According to the ISO 2859 standard, one inspection should be performed on “items of a single type, grade, class, size and composition, manufactured under uniform conditions at essentially the same time”.
In clear, if you are going to ship 15 sofas and 30 tables together, you are strongly advised to perform two separate inspections. Do not bundle different products in one inspection!
For those readers who want to jump to my conclusions, they can read this infographic: Sampling plan calculations for quality inspections. It is a simple guideline to avoid this mistake.
And for those who want the long version: read the rest of this article.
The theory: why one inspection must cover one type of products
It is possible to gather 2 or 3 references in 1 inspection, and it is often less work (because there are less samples to check).
But it is also more risky. One reference might have lots of defects, but overall the inspection might be passed if the other references “compensate” by presenting very few defects. Then the buyer realizes that he can’t sell one of the references because the customers keep returning it.
I only advise to bundle different products if they go through a similar production process, if they are roughly as difficult to make, and if they are made in the same factory (if possible on the same line).
I will use the same example throughout this article. Sofas and tables are not produced the same way. They don’t present the same “opportunities for defects”. And they are often made in different workshops. They should be checked separately.
Example of a poor sampling choice
Let’s take an example:
A client has ordered 100 pillows and 600 pillow covers. He decides to do only 1 inspection, and to bundle these 2 products for the AQL check. The total number of products is 700 pcs.
Under level II (which is chosen most often), the code letter is J.
J corresponds to 80 pcs to check. It is feasible in 1 man-day.
Among these 80 samples, how many are pillows, and how many are pillow covers? In principle the proportion must be the same as the total quantity:
- 80 x 100/700 = 11 pillows
- 80 x 600/700 = 69 pillow covers
The acceptability limits are: 5 major defects and 7 minor defects. In my example, the inspector finds:
- 4 major defects on the pillows, and none on the pillow covers.
- 5 minor defects on the pillows, and 2 on the pillow covers.
He adds up the numbers, and he is inside the limits. Based on this inspection protocol, it is impossible to reject the goods.
However, the smart reader will think “4 major defects on 11 pcs checked? The pillows are obviously not sellable.” And that’s right. The inspector found 36% of the pillows with a major defect!
Yet the other product “compensated” with a much lower proportion of the defects, and the client can’t easily tell the supplier to rework the pillows.
How this case should have been managed
Let’s take the same example. What should the inspector have done? Two separate inspections.
- For the pillows: 20 samples to check, maximum 1 major and 2 minor defects.
- For the pillow covers: 80 samples to check, maximum 5 major and 7 minor defects.
Total: 100 samples to check. It is more work for the inspector. In some cases it makes the difference between 1 man-day and 2 man-days.
This time, if there is a high proportion of defects on the pillows, it will certainly be over the AQL limits. The importer can refuse the goods, because the pre-agreed standard was not respected.
Is it clear?