Brief History of AQL and Acceptance sampling

Acceptance sampling is an important field of statistical quality control that waspopularized byDodge and Roming and originally applied by the U.S. military for the testing of bullets during World War II.

Based on the extensive work by the American military during and past World War II, US Government issued the standard for sampling procedure and tables for inspection called MIL-STD-105D in 1963. This was further modified in 1989 as MIL-STD 105 E and re-designated as ANSI/ ASQC Z 1.4 in Feb 1995.


Inspections levels:

Checking 100% of the quantity would be long and expensive. A
solution is to select samples at random and inspect them, instead of checking
the whole lot.

How many samples to select?

The objective is to keep the inspection short by reducing the number of samples to
General inspection level

The relevant standards propose a standard severity, called “General inspection levels”, there are three general levels: I, II, and III. Level II is used for more than 90% of inspections.

Suppose a product from a factory that often ships substandard quality. The risk is higher than average. You can opt for level III, and more samples will be checked.

In the case where the supplier has consistently delivered acceptable products in
the past and keeps its organization unchanged, the level I can be choose.
As fewer samples have to be checked, the inspection might take less time and be

The relevant standards give no indication about when to switch inspection levels, so most importers rely on their “gut feeling”.

The three “General” inspection levels
Level I

Has this supplier passed most previous inspections? Do you feel confident in their products quality? Instead of doing no quality control, buyers can check less samples by opting for a level-I inspection. However, settling on this level by default, in order to spend less time/money on inspections, is very risky. The likelihood of finding quality problems is lower than generally recommended.

Level II

It is the most widely used inspection level, to be used by default.

Level III

If a supplier recently had quality problems, this level is appropriate. More samples are inspected, and a batch of products will (most probably) be rejected if it is below the quality criteria defined by the buyer. Some buyers opt for level-III inspections for high-value products. It can also be interesting for small quantities, where the inspection would take only one day whatever the level chosen.

Special inspections levels

Inspectors frequently have to perform some special tests on the products they are
checking. In some cases the tests can only be performed on very few samples, for two reasons:

-They might take a long time (e.g. doing a full function test as per claims on the retail box).

-They end up in product destruction. (e.g. unstitching a jacket to check the lining fabric).

For these situations only, the inspector can choose a “special level”.

The four “Special” inspection levels
These special levels can be applied in cases where only very few samples can be checked.

“Four additional special levels, S-1, S-2, S-3 and S-4 […] may be used where relatively small sample sizes are necessary and larger sampling risks can be tolerated” (ISO 2859 standard).
Under S-3 level, the number of samples to check is lower than under S-4, and so on.

In practice: for consumer goods, quality control is usually performed under the general levels.
The special levels are used only for certain tests that either take lots of time or destroy the samples. Another situation where special levels are appropriate is a container-loading supervision. To have an idea of what is inside the cartons, without spending too much time at that checking.


The three categories of defects

Some defects are much worse than others. Three categories are typically distinguished:

  • Critical defects might harm a user or cause a whole shipment to be blocked by the customs.  Critical defects are those that render the product
    unsafe or hazardous for the end user or that contravene mandatory regulations
  • Major defects are not accepted by most consumers, who decide not to buy the product. Major defects can result in the product's failure,reducing its marketability, usability or saleability
  • Lastly Minor defects also represent a departure from specifications, but some
    consumers would still buy the product. do not affect the product's marketability or usability, but represent workmanship defects that make the product fall short of defined quality standards.
  • In order to avoid argument, buyers and sellers agree on an AQL standard, chosen according to the level of risk each party assumes, which they use as a reference during pre-shipment inspection


  • A professional inspector will notice defects and evaluate their category by himself. But it is better if the buyer himself describes the most frequent defects and assigns categories to each one.
  • Defects can be on the product itself, on the labelling or on the packaging.
  • If one sample presents several defects, only the most severe one is counted.


Four types of inspections depending on the risks identified by the buyer.


Catch problems early: Inspectors have enough time. in the table of AQLGeneral
inspection level
can be used. 


1°) Pre-production inspection

This type of inspection is necessary if you want to check the raw materials or components that will be used in production.

It can also be used to monitor the processes followed by the operators. The pre-production inspection can also focus on the processes followed as production starts and see if the buyer’s blueprints are respected (e.g. patterns for cutting fabric are received from the buyer....)


2°) During production inspection

This inspection allows you to get a good idea of average product quality, and to ask for corrections if problems are found but these samples might not be representative of the whole order. So usually it is better to do an inspection during production after
10-30% of the products are finished.
The buyer have an idea of average product quality, early in the production cycle. It is the most useful and the most under-rated tool at the disposal of importers, who often only rely on final inspections.


At the end of production, the client (e.g supermarket) requests a pre-delivery
inspection. Usually on the inspector’s checklist, this typically includes:
- Packaging conformity (barcodes, inner packing, cartons, shipping marks…)

- Product conformity (aspect, workmanship …) e.g: All products have a different colour
than requested, there is no need to count each sample as a defect. It makes more sense to refuse for product conformity.

- Specific tests defined in the inspection checklist (e.g. functional tests, functionality,
security….). They might not be performed on all inspected samples if they are
time-consuming or destructive.


Verify quality before shipment: These inspections are usually performed in hurry (just before or during the loading). Inspectors’ lack of time, in the table of AQL, only Special inspection level can be used. 

3°) Final inspection (alsocalled pre-shipment inspection)


Inspecting the goods after they are made and packed is the standard QC solution of most importers. The inspector can really check every detail, including counting the total quantity and confirming the packaging.

Loading supervision

In some cases, a buyer wants to make sure the factory ships the right products, in the right quantity, and with the right loading plan.

Its objective is really to confirm a shipment’s quality, rather than catching issues early


4°) Container loading supervision 

The inspector can arrive a little in advance, open a few cartons, and check if the products and the inner packing are conform to what the buyer is expecting.

The objective is :

- to ensure that the right kind of goods, quantity, conformity of the product, shelf live, conditioning, marking/labelling….) is shipped

- Packaging conformity (barcodes, inner packing, cartons, shipping marks…)

- Product conformity (aspect, workmanship…)

- examine the container condition (leak, smell, cleanliness….verify generating setting
for reefer …)

- He supervises the quality of the loading, handling

- Clarifies the responsibility of the forwarder & seal the container at the end of the loading