Why 7 different inspection levels?

When controlling the quality of a batch of products, it is not practical to inspect 100% of them (unless the quantity is very small). Inspecting a large number of products takes a long time: it bears a high cost, and inspectors are less effective as they get tired. Actually, a 100% check does not yield that much more information than inspecting a representative sample.

The question becomes: how many products to check?

 

There is a fairly obvious principle in statistical quality control: the greater the order quantity, the higher the number of samples to check.

But should the number of samples ONLY depend on the order quantity? What if this factory had many quality problems recently, and you suspect there are many defects? In this case, you might want more products to be checked. 

On the other hand, if an inspection requires tests that end up in product destruction, shouldn't the sample size be drastically reduced? And if the quality issues are always present on all the products of a given batch (for reasons inherent to processes at work), why not check only a few samples?

For these reasons, different levels are proposed by MIL-STD 105 E (which is the widely recognized standard for statistical quality control).

 

It is usually the buyer's responsability to choose the inspection level--more samples to check means more chances to reject bad products when they are bad, but it also means more days (and dollars) spent in inspection.

 

Has this supplier passed most previous inspections? Do you feel confident in their products quality? Instead of doing no quality control, buyers can check less samples by opting for a level-I inspection.

However, settling on this level by default, in order to spend less time/money on inspections, is risky. The likelihood to find quality problems is lower than generally recommanded.

 

If a supplier recently had severe quality problems, this level is appropriate. More samples are inspected, and a batch of products will (most probably) be rejected if it is below the quality criteria defined by the buyer.

Some buyers opt for level-III inspections for high-value products. It can also be interesting for small quantities, where the inspection would take only one day whatever the level chosen.

 

These special levels can be applied in cases where only very few samples can be checked. "Four additional special levels, S-1, S-2, S-3 and S-4 […] may be used where relatively small sample sizes are necessary and larger sampling risks can be tolerated."[1] 

Under S-4 level, the number of samples to check is higher than under S-3, and so on.

 

In practice: for consumer goods, quality control is usually performed under the general levels.

The special levels are used only for certain tests that either take lots of time or destroy the samples. Another situation where special levels are appropriate is a container-loading supervision--to have an idea of what is inside the cartons, without spending too much time at that checking.

 

Let's say you have ordered 5,000pcs of a product. In the table below, you can see how many samples would be drawn under each of the 7 inspection levels.

General inspection levels			Special inspection levels
Reduced (I)	Normal (II)	Tightened (III)	S-1	S-2	S-3	S-4
32pcs	200pcs	315pcs	5pcs	8pcs	20pcs	32pcs

As you can see, the numbers of samples to check vary from 5pcs to 315pcs. But a trained inspector might be able to do it in one day, whatever the inspection level you choose.