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ISO 17025 (§7.8.6) prohibits a simple statement of conformity when a measurement approaches the tolerance limit. The ILAC-G8 rule requires uncertainty to be taken into account before any verdict.
Principle of the Guard Band
Acceptance Limit (L_acc) = MPE − U
Micrometer example (MPE = ±0.003 mm, U = 0.0010 mm): L_acc = ±0.002 mm
The 4 Scenarios
| Case | Condition | Zone | Decision |
|---|---|---|---|
| 1 | |Deviation| + U ≤ MPE | Conforming | ✅ CONFORMING |
| 2 | |Deviation| ≤ MPE but |Deviation| + U > MPE | Uncertain on the conforming side | ⚠️ NON-CONFORMING |
| 3 | |Deviation| > MPE but |Deviation| − U < MPE | Uncertain on the non-conforming side | ⚠️ NON-CONFORMING |
| 4 | |Deviation| − U > MPE | Out of tolerance | 🔴 REJECTED |
🟢 Case 1 — Proven Conformity
Example: Deviation = +0.0017 mm, U = 0.0010 mm → 0.0027 mm ≤ 0.0030 mm ✅
🟡 Case 2 — Grey Zone (conforming side)
Example: Deviation = +0.0025 mm, U = 0.0010 mm → 0.0035 mm > 0.0030 mm ⚠️
🟡 Case 3 — Grey Zone (non-conforming side)
The deviation exceeds the MPE but the uncertainty leaves room for doubt — NON-CONFORMING decision with impact analysis.
🔴 Case 4 — Proven Non-Conformity
Immediate withdrawal from service, §7.10 record required.
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