How to Use AQL to Measure Quality in Large Batches: Example with 10,000 Units

When we import products in large quantities, verifying the quality of each unit is practically impossible. However, we also cannot afford to ignore quality control and risk receiving defective products. This is where the AQL (Acceptable Quality Limit) system comes in, a statistical methodology that allows the quality of an entire batch to be evaluated by examining only a representative sample.

This article explains in detail how to implement the AQL system to inspect a batch of 10,000 units, from sample size determination to interpretation of results, offering a practical guide for importers looking to implement effective quality control processes.

What is AQL and why use it?

AQL is defined as the maximum level of defects that is considered acceptable in a batch of products. Expressed as a percentage or number of defects per 100 units, it strikes a balance between what is statistically acceptable and what is economically viable.

Key Benefits of the AQL System:

1. Resource efficiency: Allows quality to be verified without inspecting 100% of the units.
2. Objective basis: Provides clear and quantitative criteria for accepting or rejecting a batch.
3. International Standard: Globally recognized (ISO 2859-1), facilitates communication with suppliers.
4. Flexibility: Allows you to adjust the level of rigor according to the criticality of the product.
5. Statistical predictability: Provides a known level of confidence in results.

Key components of the AQL system

To properly implement an AQL inspection, we need to understand three essential elements:

1. Sample size

The number of units that need to be examined depends on:

• The total lot size
• The level of inspection selected (which determines the stringency)

The most common inspection levels are:

• Level I: Less rigorous inspection (smaller sample)
• Level II: Normal Inspection (Recommended Standard)
• Level III: More rigorous inspection (larger sample)

2. AQL Levels for Different Types of Defects

Defects are usually classified into three categories:

• Critical (typical AQL: 0-0.065%): Defects that may cause harm to the user or violate regulations.
• Major (typical AQL: 1.5-2.5%): Defects that affect the functionality or use of the product.
• Minor (typical AQL: 4.0-6.5%): Aesthetic or finish defects that do not affect function.

3. Acceptance/Rejection Criteria

Based on sample size and AQL levels, the following are established:

• Acceptance Number: Maximum Defects Allowed to Accept Batch
• Rejection Number: Minimum defects resulting in batch rejection

Practical example: AQL inspection for a batch of 10,000 units

Let's develop a complete example for a batch of 10,000 units of a consumer electronics.

Step 1: Determine the sample size

For a batch of 10,000 units using General Inspection Level II (the most common):

1. We consult the table of sample size letter codes (ISO 2859-1):
• For 10,000 units (between 5,001-10,000) the letter code "L" corresponds
2. With the letter code "L", we consult the sample size table:
• 200 units must be examined

Step 2: Set AQL levels and accept/reject criteria

For our electronic product, we establish:

• Critical defects: AQL 0.065%
• Acceptance Number: 0
• Rejection number: 1
• Major defects: AQL 2.5%
• Acceptance Number: 10
• Rejection Number: 11
• Minor defects: AQL 4.0%
• Acceptance Number: 14
• Rejection number: 15

This means that:

• A single critical defect will result in batch rejection
• 11 or more major defects will result in batch rejection
• 15 or more minor defects will result in lot rejection

Step 3: Define inspection criteria

Before starting the inspection, it is essential to clearly document what constitutes each type of defect for this specific product:

Examples of critical defects:

• Exposed electrical components that pose a safety hazard
• Unapproved toxic material
• Total failure of core functionality
• Mislabeling of safety warnings

Examples of major defects:

• Buttons that work inconsistently
• USB ports not connecting properly
• Screens with dead pixels greater than 3
• Battery life less than 80% of the specified battery life
• Bluetooth pairing issues

Examples of minor defects:

• Surface scratches on the housing (less than 3mm)
• Color variations within defined tolerance
• Minor irregularities in surface finishes
• Packaging with slightly misaligned printing
• Labels with small cosmetic defects

Step 4: Select the units to inspect

The selection must be truly random to guarantee statistically valid results:

1. Random selection method:
• Calculate a sampling interval: 10,000 ÷ 200 = 50
• Select one unit every 50 in different boxes and pallets
• Ensure distribution between different production batches if they exist
2. Sample documentation:
• Register serial numbers or identifiers for each selected unit
• Document location of each sample (box/pallet number)

Step 5: Perform the physical inspection

Best practice for the 200 units selected:

1. Exterior Visual Inspection (100% of Sample):
• Verify packaging, labeling, and exterior condition
• Document defects found by category
2. Complete functional test (100% of the sample):
• On/off
• Main functions
• Connections and ports
3. Stress test (20% of the sample = 40 units):
• Repeated cycles of key functions
• Controlled drop test (if applicable)
• Temperature test (if applicable)
4. Accurate measurements (50% of sample = 100 units):
• Critical dimensions
• Voltage/amperage (if applicable)
• Calibration (if applicable)
5. Partial disassembly (10% of the sample = 20 units):
• Verification of internal components
• Weld quality
• Compliance with internal specifications

Step 6: Record and categorize defects

During the inspection, each defect found must:

1. Be documented with detailed description
2. Being photographed as evidence
3. Be classified according to the pre-established category
4. Be recorded on the AQL inspection sheet

Suppose that after inspecting all 200 units we find:

• 0 Critical Defects
• 8 major defects
• 12 minor defects

Step 7: Determine Batch Acceptance or Rejection

We compared our results with the established criteria:

• Critical defects: 0 found (acceptance limit: 0) → Accepted
• Major defects: 8 found (acceptance limit: 10) → Accepted
• Minor defects: 12 found (acceptance limit: 14) → Accepted

In this case, the batch would be accepted as all types of defects are below their respective rejection limits.

Special Situations and Additional Considerations

Inspection with results at the limit

If we found exactly 10 major defects (equal to the acceptance number), the batch would be technically acceptable, but it would be recommended:

1. Communicate the situation to the supplier with formal warning
2. Consider increasing sample size for greater confidence
3. Implement more rigorous inspection on the next batch

Changes in inspection level

The inspection level can be adjusted based on the supplier's history:

• Change to reduced inspection (Level I): After several consecutive batches without rejections
• Switch to Rigorous Inspection (Level III): After rejecting a batch or detecting negative trends

For our example of 10,000 units:

• Level I would require 80 units to be inspected (code J)
• Level III would require inspecting 315 units (code M)

Double or multiple sampling

To optimize resources, a double or multiple sampling system can be implemented where:

1. A smaller sample is inspected first
2. Whether the results are clearly good or bad, the decision is made
3. If the results are ambiguous, a second sample is inspected

This approach can reduce the average number of units inspected while maintaining the same level of statistical confidence.

Practical Implementation: Tips and Best Practices

Pre-Inspection Preparation

1. Clear Definition of Defects: Create a Defect Manual with Photographs and Descriptions
2. Inspector Training: Ensure that all inspectors interpret the criteria in the same way
3. Calibrated tools: Verify that all measuring instruments are calibrated
4. Proper conditions: Ensure proper lighting, space and environment

During the inspection

1. Independence: The inspector should not be influenced by the manufacturer
2. Consistent methodology: Always follow the same inspection protocol
3. Thorough documentation: Record each finding with photographic evidence
4. Transparency: Allow manufacturer representatives to observe the process

After inspection

1. Detailed Report: Provide Complete Analysis, Not Just the Final Decision
2. Root Cause Analysis: Identifying Patterns in Defects Found
3. Corrective Action Plan: Request Supplier Action for Recurring Defects
4. Follow-up: Verify the implementation of corrections in future inspections

Adapting the AQL for different products

The example above is for an electronic product, but the AQL levels should be adjusted based on:

High-risk products (medical, children's, etc.)

• Critical defects: AQL 0.010-0.040%
• Major defects: AQL 0.65-1.0%
• Minor defects: AQL 1.5-2.5%

Industrial Products or Components

• Critical defects: AQL 0.10-0.15%
• Major defects: AQL 1.0-1.5%
• Minor defects: AQL 2.5-4.0%

Decorative or non-essential products

• Critical defects: AQL 0.15-0.25%
• Major defects: AQL 2.5-4.0%
• Minor defects: AQL 6.5-10.0%

Limitations of the AQL system

It is important to understand that AQL has certain inherent limitations:

1. Statistical risk: There is always a probability of accepting defective batches (consumer risk) or rejecting good batches (producer risk).
2. Doesn't catch every problem: Systematic defects that affect all drives in the same way can go unnoticed.
3. Requires clear definitions: Without precise criteria for what constitutes each type of defect, results can be inconsistent.
4. It does not replace process improvement: It is a verification tool, not a quality improvement tool at source.

Conclusion: AQL as part of a comprehensive quality strategy

The AQL system provides a statistically sound methodology for making decisions about the acceptance of large batches like the one in our 10,000-unit example. However, it reaches its full potential when integrated into a broader strategy that includes:

• Rigorous supplier qualification
• Detailed Technical Specifications
• Process control during manufacturing
• Collaborative Relationships with Manufacturers
• Continuous analysis of quality data for improvement

Implemented correctly, the AQL system balances the need for quality control with the practical realities of cost and timing, providing a quantifiable level of confidence in the quality of the products we import, even when handling volumes as large as 10,000 units or more.

Do you need help implementing an AQL system tailored specifically to your products? Are you unsure about what inspection levels would be appropriate for your industry? Contact us for a personalized QC consultation for your imports.