What acceptance sampling and AQL are
Acceptance sampling decides whether to accept or reject a lot based on inspecting a sample. The AQL (acceptable quality level) is the worst quality level considered acceptable as a process average and is the reference for the sampling plan.
The most widely used standard is ISO 2859-1, equivalent to ANSI/ASQ Z1.4.
AQL sampling visual map
The map shows the steps and the decision for the example.
Step by step
- Define the lot size.
- Choose the inspection level (general level II is the default).
- Find the code letter in the standard's table.
- Get the sample size from the code letter.
- Set the AQL agreed with the customer or supplier.
- Read the acceptance (Ac) and rejection (Re) numbers.
Example: a lot of 1,000 pieces
| Step | Result |
|---|---|
| Lot | 1,000 pieces |
| Inspection level | General II, normal inspection, single sampling |
| Code letter | J |
| Sample size | 80 pieces |
| AQL | 1.0 |
| Ac / Re | 2 / 3 |
Decision: up to 2 defectives in the sample of 80, accept the lot; 3 or more, reject it.
Normal, tightened and reduced inspection
| Type | When |
|---|---|
| Normal | Standard condition, at the start of inspection |
| Tightened | When there are recent quality problems (for example, 2 of 5 lots rejected) |
| Reduced | When the supplier keeps a good record and the process is under control |
What sampling does not do
Sampling does not improve quality: it only makes decisions about lots. An AQL of 1.0 still accepts lots containing some defectives. The Lean path is to rely less on inspection through stable processes, poka-yoke and supplier partnerships.
Common mistakes
- Taking the sample from one spot in the lot.
- Choosing the AQL without criteria or customer agreement.
- Reading the wrong table or confusing Ac and Re.
- Ignoring supplier history.
Frequently asked questions
What is AQL?
Acceptable quality level: the quality reference used to define the sampling plan.
What sample size applies to a lot of 1,000 pieces?
At general level II, code letter J and a sample of 80 pieces.
What are Ac and Re?
The acceptance and rejection numbers: the defective limits for accepting or rejecting the lot.
Is ISO 2859-1 the same as ANSI Z1.4?
They are equivalent standards for sampling by attributes.
Does sampling guarantee a defect-free lot?
No. It decides on the lot with a known risk but does not remove defects.
Sources
- ISO 2859-1. Sampling procedures for inspection by attributes. Geneva: ISO, 1999.
- ANSI/ASQ Z1.4. Sampling Procedures and Tables for Inspection by Attributes. Milwaukee: ASQ.
- MONTGOMERY, D. C. Introduction to Statistical Quality Control. Hoboken: Wiley.
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About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.