Combined Sampling Inspection: Multiple SKUs In a Single Lot

What is Combined Sampling?

You may be importing products from China or other Asian countries and cooperate with a third-party inspection agency (such as AQI Service), then you will need to select the sample size and pay the inspection company by “Man-Days” based on the workload.

This means the sample size increases, the inspection cost will also increase, because the inspection company may take more “Man-Days” to complete the inspection.

Buyers who are concerned about product quality will know that products need to go through strict quality management procedures before they are shipped, to prevent suppliers from using substandard materials, or inaccurate processing methods, incorrect production dimensions, and other quality issues. But you may have a strict budget for quality management, and at the same time, you will take any measures to minimize the inspection costs.

How to Ensure Effective Quality Control Within a Limited Budget?

A common method to reduce the cost of the inspection is to use Combined Sampling, and the sample size is still based on AQL standards. it is determined according to the quantity of the entire lot batch. Please refer to our AQL Table for the number of samples and the inspection standard (More…).

It should be noted that Combined Sampling is to combine multiple products with different SKUs into one batch, not to sample them separately.

Below we introduce the working principle of Combined Sampling and evaluate whether it is suitable for your quality control program

How Does Combined Sampling Work In Pre-shipment Inspection?

Let’s take an example to illustrate the sampling principle of Combined Sampling. Note that the sample size to be checked in the example is always determined according to the AQL chart of the ISO2589-1 standard.

Suppose you are cooperating with a garment factory and you have ordered the following three products and plan to inspect them at the same time and place, including:

A & B: T-shirts of the same style and different designs,

C: shorts

There are two ways to sample at this time,

one is to treat each product as a separate batch and perform 3 separate inspections,

And the other is to combine them into one batch, which is Combined Sampling.

Option 1 – Treat each product as a separate batch

Combined Sampling Inspection example 1

According to the standard AQL sampling procedure, if each type of bottle is to be inspected in a separate batch, the inspector will need to inspect:

  • A: The order is 2500 pieces and the sample quantity is 125 pieces
  • B: The order is 1500 pieces and the sample quantity is 125 pieces
  • C: The order is 4000 pieces and the sampling quantity is 200 pieces

They will inspect 450 pieces in total, and this workload requires 3Man-Days to complete.

Option 2 – Combine the two T-shirts into one batch

Combined Sampling Inspection example 2

When all T-shirt styles are combined into one batch, according to the AQL standard, you need to inspect a total of 200 pieces. However, how many pieces should be inspected for each style? In principle, the sampling ratio is the same as the order quantity ratio:

1500/4000 * 200=75 pieces

2000/4000 * 200=125 pieces

It takes less time to inspect 400 pieces than to inspect 450, and the man-hours required for inspection are correspondingly reduced. In this case, this may make sense. The materials and accessories are almost the same. The production process is almost the same. Therefore, it may meet the definition of “a single type, grade, grade, size, and composition item manufactured under uniform conditions at the same time” (ISO 2859), but some people may also doubt it. They may ask whether these two styles are performed on different sewing threads. This makes sense, because, in the same workshop, the craftsmanship quality of two sewing threads may vary greatly.

So in many cases, Combined Sampling has a high risk! This sampling method is not suitable for all products.

Combined Sampling May Sometimes Not Be a Good Idea!

Combined Sampling is the use of Combined Sampling only for similar items. These products are produced in the same factory using the same type of production equipment and basically produced at the same time period. However, when the goods you order are not very similar, or when the processing originates from different factories, the combined inspection can become a problem.

For example, if you ordered 2,000 shirts and 2,000 shorts, you will need to check them separately because they are manufactured using different production processes. Or, if you have received multiple hats from two different factories, since they are not produced at the same location, it is not recommended that you combine them for sampling. Examples are as follows:

5 hats, of which 2 are from factory A, and 3 are from factory B. The acceptable limit for Combined Sampling of 200pcs (A: 125pcs, B: 75pcs) is: 10 major defects and 14 minor defects, but check The staff found:

  • There are 8 major defects and 2 minor defects on the hat of factory A
  • There are 5 minor defects on the hats of factory B

The inspector adds up the number of defects, a total of 8 major defects, 7 minor defects, this value is within the limit. According to AQL inspection standards, neither the inspector nor the buyer can reject the goods.

But if you are careful, you will find that the inspector has inspected the 125pcs products of factory A, but found 8 major defects. This bad ratio will definitely have a direct impact on product sales. However, the low defect rate in factory B compensated for the high defect rate in factory A, which made it impossible for customers to request factory rework easily;

Therefore, the key to Combined Sampling is to conduct joint inspections only on products of the same type, with the same function, using the same basic technology, materials and equipment, and produced in the same factory.

The more dissimilar the products, the greater the risk of combining them into one sample.

Therefore, use Combined Sampling wisely!


Author: AQI Service as a professional quality control service company in China specialized in providing a complete range of quality inspection and testing certification services in China & Asia for global importers, buyers, retailers, and sellers.

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