As we know, Inspection and Test Plan (known as ITP) is an inspection tool on how to ensure and control whether the product is fit to the requirement and whether the inspection activity has carried out with the agreed specification, standard and procedure. ITP plays an important role in the project quality assurance (see post: INSPECTION AND TEST PLAN (ITP) ROLE IN PROJECT QUALITY ASSURANCE). One common issue in generating or reviewing ITP is how to define the level of inspections in the ITP. Moreover if the product that we will be checking are in big quantities within a short term of inspection, it is not practical to inspect 100% of them. It will take a long time; it is expensive, and inspectors are less effective as they get tired.
In all conscience, a 100% check does not yield that much more information than inspecting a statistically representative sample. The question becomes: how many products should be checked? More samples to check means more chances to reject bad products when they are bad, but it also means more days (and certainly: budget) spent in the inspection. But should the number of samples only depend on the order quantity? What if the manufacturer had a lot of quality problems recently, and you suspect there are many rejected product? In this case, inevitably you want more quantities to be checked. But, on the other hand, if an inspection requires destruction tests, 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 of inspection should be considered to resolve this issue, which will let the purchaser to choose the suitable inspection level which fit to their purpose or quality standard.
There are 3 (three) general levels of inspection commonly utilized:
In this level, less samples are inspected. This inspection level is appropriate when the client is confident that the quality of the products is acceptable. You have to review: has the 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 very risky. The likelihood of finding quality problems is lower than generally recommended.
It is the most widely used inspection level, to be used by default, and it is chosen for 90%+ of inspections carried out at the supplier's premises.
ISO 2859 definition for normal inspection:“Normal inspection is used when there is no reason to suspect that the [quality level] differs from an acceptable level.” Unless otherwise specified, level 2 shall be used. Level 1 may be used when less discrimination is needed or level III when greater discrimination is required.
In this level, more samples are checked. This inspection level is used for suppliers that recently had severe quality problems, or for high-value products. If a supplier recently had 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 purchaser. Some purchasers 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.
ISO 2859 definition for this tightened inspection :“Use of a sampling plan with an acceptance criterion that is tighter than that for the corresponding plan for normal inspection.”
And also, there are 4 (four) special inspection level can be used, which are in the group of Special Inspection Level. This Special Inspection level is used instead of the general inspection level when the production has to be operationally inspected. The special inspection level can range from S1, S2, S3 or S4 and may be used where relatively small sample sizes are necessary or large sampling risks can be taken. Examples of this are inspections involving destructive or costly (time consuming) type inspection, or where large lots are involved, small sample sizes are desired and large risks can be tolerated such as repetitive processes (screw machine, stamping, bolting operations, etc.) performed by a quality supplier. Larger sample sizes are for inspection levels increasing from S-1 to S-4.