Can you imagine trying to determine the of a shipment of bullets by firing them one after another? How would you know what quantity to test in the total order if the method destroys each bullet you test?
The U.S. military faced this exact problem during World War II. They wanted to check the quality of their munitions so that they could be confident the bullets would function properly in the field. But they didn’t want to test so many that there were none left to actually ship to the warfront. The solution they developed was MIL-STD-105, an important standard using acceptable quality levels (AQL). Later on, ANSI Z1.4 was developed from that original military standard, it became the most popular standard inspectors use today.
Why AQL sampling is widely used for QC inspection?
1. Sampling saves time during inspection
Inspecting every unit in an order of goods offers the greatest transparency and oversight. But 100% inspection takes long time. And if you’re like most importers, you’ve probably agreed with your customers to meet certain deadlines. The time needed to inspect every unit can require longer production and shipping lead times, especially when a team outside the factory will be conducting inspection.
Factory managers, in particular, often prefer inspection of a sample because of the time and human resources needed to check 100 percent. Inspection occurring after the packaging stage requires factory workers to repack every unit before shipping. Factory staff see inspection of certain samples to be less disruptive because they need to commit less time to assisting with the process.
2. 100% inspection is not practical for low-cost items
International sporting goods companies like Nike and Adidas, for example, produce millions of garments per year through contract manufacturers in Asia. It’s difficult for them to justify inspecting every unit coming off production lines while still meeting shipping deadlines. Inspecting certain samples takes relatively less time while still offering valuable insight into the average quality and conformance of an order.
3. Inspecting certain samples is cost effective
The cost of inspection is typically directly related to the time needed to inspect. For example, third-party inspection companies usually bill services in “man-days”, a daily rate for an inspector to remain at a factory checking and testing an order. A longer or more thorough inspection may require you to send more QC staff to a factory for longer time. The more days, or inspectors, needed on-site, the higher the cost of inspection.
Another cost consideration is the inspection and testing methods themselves. Just as inspecting bullets can include destructive testing, so too can inspecting other types of products. Accurately checking fabric density in a garment, for example, requires inspectors to cut a piece of fabric from a unit to measure. It’s usually best to conduct destructive testing on one unit of an order, because the testing destroys the sample.
AQL sampling, coupled with a carton pulling procedure that selects a truly representative sample from a production lot, offers a statistically significant result showing the average order quality. Most importers rely on sampling for inspection because they can get a reasonable idea of how closely an order meets their expectations without paying to inspect every unit.
4. AQL sampling offers clear inspection results
Since acceptance sampling is based on statistics, it produces inspection results that are relatively clear and easy to interpret. AQL sets specific limits for the number of defects found in a particular sample size. The inspection result is “fail” when the number of defects found exceeds a particular limit. And you can make a relatively easy decision to accept or reject based on this result.
5. AQL standards are highly flexible
AQL is highly flexible because it allows you to customize your quality tolerance for your product and for the three types of quality defects: critical, major and minor. Consumer products often use AQLs of 0, 2.5 and 4.0 for critical, major and minor defects, respectively. You might require stricter AQLs for high-value items, such as luxury watches. You might also have customers with strict standards that require you to use lower AQLs. AQL sampling allows you to choose the quality tolerances that are best for you.
By inspecting certain samples of an order with AQL, you can apply black and white standards to a production lot without inspecting every unit. AQL provides you with statistically significant insight into your order in a clear, cost-effective and efficient manner so that you can make shipping decision quickly.