AQL Terms

FAMILIARIZE WITH THE BASICS OF QUALITY CONTROL TECHNIQUES

Generally, the standards ISO2859 / MIL-STD-105E / ANSI/ASQ Z1.4 Sampling Procedures and Tables for Inspection by Attributes are adopted for the performance of inspections. Although with different codes, these standards describe very much the same rules. There are some basic definitions to be familiar with:

Sampling, multiple Sampling inspection in which, after each sample is inspected, the decision is made to accept a lot, reject it or take another sample. But there is a prescribed maximum number of samples, after which a decision to accept or reject the lot must be reached. Note: Multiple sampling as defined here has sometimes been called “sequential n sampling” or “truncated sequential e sampling.” The term “multiple sampling” is recommended. Sampling, double Sampling inspection in which the inspection of the first sample leads to a decision to accept a lot, reject it or take a second sample; the inspection of a second sample, when required, then leads to a decision to accept or to reject the lot. Sampling at random  As commonly used in acceptance sampling theory, the process of selecting sample units so all units under consideration have the same probability of being selected. Note: Equal probabilities are not necessary for random sampling; what is necessary is that the probability of selection be ascertainable. However, the stated properties of published sampling tables are based on the assumption of random sampling with equal probabilities. An acceptable method of random selection with equal probabilities is the use of a table of random numbers in a standard manner. MIL-STD-105E  A military standard that describes the sampling procedures and tables for inspection by attributes. Inspection, tightened  Inspection in accordance with a sampling plan that has stricter acceptance criteria than those used in normal inspection. Tightened inspection is used in some inspection systems as a protective measure when the level of submitted quality is sufficiently poor. The higher rate of rejections is expected to lead suppliers to improve the quality of submitted product. Note: The criteria for determining when quality is “sufficiently poor” must be defined in objective terms for any given inspection system. Inspection, reduced Inspection in accordance with a sampling plan requiring smaller sample sizes than those used in normal inspection. Reduced inspection is used in some inspection systems as an economy measure when the level of submitted quality is sufficiently good and other stated conditions apply. Note: The criteria for determining when quality is “sufficiently good” must be defined in objective terms for any given inspection system. Inspection, normal  Inspection used in accordance with a sampling plan under ordinary circumstances. Inspection, 100% Inspection of all the units in the lot or batch. Acceptable Quality Level (AQL): The acceptable level (AQL) is defined as the maximum percent defective that, for purpose of sampling inspection, can be considered satisfactory as a process average. Inspection Levels: The standards provides for three general inspection levels and four special inspection levels. These seven levels permit the user to balance the cost of inspection against the amount of protection required. Sampling Plans: A lot sampling plan is a statement of the sample size or sizes to be used and the associated acceptance and rejection numbers. Critical Defects: likely to result in unsafe condition or contravene mandatory regulation or reject by import customs. Major Defects: reduces the usability/function and/or sale of the product or is an obvious appearance defect. Minor Defects: doesn't reduce the usability/function of the product, but is a defect beyond the defined quality standard more or less reduces the sale of the products.
Commonly, the Critical/Major/Minor defects are defined as above and in practice specific circumstances and client requirements may also be taken into consideration.

There are basically two AQL tables. The first one tells you which 'code letter' to use based on the selected sample plan, such as S-4, G-I, G-II, etc. (the most commonly used inspection level for consumer goods is General Inspection Level II). If you follow my example, I assume your 'lot size' (which means quantity) is between 3201pcs and 10,000pcs, and that your inspection level is 'II'. Consequently, the code letter is "L".

Then, the second table will give you the sample size and the maximum numbers of defects that can be accepted based on the selected code letter. Your code letter is "L", so you will have to draw 200pcs randomly from the total lot size according to TABLE II. Acceptable Quality Levels 0.10, 0.25, 0.40, 0.65, … determines the limits of Critical, Major and Minor defects acceptable. For example, if we assume 0/2.5/4.0 for Critical/Major/Minor defects, the products are acceptable if no more than 0 Critical, 10 Major and 14 Minor defects are found.

WHAT TO DO WHEN AN INSPECTION FAILS?

1.Do not panic, review with calmness the details the inspection reports/alerts. Sometimes, an inspection report with failure conclusion doesn’t necessarily mean that the batch is absolutely unacceptable based on the specific business circumstances, such as price, target client, contract term, etc. The best practice is to review the report in details and then make the decision.

2.Consider the implications of accepting the goods the way they are but do not compromise your business integrity due to pressure on delivery, is better to delay a shipment than losing a customer for ever and getting into potential legal issues.

3.Share the inspection results with your supplier and negotiate an immediate action (e.g. stop production, ask for a few extra products to cover potential defectives, ask for discounts, delay the shipment, arrange re-inspections and ask Supplier to bear the cost of them, etc...)

4.Ask or implement Long Term Corrective Actions. (Add penalty clauses in your purchasing contracts, define defectives limits and product specs on time, perform re-inspections in all future orders, monitor your products in early stages of production, etc).