The quality control industry is one that is shrouded in misconceptions, particularly when it comes to QC in China. For instance, many people assume that every single item is checked for its quality during a product inspection.
An inspection covering 100 percent of an order is sometimes necessary for certain buyers and products. But for most importers, a product inspection conducted by verifying a random sample of an order is typically a more cost-effective approach. AQL is the most common sampling method used in the product inspection industry among importers, suppliers and third-party inspeciton companies (related: The Importer's Guide to Managing Product Quality with AQL [eBook]).
Sample sizes can range from anywhere from 2 to 2000 units. In this article, we break down how to determine a sample size using different inspection levels to help guide you to the scope of inspection that is best for you and your product.
Acceptance sampling for quality control inspection
“Acceptance sampling” is a method used to determine how many pieces of a batch you should check to provide a statistically valid representation of the order as a whole. When a random sample of product is checked using acceptance sampling, you can make a decision to accept or reject the entire lot based on the quality level of the inspected sample (related: How AQL Sampling Affects Your Product Inspection Results). When using AQL for inspection, acceptance sampling decides the sample sizes for different order quantities.
General inspection levels
There are three main inspection levels used for acceptance sampling, in addition to lesser-used special inspection levels. These main inspection levels – “GI”, “GII” and “GIII” – stand for general inspection level 1, 2 and 3, respectively. Any professional AQL chart will show the GI, GII, and GIII sample size of each group of lot sizes.
These three different levels indicate sample sizes for a non-destructive inspection. The inspector inspects the entire sample size for quality issues related to performance, function and visual apperance. Any quality defects found during inspection contribute to the final AQL result of the inspection.
GI vs. GII vs. GIII inspection levels
A higher inspection level corresponds to a larger sample size. For example, let’s look at a lot size of 5,000 pieces shown in the table below. The three inspection levels each have a different sample size, which means you need to decide how many samples you would like to check for this lot.
|Lot Size||General Inspection I
|General Inspection II
|General Inspection III
However, the lot size is not the only factor at work when determining the sample size. There are many other elements that need to be taken into account, such as the quality history at a factory, required testing, available man-power and budget.
GI inspection level
A GI inspection level is a reduced sample size, or the smallest sample size of the three options. GI may be adequate in cases where the suppliers have a good quality background, where destructive testing is used or where you have strict budgetary constraints.
GII inspection level
GII level is sometimes also referred to as a normal sample size, as it's the most commonly-used inspection level for quality control inspection. Many importers start with a GII inspection level when first using AQL for inspeciton.
GIII inspection level
The GIII inspection level provides a sample with the highest percentage of pieces from the lot out of the three inspection levels. A stricter inspection level allows for less risk of accepting an shipment with excessive defects, which is especially important if your supplier has a history of poor quality. Next to 100 percent inspection, this general inspection level offers the largest scope and most assurance of order status and quality.
Some buyers mistakenly think that product inspection means verifying the quality of every piece in a lot of goods.
A random sample of product pulled from a lot based on general inspection levels will be suitable in most cases (related: Why QC Professionals Use AQL Sampling for Product Inspection). Your decision to use one sampling method or another for QC inspection, or one sample size over another, will depend on your unique circumstances, budget, product and quality history with your supplier.
Learn more about how to select an appropriate sample size for quality control inspection in our eBook below!