Scenario
Procurement said: "This batch of parts was accepted using AQL 1.0, it's qualified." You pressed for details and found out he sampled 20 pieces from 1000, found no defects, and released the batch.
However, for an AQL 1.0 lot size of 1000 pieces, the correct sample size should be 80 pieces, and the acceptance criteria is ≤ 2 defective items.
His sample size was incorrect, and his acceptance criteria was also incorrect. The statement "using AQL 1.0" was false.
The Correct Definition of AQL
AQL (Acceptable Quality Level) = 可接受品質水準 (Acceptable Quality Level)
The meaning of AQL is: In the long term, the highest defect rate that this process can accept.
For example, AQL 1.0 = the supplier's long-term defect rate can be at most 1%; exceeding this level is unacceptable.
Important Misconception: AQL does not guarantee "this batch's defect rate ≤ 1%"
It is a sampling risk framework that balances two types of errors:
- Producer's Risk (α): Good product being rejected (usually 5%)
- Consumer's Risk (β): Bad product being accepted (usually 10%)
Consulting the ANSI Z1.4 Sampling Tables
Step One: Determine Lot Size
Step Two: Select Sample Size Code Letter
Look up the table based on lot size, e.g., Lot Size 1000 pieces → Code Letter J
Step Three: Consult the Normal Inspection Table
Code Letter J, AQL 1.0:
- Sample Size = 80 pieces
- Acceptance Number (Ac) = 2 pieces
- Rejection Number (Re) = 3 pieces
Meaning: Sample 80 pieces, if defective items ≤ 2 pieces → Accept; if ≥ 3 pieces → Reject.
Normal / Tightened / Reduced Inspection Switching Rules
Sampling plans do not always use normal inspection:
Switch to Tightened Inspection:
2 out of 5 consecutive lots were borderline (close to rejection) under normal inspection.
Switch back to Normal Inspection:
5 consecutive lots passed under tightened inspection.
Switch to Reduced Inspection:
10 consecutive lots passed under normal inspection, and the process is stable.
Reduced inspection can decrease sample quantity but must be supported by process control data.
How to Select AQL Levels
| AQL Level | Applicable Scenarios |
|---|---|
| AQL 0.065-0.25 | Safety-related parts, medical devices |
| AQL 0.65-1.0 | Precision machinery, electronic components |
| AQL 2.5-4.0 | General industrial parts |
| AQL 6.5-10.0 | Packaging materials, low-risk consumables |
Golden Quote
"AQL is a risk management tool, not a quality guarantee certificate. A passing sample only means this batch has met the threshold for risk assessment; it does not mean there are no defective items within the batch. The ultimate safeguard for incoming inspection is the supplier's process control capability, not how many pieces you sampled."