Also called acceptable quality limit, acceptable quality level (AQL) is the percentage of defective goods that a client or buyer considers satisfactory or acceptable within a production sample representative of total output. In other words, AQL is the maximum faulty units a sample could have. If the sample has defects higher than the determined AQL, the entire batch would be rejected.
For example, a company is making 1000 mobile phone panels for a client in batches, with each batch comprising 100 panels. The panels are made as per buyer specifications. However, the buyer states it’ll accept a finished batch if there are no defective panels, or if the number of faults (minor or major) isn’t more than three per batch. The particular batch’s AQL is therefore 3 percent. Though three or lesser defective panels are considered acceptable, the manufacturer’s goal would always be to ensure zero defects.
Sample Size and Number
As aforementioned, product inspection for AQL is carried out through samples where units for testing are picked up randomly from a lot. The number of samples and number of units per sample are chosen based on the total units produced and product types (if any). For instance, if a particular item is produced in the thousands, multiple samples would be created and tested separately. But if the number of units is just a few 100, then only a sample would suffice. Typically, the batch or sample size is 10 to 20 percent of the total units produced. However, it’s perfectly fine for the sample size to be smaller or larger.
If a supplier is making different products for the same buyer, then the different items would have their own individual samples. For example, if a leather factory is making shoes, belts and wallets to be delivered on the same date, the buyer would create a sample each for the different products that only contain the particular item. Even if the number of units to be produced are few, all the items would not be put together to create an all-encompassing sample.
AQL varies with different processes, products, buyers and also the type of defect. In the above leather company example, the AQL could differ for each product. Similarly, the quality limit could be higher or less stringent for shirts than airplane components.
Usually, a manufacturer sets the AQL a level stricter or lower than a client’s requirements. This is usually a precautionary measure to increase the chances of a sample clearing the client’s final inspection. Also, if a manufacturer’s average rate of producing defective goods is 5 percent and the client has set the AQL at 3 percent, the manufacturer may conduct 100 percent inspection to ensure the goods clear client inspection. Also, if the AQL set by the buyer is too strict, the supplier may ask for some relaxation or forgo the order.
Types of Defects
As aforementioned, product defects get categorized as minor, major and critical (based on the product’s application) by the buyer. AQLs are set differently for minor and major defects. Usually, the level set for major defects is more stringent or lower. A major defect is something that would prompt the customer to take a product back to the seller for repair, replacement or refund. A minor defect, on the other hand, is something the buyer may be dissatisfied with but would not return the product for.
A “critical” defect, on the other hand, has zero tolerance levels. A critical product defect is something that could harm the user or when a product is not made as per industry or government regulations. Critical defects do not apply to every industry. For instance, the medical, dairy, electronics and automobiles industry may categorize product defects as “critical” but the garments industry may not.
AQL is a quality standard that helps determine a production lot’s quality without the need to inspect each and every item, thereby helping the buyer save money and time. Besides helping ascertain a production batch’s quality, AQL also provides insights into a production process’ effectiveness or efficiency. If units churned out by a particular machine has been historically failing the AQL test, it certainly indicates the equipment needs to be looked into.
AQL is not the leverage that lets a manufacturer send a product batch with acceptable number of defects to the client. It essentially means a client would accept a particular batch if the sample has a defect lower than AQL. And the particular units that didn’t clear the inspection would not be lapped, irrespective of the sample getting approved or rejected. In case the sample has defects exceeding the AQL, the entire lot would be rejected, including the good units.
When Should Be AQL Applied?
On any given day, zero defects would be preferred. But there are instances when defects cannot be completely eliminated and when these defects are acceptable to an extent. When quality is paramount and no defect could be tolerated, AQL is not recommended. Generally, the AQL technique is applied when the buyer doesn’t expect production to be completely defect-free. This levy is provided when a marginal number of defects may not hurt the buyer’s business much. AQL is set only to ensure there aren’t too many defects.