What is AQL sampling inspection?

What is AQL?

AQL (Acceptable Quality Limit) sampling inspection is a statistical tool used for quality control, mainly used to decide whether a batch of products can be accepted or rejected.

What is the use of AQL sampling inspection?

AQL sampling inspection is mainly used in large-scale production. By sampling a part of a batch of products, it is inferred whether the quality of the entire batch of products meets the requirements. The core idea of the AQL sampling method is that it is not necessary to inspect every product, but to estimate the quality level of the entire batch by sampling a part of the products, thereby saving cost and time.

AQL sampling inspection is based on the standard

AQL (Acceptance Quality Limit) sampling standards are:

ANSI/ASQC Z1.4

ANSI/ASQ Z1.4 is a quality control standard jointly developed by the American National Standards Institute (ANSI) and the American Statistical Association (ASQ). Its full name is "Sampling Procedures and Tables for Inspection by Attributes".

MIL-STD-105E

MIL-STD-105E is a sampling inspection standard developed by the U.S. Department of Defense. Its full name is "Military Standard 105E - Sampling Procedures and Tables for Inspection by Attributes".

BS 6001

BS 6001 is a sampling inspection standard published by the British Standards Institution (BSI). Its full name is "BS 6001 - Sampling Procedures for Inspection by Attributes". It is used to implement sampling inspection in the production and manufacturing process.

ISO 2859

ISO 2859 is a set of quality management standards developed by the International Organization for Standardization (ISO). Its full name is "Sampling Procedures for Inspection by Attributes". It aims to provide guidelines and procedures for attribute-based sampling inspection.

GB/T 2828.1

GB/T 2828.1 is a Chinese national standard, formally known as "Sampling procedures for inspection by attributes - Part 1: Sampling plans for batch inspection based on acceptance quality limit (AQL)". This standard provides enterprises with a set of quality control procedures based on sampling inspection, which is suitable for quality inspection in mass production.

How to read the AQL sampling standard table?

Table 1: Sample size code letters

Lot Size

This is the first column of the table, the number of unit products in the inspection batch.

Special Inspection Levels

It is divided into four levels: "S-1", "S-2", "S-3" and "S-4", and the discrimination ability is: S-1＜S-2＜S-3＜S-4.

General Inspection Levels

It is divided into three levels: "I", "II" and "III", with the discrimination ability: Ⅰ＜Ⅱ＜Ⅲ.

General inspection level ＞ Special inspection level

Principles for selecting test levels

1) If there is no special provision, the general inspection level II shall be adopted first;

2) Compare the inspection costs. If the inspection cost of a single sample is a, and the cost of processing a sample when the batch is judged unqualified is b, the inspection level selection should follow:

a＞b Select test level I

a＝b Select test level II

a＜b Select test level III

3) To ensure the AQL and minimize the number of product batches that are inferior to the AQL, a high inspection level should be selected;

4) When the inspection cost (including manpower, material resources, time, etc.) is low, choose a high inspection level;

5) When product quality is unstable and fluctuates greatly, a high inspection level should be selected;

6) For destructive testing or testing that seriously reduces product performance, a low inspection level should be selected;

7) When the inspection cost is high, choose a low inspection level;

8) When product quality is stable and the variance is small, a low inspection level is selected;

9) For trial products with little or no historical data, a higher inspection level must be selected for safety reasons; for products in intermittent production, a higher inspection level should be selected.

Selection conditions for general inspection level I

a) Even if the accuracy of the judgment is reduced, it will not have a significant impact on the customer's use of the product;

b) The price per unit of product is lower;

c) The product production process is relatively stable and has little impact from random factors;

d) The quality status of each inspection batch does not fluctuate much;

e) The quality of the inspection batch is relatively uniform;

f) When a batch of products fails, the risk is relatively small.

Selection conditions for general inspection level III

a) The purchaser has special requirements on the use of the product;

b) The price per unit of product is higher;

c) The quality of products is easily affected by random factors during the production process;

d) There are large fluctuations in the quality status between each inspection batch;

e) There are large differences in quality between the inspection batches;

f) When a batch of products fails, the average cost of handling it far exceeds the cost of inspection;

g) For new products whose quality status is not well known.

Special inspection levels are generally used in the following scenarios

1. Product destructive testing;

2. The product itself is of high value;

3. The inspection fee is high.

Directions:

After finding the batch size and inspection level, you can find the corresponding sample size code letter through the table.

For example, if the batch size is between 1201 and 3200 and the test level is II, then the corresponding sample size code is K.

Table 2: Single sampling plans for Normal Inspection

Sample Size Code Letter

Corresponding to the sample size code obtained in Table 1.

Sample Size

According to the sample size code obtained in Table 1, the sampling quantity can be clarified.

AQL value

It is a string of 21 numbers (0.010, 0.015...65, 100). The lower the AQL value, the higher the required quality standard.

Ac Acceptable number/Re Rejected number

Based on the AQL value, the table will give the number of defects accepted and rejected. If the number of defects in the sample is less than or equal to the acceptance number, the batch can be accepted; if it is greater than the rejection number, the batch is rejected.

Directions:

According to the sample size code obtained in Table 1, after the sampling quantity is determined, the corresponding acceptance number and rejection number are found according to the AQL value. If the number of defects in the sample is less than or equal to the acceptance number, the batch can be accepted; if it is greater than the rejection number, the batch is rejected.

For example, if the sample size code obtained from Table 1 is K, then the corresponding sample size is 125. Assume that the AQL is set to 2.5 (%) for major defects and 4.0 (%) for minor defects. Therefore, it can be concluded from Table 2 that if no more than 7 major defects and no more than 10 minor defects are found in the product, the batch of products is accepted.

For example: If 9 major defects and 9 minor defects are found in the product, then this batch of products will be rejected; if 6 major defects and 12 minor defects are found in the product, then this batch of products will be rejected; if 5 major defects and 7 minor defects are found in the product, then this batch of products will be accepted.

Tips:

Defect Type

Fatal flaw

Fatal defects refer to defects that may cause harm or unsafe conditions to consumers during the use and maintenance of the product, or defects that cause the product to fail to comply with relevant laws and regulations, or defects that damage the important and ultimate basic functions of the product.

Main flaws

The main defects are some functional defects that will reduce the usability and reliability of the product without affecting safety, or obvious appearance defects that will affect sales.

Minor defects

Minor defects are defects that do not significantly reduce the expected performance of the product, or defects that deviate from the standard but only slightly affect the effective use or operation of the product, but may affect the sale of the product.

If multiple defects are found in the same product (set), only the most serious defect will be recorded in the defect list, but other defects should also be noted. If any fatal defects are found, the inspection result of the entire batch of goods should be failed, and it is up to the customer to decide whether to accept it.

Default sampling plan

The default sampling plan is Class II sampling, and the AQL acceptance criteria are: fatal defects (not allowed), major defects (2.5), and minor defects (4.0). The above values are not fixed and can be adjusted according to the type of product or the relationship between the buyer and the manufacturer.

How to design an AQL sampling test plan?

Step 1

First of all, all table lookups must first obtain known conditions, which must include:

1. Total quantity (batch), the total quantity of products to be inspected this time - the amount obtained from the order;

2. Inspection level - confirmation between the manufacturer and the brand (buyer) before placing an order;

3. Obtain the AQL value - the manufacturer and the brand (buyer) confirm in advance before placing an order.

These three known conditions are indispensable.

Step 2

According to the batch quantity, select the corresponding sample size code in the sample size code table.

Step 3

Select the corresponding sampling quantity in the normal inspection single sampling plan table through the sample size code.

Step 4

Based on the AQL value, combined with the sampling quantity, the acceptance number is obtained in the normal inspection single sampling plan table.

Tips:

When querying the AQL table, what do the up and down arrows represent?

If you encounter an arrow, select the acceptance/rejection number in the direction of the arrow and reselect the corresponding sample size. If the sample size is equal to or exceeds the batch size, perform a 100% inspection (only applicable to a single defect level).

AQL Sampling Test Program Example

A company orders 13,000 suits from a clothing manufacturer in China. All suits are produced in a single batch. The specified AQL levels are: 0.0% for critical defects, 2.5% for major defects, and 4.0% for minor defects; the inspection level is General Inspection Level II.

Table 1: Sample size The code table shows that the intersection of the column and the row is 'M'.

Table 2: Normal inspection single sampling plan table, M corresponds to a sample size of 315. The AQL for fatal defects is 0%, so if any fatal defects are found, the entire batch will be rejected.

The intersection of the M row and the 2.5% column shows the numbers 14 and 15. The lower number 14 means that the maximum number of major defects allowed is less than or equal to 14. The intersection of the M row and the 4.0% column shows 21 and 22. The lower number 21 means that the maximum number of minor defects allowed is less than or equal to 21.

If the number of major defects is greater than or equal to 15 or the number of minor defects is greater than or equal to 22, the batch is rejected.

Frequently Asked Questions (FAQs)

1. What is AQL sampling inspection?

AQL sampling inspection is a statistical method used to infer whether the quality of the entire batch of products meets acceptable standards through sampling in large-scale production.

2. How to use AQL standard table?

First, the sample size is determined based on the batch size and inspection level, then the acceptance and rejection criteria are determined based on the AQL value, and finally, sample spot checks are carried out.

3. How to set the AQL value?

The AQL value is set by negotiation between the manufacturer and the customer, and is usually selected based on the quality requirements of the product. A lower AQL value indicates a higher quality requirement.

4. What are the advantages of AQL sampling inspection?

AQL sampling inspection can reduce inspection costs, improve inspection efficiency, and ensure that product quality meets standards.

5. How to determine the sampling size?

The sampling quantity is determined from the AQL standard table based on the batch size and inspection level. Larger batches usually require more samples for testing.

6. What industries does the AQL standard apply to?

AQL sampling inspection is widely used in quality control in many industries such as manufacturing, electronics, consumer goods, and clothing.