Receiving Inspection Sampling Plans (AQL): ISO 2859-1 Explained with Examples

Essentials: Why AQL Sampling Matters in Receiving Inspection

• AQL (Acceptance Quality Limit) sets the maximum % of defects allowed in a lot before rejection.
• ISO 2859-1 defines how to select sample sizes and accept/reject numbers using attributes inspection.
• For receiving inspection, AQL helps avoid checking every item while keeping statistical confidence.
• A 5-step method (lot size → inspection level → code letter → sample size → AQLs) guides inspectors from raw numbers to a decision.
• Pairing AQL with defect classification (Critical/Major/Minor) ensures inspectors make consistent calls and raise an NCR when needed.

One-Line Definitions:

• AQL: Maximum defective percentage accepted in a batch under defined sampling.
• ISO 2859-1: International standard for sampling by attributes, mapping lot size + inspection level to sample size and accept/reject limits.

 

Quick Definitions of AQL & ISO 2859-1

AQL in One Line

The Acceptance Quality Limit (AQL) is the maximum % of defective units permitted in a batch while still accepting the lot. It is a statistical threshold, not a license for suppliers to ship defects.

ISO 2859-1 in One Line

ISO 2859-1 is the global attributes sampling standard, linking lot size and inspection level to a code letter, which defines the sample size and corresponding accept/reject numbers.

(For background on certificate-based inspections, see [EN 10204 3.1 vs 3.2: Differences Explained] (W1.3)).

 

5-Step AQL Quick Guide for Inspectors

When applying AQL sampling during receiving inspection, follow these steps:

1. Identify lot size → total units delivered.
2. Pick inspection level → typically General Level II for standard incoming inspections.
3. Find code letter → from ISO 2859-1 table (lot size + inspection level).
4. Read sample size → code letter defines how many items to check.
5. Choose AQLs (Critical/Major/Minor) → find accept/reject numbers from the table.

📌 Tip: Using an AQL Calculator (Excel) saves time compared to flipping ISO tables during inspection.

(Pair this with [Visual & Dimensional Checks: Step-by-Step] (W3.2) before applying sampling to measured items.)

 

Defect Classes & Typical AQL Ranges

Critical, Major & Minor Defects Explained

• Critical defects → Unsafe or non-functional. Example: a cracked pressure fitting or a mismatched [MTC Interpretation Guide + Checklist (PDF)] (W1.5). Tolerance is always AQL = 0.
• Major defects → Affect usability or compliance. Example: dimension out of tolerance or coating blister. Typical AQL = 0.65%–1.0%.
• Minor defects → Cosmetic or low-risk deviations. Example: scratches, shade mismatch, small dents. Typical AQL = 2.5%–4.0%.

Why AQL Ranges Differ by Defect Class

• Safety-critical products (valves, raw materials after [PMI Testing: What, When & How to Document] (W2.3)) require the lowest tolerance.
• Major defects are limited to protect contractual compliance and client satisfaction.
• Minor defects allow higher tolerance since they don’t affect function but still need monitoring.
• This classification ensures inspectors focus resources where they matter most: safety → function → cosmetics.

 

From Lot Size to Code Letter: Real Examples

ISO 2859-1 provides lookup tables converting lot size + inspection level into a code letter. That letter then defines the sample size for inspection.

Example 1: Small Lot (≈120 pcs)

• Lot size: 120 units
• Inspection level: General II
• Code letter: E
• Sample size: 13 units
  👉 With AQL 0.65% Major and 2.5% Minor, the results are:
• Major → accept 0 / reject 1
• Minor → accept 1 / reject 2

Example 2: Medium Lot (≈2,400 pcs)

• Lot size: 2,400 units
• Inspection level: General II
• Code letter: K
• Sample size: 125 units
  👉 Acceptance results:
• Major (0.65%) → accept 2 / reject 3
• Minor (2.5%) → accept 7 / reject 8

Example 3: Large Lot (≈35,000 pcs)

• Lot size: 35,000 units
• Inspection level: General II
• Code letter: N
• Sample size: 500 units
  👉 Acceptance results:
• Major (0.65%) → accept 10 / reject 11
• Minor (2.5%) → accept 32 / reject 33

(Recording these values in a Receiving Inspection Form (Excel) ensures audit-ready traceability.)

 

How to Apply AQL in Your Receiving Inspection Process

Step 1: Perform 100% Visual & Dimensional Checks

Before sampling, always verify critical dimensions and visual features. For guidance, see [Visual & Dimensional Checks: Step-by-Step] (W3.2). This ensures that safety-critical attributes are screened before statistical sampling begins.

Step 2: Record Results in Receiving Inspection Form

Every AQL sampling result should be logged in a Receiving Inspection Form (Excel), including:

• Lot size, inspection level, and code letter
• Sample size inspected
• AQLs applied (Critical/Major/Minor)
• Accept/reject limits and actual defects found
• Final decision (Accept, Conditional, Reject)

This documentation supports traceability, audits, and supplier feedback.

Step 3: Raise an NCR When Needed

If defects exceed AQL limits, raise an NCR (Non-Conformance Report) immediately. This formal step:

• Creates a clear record for disputes
• Informs supplier quality teams
• Escalates issues for corrective action

(See also [MTC Interpretation Guide + Checklist (PDF)] (W1.5) for related documentation practices.)

 

Disposition Flow: From AQL Results to Final Decision

Accept, Conditional, or Reject

Based on AQL results, every lot follows one of three paths:

• Accept → Defects are within AQL limits; lot is cleared.
• Conditional (100% re-check) → Minor defects exceed tolerance, but major/critical are within limits. The supplier may rework or re-screen.
• Reject → Any critical defect, or defects exceeding major/minor AQL. The lot is rejected, NCR required.

Think of it as a simple flow: Inspect → Compare with AQL → Decide → Document.

Supplier Feedback & Escalation

Inspection decisions are only effective if shared with suppliers:

• Provide clear feedback on defect categories found.
• Track repeat issues with NCRs and corrective action requests.
• Escalate unresolved cases to vendor performance reviews, potentially impacting future sourcing.

 

Common Pitfalls to Avoid in AQL Sampling

Mistake 1: Confusing AQL with Actual Defect Rate

AQL does not represent the defect percentage of the entire lot. It’s a statistical acceptance threshold. A lot accepted at AQL 2.5% could have fewer or more actual defects—acceptance simply reflects confidence at the chosen level.

Mistake 2: Wrong Inspection Level Selection

Selecting the wrong inspection level (e.g., General I instead of General II) can lead to under-sampling and missed defects, or over-sampling and wasted resources. Always align the inspection level with product risk and contractual requirements.

Mistake 3: Weak Documentation & Traceability

If results aren’t captured in a structured form, NCRs can be disputed by suppliers. Always record:

• Lot size and inspection level
• Sample size and AQLs used
• Actual defect counts

For consistency, align with certificate reviews explained in [EN 10204 3.1 vs 3.2: Differences Explained] (W1.3).

 

🔗 Cross-reference also: [PMI Testing: What, When & How to Document] (W2.3) to understand how AQL complements raw material verification.

 

FAQs: Receiving Inspection & AQL

1. What is AQL in receiving inspection?

AQL (Acceptance Quality Limit) is the maximum defect percentage allowed in a batch for it to be accepted during incoming/receiving inspection.

2. How does ISO 2859-1 define sampling plans?

ISO 2859-1 links lot size and inspection level to a code letter, which gives the sample size and accept/reject numbers for defects.

3. What is the difference between critical, major, and minor defects?

• Critical defects: safety/function risks → AQL = 0
• Major defects: functional or spec issues → AQL ≈ 0.65–1.0%
• Minor defects: cosmetic issues → AQL ≈ 2.5–4.0%

4. Which inspection level should I use for AQL sampling?

Most receiving inspections use General Level II. Stricter levels (III) or lighter (I) may be used depending on risk, product type, or client requirements.

5. Does AQL mean the lot has exactly that defect rate?

No. AQL ≠ actual defect rate. It only provides a statistical acceptance threshold, not the real percentage of defects in the lot.

6. What happens if my inspection results exceed the AQL limit?

The lot is rejected or put under conditional re-check. Inspectors must document the finding in a Receiving Inspection Form and issue an NCR for supplier action.

7. Can I automate AQL sampling in inspections?

Yes. Many QA/QC teams use an AQL Calculator (Excel) or inspection software that applies ISO 2859-1 automatically, reducing calculation errors.

📂 Downloadables & Tools for Inspectors

• AQL Calculator (Excel) → Quickly calculate code letters, sample sizes, and accept/reject limits without flipping ISO tables.
• Receiving Inspection Form (Excel) → Standardized template to log lot size, defects, AQLs, and final disposition.
• Defect Classification Cheatsheet (PDF) → Quick reference guide for Critical / Major / Minor defects with examples.

👉 These tools reduce calculation errors, improve consistency, and provide audit-ready documentation for vendor quality dossiers.

 

🚀 Master Receiving Inspection with NTIA

Applying AQL sampling correctly turns receiving inspection into a data-driven quality gate, not a guessing game. With ISO 2859-1, you get:

• Consistent decision-making (Accept / Conditional / Reject)
• Supplier accountability through NCRs and corrective actions
• Traceable documentation for audits and contracts

📌 But here’s the truth: templates alone are not enough. Without fully understanding why and how AQL works, inspectors risk misusing the numbers, raising wrong NCRs, or missing critical defects.

👉 That’s why we built the NTIA Raw Course (eLearning):

• Learn step-by-step how to choose, apply, and document AQL plans
• Practice with real inspection cases and defect photos
• Understand how AQL connects with MTC review, PMI testing, and vendor reporting
• Earn a certificate trusted by EPC contractors and QA/QC managers

💡 With the Raw Course, you won’t just fill in forms—you’ll master the logic behind them. That’s the difference between an inspector who checks boxes and one who builds credibility with every report.

👉 [Join the Raw Course Today] and take your inspection skills to the level where clients trust your decisions, not just your templates.