To use AQL (Acceptable Quality Limit) Tables to determine the right sample size for inspection, follow these structured steps. AQL tables are standardized tools (e.g., ISO 2859-1, ANSI/ASQ Z1.4) that help balance quality control costs and risks by defining how many items to inspect and how many defects are permissible.

Define Key Parameters

• Lot Size (N): Total number of items in the batch (e.g., 5,000 units).
• Inspection Level: Choose based on risk tolerance:
  • Level II (Default): Balanced for most scenarios.
  • Level I: Reduced sample size (less strict).
  • Level III: Larger sample size (stricter).
  • Special Levels (S-1 to S-4): For destructive/critical tests (small samples).
• AQL Value: Maximum acceptable defect rate (e.g., 1.0%, 2.5%, 4.0%).
  • Common Values:
    • Critical defects: 0.1–0.65%
    • Major defects: 1.0–2.5%
    • Minor defects: 4.0–6.5%

Find the Sample Size Code Letter

• Use the Lot Size and Inspection Level to locate the code letter in the table below:

Example:

• Lot size = 500 units, Level II → Code Letter H.

Determine Sample Size (n) and Acceptance/Rejection Numbers

• Use the Code Letter and AQL Value to find the sample size (n), acceptance number (Ac), and rejection number (Re) in the AQL table below:

Example:

• Code H, AQL 0% → Sample Size = 50, Ac = 1, Re = 2.
  • Interpretation:
    • Accept the lot if ≤1 defect is found in 50 samples.
    • Reject if ≥2 defects are found.

Example Calculation

Scenario:

• Lot size = 1,500 units
• Inspection level = II
• AQL = 5% (for major defects)

Steps:

1. Code Letter: Lot size 1,201–3,200 + Level II → Code K.
2. Sample Size: Code K → Sample Size = 125.
3. Acceptance/Rejection:
   • AQL 2.5% → Ac = 7, Re = 8.
   • Decision: Accept if ≤7 defects; reject if ≥8 defects.

Key Notes

1. Defect Classification:
   • Test items for critical, major, and minor defects separately.
   • Use different AQL values for each (e.g., 0.1% for critical, 1.5% for major).
2. Random Sampling:

   Ensure samples are randomly selected to avoid bias.

3. Switching Rules:
   • If quality is poor, switch to tightened inspection (larger samples).
   • If quality improves, switch to reduced inspection (smaller samples).
4. Limitations:
   • AQL tables assume attributes inspection (pass/fail), not continuous data.
   • For high-risk items (e.g., medical devices), use zero-defect AQL (0.0%).

When to Use AQL Tables

• Manufacturing quality control.
• Incoming/outgoing inspections.
• Compliance with industry standards (e.g., automotive IATF 16949, ISO 9001).

By following these steps, you ensure statistically sound decisions that minimize risk while controlling inspection costs. For complex scenarios, consult quality engineers or standards like ISO 2859-1.