Incoming inspection strategy: AQL, skip-lot or ship-to-stock
How to decide the incoming inspection for each part and supplier: full inspection, AQL sampling, skip-lot and ship-to-stock, with switching rules and the zero-defect expectation of IATF 16949.
Inspection by risk, not by habit
Incoming inspection cannot inspect quality into a part; it only finds some of the defects the supplier already made. Its job is to protect your process while the supplier proves stable, and to give data for the scorecard. The level should depend on the part risk and the supplier’s history.
The four levels
| Level | When | What |
|---|---|---|
| 100 % inspection | Launch, escalation, known defect | Every part on the critical characteristics, often by the supplier under controlled shipping |
| Sampling (AQL) | New supplier or part, medium risk | Sample size and acceptance number from ISO 2859-1 / ANSI Z1.4 by lot size and AQL |
| Skip-lot | Stable history, e.g. 10 accepted lots in a row | Inspect every n-th lot; return to normal after a rejection |
| Ship-to-stock | Class A supplier, capable process, certificates | No routine inspection, periodic requalification and certificate checks |
AQL and zero defects
An AQL of 1.0 % does not mean that 1 % defects are acceptable. It is the quality level that the sampling plan accepts most of the time. In automotive, IATF 16949 expects zero defects as the acceptance level for attribute sampling (c = 0 plans), and the customer contract normally forbids delivering known defects. Use the operating characteristic (OC) curve to see what a plan really protects against.
Switching rules
- Normal to tightened: 2 of 5 consecutive lots rejected.
- Tightened to normal: 5 consecutive lots accepted.
- Normal to reduced or skip-lot: 10 consecutive lots accepted, stable production and approval by quality.
- Any rejection on reduced or skip-lot: back to normal inspection immediately.
Written by Ľubomír Blažek, automotive quality manager. Standards are summarised in our own words; use the official publications for formal work.