are more than 10 defects in the sample lot of 100, then the entire batch of 5,000 is considered not acceptable.

3 best practices when thinking about acceptance numbers

How you design, conduct and follow-up on your acceptance sampling process is vital to ensuring good quality-control measures. These standards have profound implications on your long-term viability, particularly when it comes to your reputation with customers.

1. Develop a strong sampling plan

Design your sampling process with care, consideration and support from statistical professionals. There are several ways you can set up this process to minimize the risk of errors without adding too much time or financial cost.

2. Evaluate and reconsider

Don’t be afraid to change your process, especially if you are still developing a reliable sampling technique. Talk to your product inspectors, clients and internal auditors to make sure you are getting the most out of your efforts.

3. Leverage the sampling data

As you collect data from all of the batch samples, don’t forget to apply conventional statistical analysis to dig deeper into the issues. Leveraging the data can help you improve overall defect prevention and identify problems in your own production processes.

Frequently Asked Questions (FAQ) about acceptance numbers

1. How do you find your acceptance number?

You need to know your acceptance quality level (AQL), which is the level of batch quality expected by your customers. The AQL and margin of error for your own sampling practices are both factors in finding the right number.

2. What’s the difference between an acceptance and rejection number?

When using integer values, the difference is 1. Rejection numbers are the minimum number of defective items that would render a batch unacceptable, which means they are always one degree of measurement higher than corresponding acceptance numbers.

3. What are the basic types of acceptance inspections?

Sampling strategies can be customized and tailored according to specific needs, but most plans are based on one of several fundamental strategies. These types include: single, double and sequential-sampling.

Acceptance and rejection

A reliable and efficient process for the acceptance or rejection of product batches is a must-have for large-scale manufacturing. It scales well and is often the only viable way to assess quality without excessive inspection costs. However, operators need to know the natural limitations of this technique and take steps to properly implement their sampling techniques to maintain a strong quality standard.

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