AQL Sampling Explained: A Worked Example for Your First Lots
Your manufacturer is about to ship 1,000 units. You will not inspect all of them. So how many do you check, and how many defects are too many?
That is what AQL sampling answers. This guide explains it in plain language, walks through a full example, and clears up two misunderstandings that show up in a lot of AQL content, including what "AQL 2.5" actually promises.
The example follows Northline, a fictional hardware startup shipping an aluminum laptop stand in three lots of 1,000 units, and Rainwell, a fictional skincare brand shipping a 6,300-unit serum lot.
What AQL means
AQL stands for acceptance quality limit. It is the worst long-run percent defective you are prepared to tolerate as a process average.
It is not the pass rule itself. You use it to pick a sampling plan from standard tables, usually ISO 2859-1 or ANSI/ASQ Z1.4, which work the same way. You choose three things:
- Lot size: how many units are in the lot being accepted
- Inspection level: how much sampling relative to lot size. General level II is the usual default. Levels I and III sample less and more.
- An AQL for each class of defect: a lower number is stricter
The table then gives you a sample size and an acceptance number (Ac): the most defects allowed in the sample for the lot to pass. The rejection number (Re) is one more.
A common mix-up: inspection levels (I, II, III) are not the same thing as normal, tightened, and reduced inspection. Levels set sample size relative to lot size. Normal, tightened, and reduced are switching states that change based on recent lot history. More on that below.
Step 1: Classify your defects
This is a decision, not a table lookup, and it is the step most people rush.
| Class | Northline NL-100 examples | AQL |
|---|---|---|
| Critical | Tips under 3 kg on a 15° slope. Sharp edge. Hinge breaks. | None. Tested on every unit where possible, and zero allowed in any sample. Criticals are not an AQL. |
| Major | Hinge torque outside 1.2 to 1.6 N·m. Pad missing or lifting. Flatness over 0.3 mm. Anodize defect visible at 40 cm under D65 light. Wrong color. | 2.5 |
| Minor | Scuff on an inside face not seen in use. Carton corner dent. Insert misprint. | 4.0 |
Write the classification into your quality agreement with the manufacturer so nobody argues about it on shipping day.
Step 2: Find the sample size and acceptance numbers
Northline's launch order is 3,000 units shipped as three lots of 1,000, at general inspection level II, normal inspection.
- A lot of 1,000 falls in the 501 to 1,200 band. At level II, the table gives code letter J.
- Code J means a sample of 80 units, pulled at random from across all cartons, not from the top of one pallet.
- Majors at AQL 2.5: accept with 5 or fewer, reject with 6 or more.
- Minors at AQL 4.0: accept with 7 or fewer, reject with 8 or more.
- Criticals: checked on every unit at final inspection. One critical in the sample rejects the lot.
Step 3: Inspect and decide
On the first lot, the inspector found 0 critical, 2 major, and 4 minor defects. Two is 5 or fewer, and four is 7 or fewer. The lot was accepted.
Had the inspector found 6 majors, the lot would be rejected. Under Northline's agreement, the manufacturer would sort all 1,000 units for that defect at its own cost, and the lot would be re-inspected.
What AQL does not mean
AQL 2.5 does not mean the lot contains 2.5 percent or fewer defects. It means a process running at about 2.5 percent defective will pass most of the time. Some worse lots will pass too. Sampling is a bet with known odds, not a guarantee.
A sample of 80 will rarely catch a defect that occurs once in a thousand. That is why critical features should not rely on AQL. Test them 100 percent, or prove the process is highly capable and keep it under control.
AQL is for lot acceptance, not process proof. It tells you whether to accept a lot. It does not tell you whether the process is capable. That comes from validation: stable control charts and capability indices such as Cpk and Ppk on critical features.
Capability is built up through the pilot builds and proven in process validation before launch. See EVT, DVT, PVT explained.
Normal, tightened, and reduced inspection
A sampling plan starts at normal inspection and changes only by rule. Under ANSI/ASQ Z1.4:
| Switch | Rule |
|---|---|
| Normal to tightened | 2 lots rejected within 5 or fewer consecutive lots on original inspection |
| Tightened to normal | 5 consecutive lots accepted on original inspection |
| Tightened for too long | If 5 lots in total are rejected while on tightened inspection, stop accepting under the plan until the supplier's corrective action is verified |
| Normal to reduced | The switching score reaches 30, production is steady, and you approve |
| Reduced to normal | Any lot rejected, production becomes irregular, or you decide normal is warranted |
The switching score starts at 0. For plans with an acceptance number of 2 or more, add 3 for each lot that would still have passed at the next stricter AQL; otherwise reset to 0. For plans with an acceptance number of 0 or 1, add 2 for each accepted lot and reset on any rejection. At one lot a month, reaching 30 takes about a year of clean lots. Do not let anyone move you to reduced inspection because "things have been fine lately."
Formulated products: what AQL covers and what it doesn't
Rainwell's first filled lot of 6,300 units falls in the 3,201 to 10,000 band, code letter L: sample 200 units. A pump that will not dispense makes the product unusable, so Rainwell set pump function at AQL 1.0, stricter than Northline's majors: accept with 5 or fewer failures, reject at 6.
Micro, pH, and viscosity are not AQL checks. They are tested on the batch and reported on the certificate of analysis. Fill weight is controlled by fill-weight checks across fill heads, not by the AQL sample. AQL covers what you inspect unit by unit: function, appearance, labeling, packaging.
Quick reference: setting up your first AQL plan
- Define critical, major, and minor defects with examples and photos
- Take criticals out of AQL: 100 percent test or proven capability, zero accepted
- Choose lot size to match how product is actually built (one set of material lots per lot)
- Start at general inspection level II, normal inspection
- Set AQL per class (a common starting point for consumer goods is 2.5 major and 4.0 minor)
- Specify random sampling across all cartons
- Write who pays for sorting and re-inspection on a rejected lot
- Put the whole plan in the quality agreement
Frequently asked questions
What does AQL 2.5 mean?
It means the plan is designed so that a process averaging about 2.5 percent defective will usually be accepted. It is a process-average concept used to select a sampling plan. It does not guarantee that any single accepted lot has 2.5 percent or fewer defects.
What sample size do I need for a lot of 1,000 units?
At general inspection level II under normal inspection, a lot of 1,000 falls under code letter J, which means 80 units. At AQL 2.5, accept with 5 or fewer defects and reject with 6.
Should critical defects have an AQL?
No. Critical defects (anything that could hurt someone or make the product unusable) should be tested on every unit where possible, with zero allowed in any sample. A sample rarely catches rare defects.
Who performs AQL inspection?
Either the manufacturer's quality team, a third-party inspection firm you hire, or your own staff. Many brands use third-party pre-shipment inspection for early lots and tie the payment balance to the lot passing.
Next steps
- See what your first-pass yield and defect rate do to unit cost in the product cost calculator.
- Check whether your defect classes and critical features are defined with the free manufacturer readiness check.
- The Contract Manufacturing Playbook covers validation and first-lot control in full, including Cpk and Ppk worked from raw data, success-run sample sizes for pass/fail tests, and a Validation Plan & Protocol template.