ISO 2859-1 switching rules: normal, tightened, reduced
When AQL sampling switches from normal to tightened, back to normal, to reduced and when to stop: the 2-of-5 and 5-in-a-row rules, the switching score and a worked example.
An AQL sampling plan from ISO 2859-1 (or ANSI/ASQ Z1.4) is not a single plan. It is a scheme with three severities, normal, tightened and reduced, and rules for moving between them based on the results of recent lots. The switching rules are what makes AQL sampling protect the buyer: a supplier who delivers worse than the agreed AQL is moved to tightened inspection quickly, and one who keeps failing is stopped.
The rules at a glance
| Switch | Condition |
|---|---|
| Normal → tightened | 2 lots out of 5 or fewer consecutive lots not accepted on original inspection. |
| Tightened → normal | 5 consecutive lots accepted on original inspection. |
| Normal → reduced | Switching score at least 30, production at a steady rate, and reduced inspection approved by the responsible authority (usually the customer or quality). |
| Reduced → normal | One lot not accepted, production irregular or delayed, or other conditions that make normal inspection necessary. |
| Tightened → discontinue | The number of lots not accepted on tightened inspection reaches 5 (counted from the start of tightened inspection). |
| Discontinue → tightened | The supplier has taken action to improve quality and the responsible authority agrees. Inspection restarts on tightened, not normal. |
Always start on normal inspection unless the responsible authority says otherwise. "Original inspection" means the first inspection of a lot: resubmitted lots after sorting do not count.
What changes on each severity
- Normal: the plan from Table 2-A, the one the AQL calculator gives you.
- Tightened (Table 2-B): mostly the same sample size with a smaller acceptance number, for some plans a larger sample. A lot that would just pass on normal can fail on tightened.
- Reduced (Table 2-C): a sample of roughly 40 % of the normal size. Cheaper, but it discriminates less, which is why any rejection sends you back to normal.
The switching score (normal to reduced)
Since the 1999 edition ISO 2859-1 uses a switching score instead of the older rule based on the total number of defects in the last 10 lots:
- Start at 0 at the beginning of normal inspection.
- Acceptance number 0 or 1: add 2 if the lot is accepted, otherwise reset to 0.
- Acceptance number 2 or more: add 3 if the lot would also have been accepted with the AQL one step tighter, otherwise reset to 0.
- At 30 points, with steady production and approval, switch to reduced inspection.
Example with a plan n = 80, Ac 1 (AQL 0.65, lot 281 to 1,200 at level II):
| Lot | Defectives | Result | Score change | Score |
|---|---|---|---|---|
| 1 | 0 | Accepted | +2 | 2 |
| 2 | 1 | Accepted | +2 | 4 |
| 3 | 0 | Accepted | +2 | 6 |
| 4 | 2 | Not accepted | reset | 0 |
| 5 | 0 | Accepted | +2 | 2 |
With Ac 1 a supplier needs 15 accepted lots in a row to reach 30 points. Lot 4 also counts towards tightened inspection: one more rejected lot among the next four would trigger it.
Worked example: from normal to tightened and back
- Lots 1 to 6 on normal inspection: lots 3 and 6 are rejected. That is 2 rejections within 4 consecutive lots, so lot 7 is inspected on tightened.
- Lots 7 to 11 on tightened are all accepted: 5 consecutive accepted lots, so lot 12 goes back to normal.
- Had lots 7 to 11 contained rejections, the count of rejected lots on tightened would keep growing until it reached 5, when acceptance sampling stops until the supplier shows corrective action.
Practical tips
- Keep a lot log per part number and supplier with the severity, n, Ac, defectives and decision, so the switching is traceable in an audit.
- A switch to tightened is a quality signal: open a complaint or 8D with the supplier and reflect it in the supplier scorecard.
- In automotive many customers require Ac 0 and do not accept reduced inspection; see the incoming inspection strategy for skip-lot and ship-to-stock alternatives.
- The free AQL calculator and the free AQL LITE Excel template cover normal inspection. The full Excel version on Etsy adds tightened inspection and a lot log (AQL Sampling Calculator, full version).
FAQ
What are the switching rules in ISO 2859-1?
Start on normal inspection. Switch to tightened when 2 of 5 or fewer consecutive lots are not accepted, back to normal after 5 consecutive accepted lots on tightened, to reduced when the switching score reaches 30 with steady production and approval, back to normal from reduced after one rejected lot, and discontinue sampling inspection when 5 lots are rejected on tightened.
Are the switching rules mandatory?
ISO 2859-1 treats them as part of the system: the protection given by the AQL tables assumes the switching rules are used. Using only the normal plan gives a buyer much weaker protection against a supplier whose quality is worse than the AQL. In practice the contract or the customer quality manual decides.
What is the switching score?
A counter for the move from normal to reduced inspection. It starts at 0. With an acceptance number of 0 or 1, add 2 points for every accepted lot. With an acceptance number of 2 or more, add 3 points if the lot would also have been accepted at one AQL step tighter. Any other result resets the score to 0. At 30 points reduced inspection can start.
Do ANSI/ASQ Z1.4 and ISO 2859-1 use the same switching rules?
Yes in principle: both have normal, tightened and reduced inspection with the 2-of-5 and 5-consecutive rules and discontinuation after 5 rejected lots on tightened. Check the edition your customer refers to for the details of the move to reduced inspection.
Do resubmitted lots count?
No. The rules count lots on original inspection. A lot that is sorted and resubmitted does not count as an accepted lot.