Inspection: what a sampling plan does and does not prove
An inspection report is evidence about one lot, not a certificate of a factory. Here is what the sampling standards actually measure, and how to write the defect definitions before anyone inspects.
What sampling proves
An inspection by attributes draws a sample from the lot and decides accept or reject against a defined defect level. It tells you about the lot in front of the inspector; it does not tell you what the next lot will be like, and it does not measure how the product behaves after a year of use.
That distinction matters when you use the report as an argument with the factory. The report supports the sentence 'this lot has this defect rate', not the sentence 'this factory makes bad products'.
The standard behind the numbers
Sampling schemes indexed by acceptance quality limit are standardised in ISO 2859-1 (sampling procedures for inspection by attributes, part 1). The standard defines the sample size and the accept/reject numbers for a given lot size and level.
Inspectors normally work at a general inspection level and quote an AQL per defect class. Buyers often set a stricter level for critical defects and a looser one for cosmetic ones, which is a commercial decision rather than a standard requirement.
Write the defect classes down first
Critical, major and minor have to be defined in the contract with your product in mind, with photographs of each: a critical defect is one that makes the product unsafe or unusable, a major one is a defect the end buyer would reject, a minor one is cosmetic and acceptable to sell.
Do not inherit someone else's list. A defect class copied from a different product quietly accepts the exact fault you care about.
When to have it done
Inspect before the goods are packed and sealed, not after. Once the cartons are taped and on a pallet, a problem becomes a repacking negotiation.
Say who attends. Third-party inspection, factory self-inspection and a buyer's own visit answer different questions, and the cheapest option in the quote is not always the one that produces evidence you can act on.
Evidence you can use later
Require dated photographs with carton numbers visible, and the inspector's measurement records, not a summary page. A report without measurements cannot settle an argument about tolerance.
Keep the sealed retained samples from the sample-approval stage next to the inspection report. The comparison you will want, months later, is sample against delivery.
Our own buyer-side working practice. Defect classes, levels and AQL values are commercial decisions you set in your own contract.