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 (the widely recognized standard for statistical quality control).
It is usually the buyer's responsibility 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.
