I regularly receive emails from readers who feel that the AQL tables push them to inspect too many samples.
First, a warning: if you need to be 95% confident in an inspection’s findings, it is surprising how many samples you should check. You should not follow your gut feelings, but rather cold statistics.
If you feel you should decrease the number of samples you check, there are basically two ways to do this and still follow the commonly-accepted standards such as ISO 2859-1.
1. At the product level: decrease the level
You can follow level I or S-3, for example, instead of level II. It means you accept to take a higher risk of not detecting quality issues.
An inspection in a lower level is only useful in catching very widespread problems. It is justified if you are mostly afraid of problems regarding the components, or if your products are made by machines (and any quality issue is visible on many products).
You can refere to What is an inspection level? for more details.
2. At the supplier level: decrease the severity
The ISO 2859-1 standard actually gives a method to switching from normal severity to reduced severity, when a factory gets to a certain score.
If you want to follow this rigorous process, here are some excerpts from the ISO 2859-1 standard:
Normal inspection shall be carried out at the start of inspection, unless otherwise directed by the responsible authority.
When normal inspection is being carried out, reduced inspection shall be implemented provided that all of the following conditions are satisfied:
- the current value of the switching score (see 220.127.116.11) is at least 30;
- and production is at a steady rate;
- and reduced inspection is considered desirable by the responsible authority.
The calculation of the switching score shall be initiated at the start of normal inspection unless otherwise specified by the responsible authority.
The switching score shall be set at zero at the start and updated following the inspection of each subsequent lot on original normal inspection.
- when the acceptance number is 2 or more, add 3 to the switching score if the lot would have been accepted if the AQL had been one step tighter; otherwise reset the switching score to zero;
- when the acceptance number is 0 or 1, add 2 to the switching score if the lot is accepted; otherwise reset the switching score to zero.
Oh, and don’t forget… The score changes and you switch back to normal severity in any of these cases:
- a lot is not accepted; or
- production becomes irregular or delayed; or
- other conditions warrant that normal inspection shall be re-instated