GlossaryKind of term: Standard and conventionSampling inspection (AQL)
What is sampling inspection?
Sampling inspection means inspecting part of a lot to a set sampling plan and accepting or rejecting the whole lot on that result.
In sampling inspection you do not inspect every part, but a sample from the lot. A sampling plan sets how many parts you inspect and how many rejects the sample may contain for the lot still to be accepted: the acceptance number. AQL stands for acceptance quality limit, formerly acceptable quality level, the worst process average that is still acceptable over a continuing series of lots. The AQL is a setting of the plan, not a reject rate you may ship. Sampling by attributes counts good and bad; sampling by variables works with measured values.
What does the standard say about sampling inspection?
The usual tables for inspection by attributes are in ISO 2859-1, which is meant for a continuing series of lots. Lot size and inspection level give a code letter; code letter and AQL give the sample size and acceptance number. In the widely used tables, a lot of 500 parts at general inspection level II, with single sampling and normal inspection, gives code letter H and so a sample of 50 parts; for some AQL values the table points to a different plan. The standard also has rules for switching to tightened or reduced inspection. For an isolated lot ISO 2859-2 applies, for inspection by variables the ISO 3951 series. Which plan applies is agreed with the customer.
How does sampling inspection work in practice?
The customer usually specifies the plan: the standard, the inspection level and the AQL, sometimes per feature or per class of nonconformity. Some customers require a zero-acceptance plan: one reject in the sample and the lot is rejected or sorted. Inspection time per lot is roughly the sample size times the inspection time per part. Because the sample grows more slowly than the lot, inspection time per delivered part falls on larger lots. A rejected lot often means inspecting or sorting every part after all, and that risk belongs in the trade-off.
What is sampling inspection often confused with?
Sampling inspection is not statistical process control (SPC): SPC follows the process during production to catch drift early, a sampling plan judges a lot afterwards. Nor is it process capability: Cpk says how well a process fits within the tolerance, a sample only whether a lot is accepted.
Frequently asked questions
How many parts do I inspect in a sample?
That follows from the plan the customer specifies. Lot size, inspection level and AQL set the sample size and acceptance number through the ISO 2859-1 tables. Ask for those three before you estimate the inspection time.
What does AQL 1.0 on a purchase order mean?
That the sampling plan is set up around a process average of 1 percent nonconforming as the limit of what is acceptable. It is not a licence to ship 1 percent bad parts; any reject you find is still a reject.