If you have ever reviewed a quality inspection report based on AQL sampling, you have probably seen the abbreviations Ac and Re next to the defect counts. Ac stands for Acceptance Number — the maximum number of defective units allowed for the lot to pass. Re stands for Rejection Number — the minimum number of defective units that causes the lot to fail. Understanding how these numbers work is essential for properly interpreting inspection results.

What Are Ac and Re?

In statistical sampling inspection, Ac and Re are the decision boundaries. After inspecting the sample units, you count the number of defects found and compare them against these two numbers:

• Ac (Acceptance Number): If the number of defective units is less than or equal to Ac, the lot is considered acceptable for that defect category.
• Re (Rejection Number): If the number of defective units is greater than or equal to Re, the lot is considered rejected for that defect category.

For standard single-sampling plans, Re is always Ac + 1. There is no middle ground — either the lot passes or it fails. If you find exactly Ac defects, it passes. If you find Re defects (one more), it fails.

Where Do Ac and Re Come From?

Ac and Re values are not arbitrary. They are derived from the AQL (Acceptable Quality Limit) tables defined in the ISO 2859-1 / ANSI Z1.4 standard. To find the Ac and Re numbers for a specific inspection, you need three pieces of information:

1. Lot size — the total number of units in the production batch
2. Inspection level — usually General Inspection Level II for normal consumer products
3. AQL level — the acceptable defect rate you specify (e.g., 2.5 for major defects, 4.0 for minor)

The process is:

1. Use the lot size and inspection level to find the sample size code letter (from Table I)
2. Use the code letter to find the sample size (from Table II-A or II-B)
3. Read across to the column for your AQL level to find the Ac and Re numbers

For example, if you have a lot size of 3,200 pieces at General Inspection Level II with AQL 2.5 for major defects:

• Lot size 3,200 + Level II = sample size code letter K
• Code letter K = sample size of 125 units
• AQL 2.5 column = Ac 7, Re 8

This means that after inspecting 125 samples, if you find 7 or fewer major defects, the lot passes. If you find 8 or more major defects, the lot fails.

Reading an Inspection Result

Here is a typical summary you might see in an inspection report:

Sample size: 200 units
Critical defects (AQL 0): Found 0 — Ac 0, Re 1 — PASS
Major defects (AQL 2.5): Found 6 — Ac 10, Re 11 — PASS
Minor defects (AQL 4.0): Found 14 — Ac 14, Re 15 — PASS

In this example, all three categories pass. The lot is accepted. But notice how close minor defects came to failing — 14 found with an acceptance number of 14. That is right at the boundary, which should raise a flag even though technically it passes.

On the other hand, if the major defect count were 11 (instead of 6), the lot would fail for major defects even if critical and minor both passed. A failure in any single category means the overall inspection result is a fail.

Common Misconceptions

Misconception 1: "AQL 2.5 means 2.5% of my products can be defective."
This is not exactly right. AQL is the quality level that, for sampling purposes, represents the worst tolerable process average. It is not a guarantee that no more than 2.5% of your entire lot is defective. Because sampling is statistical, there is always some risk — both of accepting bad lots (consumer's risk) and rejecting good lots (producer's risk).

Misconception 2: "If the inspection passes, there are no defects."
No. A passing result means the defect rate is likely below the AQL threshold you set. AQL sampling is designed to catch lots that are significantly worse than the AQL, not to guarantee zero defects.

Misconception 3: "Ac means the factory is allowed that many defects."
Ac is a statistical decision threshold, not a target or permission. Good factories aim for far fewer defects than the Ac number allows.

How to Choose the Right AQL Levels

Selecting appropriate AQL levels depends on your product, your customers' expectations, and your risk tolerance:

• Critical defects: AQL 0 — anything hazardous or that could cause injury is never acceptable
• Major defects: AQL 2.5 — standard for most consumer goods where functional or significant cosmetic defects would result in customer returns
• Minor defects: AQL 4.0 — common for minor cosmetic issues that do not affect product function

For high-value products, products with strict safety requirements, or for initial orders with a new supplier, you may want tighter AQL levels (such as 1.0 for major defects). For low-value commodity items where minor imperfections are acceptable, you might loosen to AQL 4.0 for major defects.

Common Pitfall: Changing AQL After the Fact

A common mistake is to negotiate AQL levels downward after a failed inspection. Some buyers, wanting to avoid rework costs and shipment delays, say things like "well, maybe AQL 4.0 is fine for major defects too" after seeing that the lot fails at 2.5.

Best practice: Define your AQL levels before the inspection and stick with them. If you consistently find that your standards are too tight for the product category or factory capability, adjust them for future orders — not after seeing the results of a specific inspection. Changing the rules after you see the numbers defeats the purpose of statistical sampling.

Conclusion

Ac and Re are the decision numbers that make AQL sampling work. They translate statistical quality standards into clear pass/fail criteria you can apply on the factory floor. Understanding how they are derived and what they really mean helps you make better decisions about your shipments — and have better discussions with your suppliers.

If you need help setting up AQL standards for your products or interpreting inspection results, contact QCANT at cs@QCANT.com. Our quality engineers can help you define the right inspection criteria for your product categories and risk profile.