Let’s say you intend to conduct a final random QC inspection, after all the products are packed.
(I already explained why the whole order should be ready for final QC).
You need to select a few cartons at random. What are the best practices to ensure that the samples you will check represent the whole batch?
1. Don’t come if nothing is packed.
When the manufacturer is rushing the job and products are still under production (or rework), it is impossible to do a good job. You will never be sure that your findings are representative of the whole order.
You cannot count how many pieces are on the lines. And chances are they will not let you interfere with their production processes.
2. Make sure you can count and pick cartons randomly
If the warehouse is full and the factory prepared this kind of pile, you are in trouble.
It is impossible to separate each reference in a different pile, because of lack of space. Good luck to pick cartons from all sides of the pile…
You’ll need to insist heavily on this point before going for the inspection. Sometimes they will make an effort for you when they stack the cartons up.
3. Pick cartons in a “stratified random” manner
If 100% of the cartons are ready and they are nicely stacked, you can use the packing list to select the carton numbers: 3, 10, 17, 23, 32, and so on. Two pieces of advice:
- Avoid no arithmetical series in the list of numbers you select (2, 4, 6, 8).
- The number of picked cartons should be at least the square root of the total number of cartons.
However, most of the time this is not practical. Warehouse workers spend a lot of time searching the right carton numbers because their pile is a mess!
Most of the time, QC inspectors follow this logic:
Then the factory workers take the cartons that were selected, and bring them to the inspection area (under the inspector’s supervision, of course).
4. Following step: pick the products
In many cases, there will be more products in the inspection cartons than the number of inspection samples you need to check.
The same logic applies: don’t take all the products in one carton, or in the same place inside the cartons.
Is it clear?