Sampling schemes offer more than one inspection level. A general level suits everyday products; a stricter level with larger samples is chosen when the cost of a bad lot is high, such as a safety garment. A looser level with smaller samples is used for low-risk, low-cost checks. The level is set in the inspection instruction and does not change lot by lot.
What does change lot by lot is the inspection severity, and this follows a supplier's track record. Normal inspection is the starting point. If lots keep failing, you switch to tightened inspection, which uses the same sample size but a smaller accept number, making it harder to pass. If lots keep passing over a long run, you may switch to reduced inspection, which uses a smaller sample and saves time.
The switching rules are set out in the scheme and should be written into the factory procedure. The logic is simple: poor history earns closer scrutiny, good history earns relief, and either can be reversed. Reduced inspection is a privilege based on evidence, never a shortcut taken because the inspection room is busy.
Standard Procedure
- 1Fix the inspection level in the instruction based on product risk.
- 2Start every new supplier or new style at normal severity.
- 3Track accept and reject results lot by lot on a log.
- 4Apply the written switching rules to decide the severity for the next lot.
- 5Return to normal only when the procedure's conditions are met.
Lesson Summary
- ✓Inspection level reflects product risk; severity reflects supplier history.
- ✓Tightened keeps the sample size but lowers the accept number; reduced shrinks the sample after a sustained good record.