In QC inspections, a sampling plan will commonly use random sampling so quality inspectors can check larger amounts of products quickly and effectively. This slideshow provides you with a useful introduction to sampling plans, allowing you to start using them in your quality improvement activities in order to get better results from your suppliers. It accompanies this blog post: https://qualityinspection.org/sampling-plans-china/

1. The Different Types Of Sampling
Plans For QC Inspections

2. In this slideshow we’re going to examine:
• What a sampling plan is
• Random sampling
• 3 common cases where a sampling plan is used
Read the post at: https://qualityinspection.org/sampling-plans-china/

3. What is a sampling plan?
A sampling plan allows an auditor or researcher to study a group (e.g. a
batch of products, a segment of the population) by observing only a
part of that group, and to reach conclusions with a pre-defined level of
certainty.
In QC inspections, a sampling plan will commonly use random sampling
so quality inspectors can check larger amounts of products quickly and
effectively.

4. Is random sampling always necessary?
Not at all, it really depends on the value and amount of goods being
inspected.
For example, an order of 50 luxury watches should be inspected one-
by-one.

5. Whereas 4 container loads of Christmas baubles costing 25 cents each
is a huge number of goods to check and could lead to many products
being destroyed!
Therefore, in this case, checking a smaller sample of them makes far
more sense.

6. Random sampling
A good sampling plan relies on statistics to help a quality inspector to:
1. Decide how many pieces to pick for inspection
2. Decide what number of defective units is “too many”
3. How certain they are to make the right decision in their findings

7. Statistical sampling plans
You might say: “Why not just choose 10% of the batch at random and check those pieces?”
However, if you do not use statistics to help predict sample size and defect rate, you are at
a disadvantage.
For example:
1. You can’t make the link between this plan and your risk as the buyer
2. You have no previously-agreed rule to decide if the supplier should take action if a
percentage of the samples are found to be defective
Fortunately, statisticians have been hard at work on this topic, and have been coming up
with simple tools, like the AQL tables, for practitioners, since the 1930s.

8. 3 situations that might be familiar to importers and manufacturers

9. 1. You buy a batch of products and you want to
use the most common sampling plan
The most popular plan was developed in the 1930s in the USA and was
formalized in standards MIL-STD 105E, ISO 2859-1, and ANSI Z1.4.
Many importers call this an“AQL inspection”.

10. What is AQL inspection?
In short, the ‘Acceptance Quality limit’ is the “quality level that is the
worst tolerable” in ISO 2859-1.
It represents the maximum number of defective units allowed to be
found in the inspection, beyond which a batch is rejected. Importers
usually set different AQLs for critical, major, and minor defects.
Most Asian exporters are familiar with this type of setting in inspections.

11. AQLs can be defined by checking the AQL tables:
Your quality inspector will use them to decide:
• How many samples to check out of the batch
• Number of critical defects allowed (totally unacceptable quality)
• Number of major defects allowed (products usually considered unusable by the end user)
• Number of minor defects allowed (most end users would accept this quality)

12. Typically used AQL settings
for consumer goods in
China are:
• 0% critical defects
• 2.5% major defects
• 4% minor defects
An example of this would be
the following:
TIP: Instead of using the AQL tables, you can
use Sofeast’s AQL calculator here:
https://www.sofeast.com/glossary/aql-
calculator/

13. Watch our video to learn more about the AQL inspections in detail (make
sure you’re connected to the internet).

14. 2. You are a manufacturing organization who wants
to follow good practices
Manufacturers can follow three plans and their inspection locations to weed out defects during production:
Now, let’s look at each inspection location in turn…

15. A) Incoming inspection
You need a cost-effective way of checking a number of batches.
Recommended approach - An “AQL inspection” works well here.
(We covered this earlier in the slideshow)

16. If you purchase batches of products from an external company, and those batches are made in a
continuous or semi-continuous manner with no changes in process or components, it does make
sense.
If you are curious about the way the statistics behind this plan work, you can watch this video (make
sure you’re connected to the internet):

17. B) In-process control
The recommended approach here is usually a combination of:
• Statistical process control (e.g. a C or P control chart) – (beyond the
scope of this slideshow).
• Product controls (e.g. a continuous sampling plan).

18. What is a continuous sampling plan? (MIL-STD-
1235B)
This approach makes sense when these conditions are met:
• Inspection is quick and results are known very fast
• No destructive test is involved
• Product quality is known to be relatively stable
• Products are identical (same materials going through the same
process under the same specifications) — they can be made in a
continuous flow or in batches

19. A continuous sampling plan consists of several phases:
• In the beginning, each piece is checked (that’s “100% check” or
“screening”).
• After a certain number of pieces were found satisfactory, only certain
pieces are checked randomly (that’s the “sampling”).
• If a piece is defective: back to screening.
• If screening lasts for a long time (meaning defective units are often
found), the priority is to improve the process and/or to set up testing
at the source to catch issues immediately.

20. This flowchart summarizes a continuous sampling plan well:
(Source: Quality Progress)

21. Watch our video which explains how continuous sampling plans work
(make sure you’re connected the internet):

22. C) Final inspection
You still want a filter here, in order to stop batches that still have
defects.
The right approach depends on your situation:
• If you are in general consumer goods, setting AQL limits a bit tighter
than what your customer would select is often a “good enough”
solution.
• If you can’t afford to ship even a few defective goods, an acceptance
on zero plan makes more sense.

23. What is an ‘Acceptance on zero’ sampling plan?
Some importers, who are sensitive to legal litigation by their customers or
who have high-quality standards, accept batches only if no defective unit is
found. This is common in industries such as automotive or pharmaceutical.
In some cases, the producer itself adopts this type of approach for its
outgoing quality control.
What are the differences between ‘acceptance on zero’ and the traditional
acceptance plan (ISO 2859-1 / ANSI Z1.4)?
1. There are no inspection levels.
2. Rejection is always as soon as 1 defective unit is found.

24. One big advantage of an acceptance on zero sampling plan is that fewer samples need to be
checked.
In principle, it only makes sense if the process has a capability index (Cp) of at least 1.67. (In simple
terms, the key characteristics of the product are measured and they fall within the specifications in
the vast majority of cases.)
Watch our video where we explain this example (make sure you’re connected to the internet):

25. More sampling plans to be aware of
We won’t cover all types of sampling plans used for quality control purposes,
but here are some others:
• The plans mentioned earlier are “by attributes”, which classify the samples
as either “non-defective” or “defective”.
If a plan is “by variables”, it allows for a finer evaluation.
For example, the length of the product is measured, and the exact findings
are taken into account when a decision is made.
• A “rectifying” is applicable if the defects that are found can be corrected
immediately. It takes into account the fact that the batch is of higher
quality after the inspection…and, in case of inspection failure, the whole
batch should be inspected.

26. Do you need a Quality Assurance partner who can advise you
on your sampling plan?
Sofeast provides Quality Assurance solutions to clients across China and SE Asia who are
both buyers and manufacturers – follow the links to learn more and get a quotation if you’d
like:
• Product Inspection Solutions - https://www.sofeast.com/quality-assurance/product-inspection-
solutions/
• 100% inspection, repacking, and shipping - https://www.sofeast.com/quality-assurance/100-
inspection-repacking-and-shipping/
• Quality assurance consulting - https://www.sofeast.com/quality-assurance/quality-assurance-
consulting/
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27. Don’t miss any blog posts & resources from
the companies we’re related to!
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Catch our regular blog posts on QualityInspection.org here:
https://qualityinspection.org/blog/
We also blog and have a lot of free resources for importers and
manufacturers at Sofeast, too:
Blog - https://www.sofeast.com/knowledgebase/
Free resource library - https://www.sofeast.com/resources/
Finally, check the SynControl (our quality inspection software company) blog,
too: http://www.syncontrol.com/blog

28. The Different Types Of Sampling
Plans For QC Inspections