1 December 2010,
 1

FINAL GOODS INSPECTION  USING AQL
(Acceptable Quality Level)

What is AQL?

The AQL of a sampling plan is a level of quality routinely accepted by the sampling plan. It is generally defined as the percent defective – Non Conformances- (defectives per hundred units X 100%) that the sampling plan will accept 95% of the time. This means lots at or better than the AQL are accepted at least 95%  of the time and rejected at most 5% of the time.

Associated with the AQL is a confidence statement one can make. If the lot passes the sampling plan, one can state with 95% confidence that the quality level of the lot is equal to or better than the AQL (i.e., the defective rate of the lot < AQL). On the other hand, if the lot fails the sampling plan, one can state  with 95% confidence that the quality level of the lot is worse than the AQL.

How to choose between general and special inspection levels?
  • The general inspection levels (I to III) are commonly used for non-destructive inspection. Level II is considered the norm (except for small sample sizes). Level I is required only 40 percent of inspection level II and can be used where less discrimination is needed. Level III equals 160 percent of the amount of inspection Level II. Level III will give a lower risk of accepting a lot with excessive number of defects. However, inspection of larger samples is required. Unless otherwise specified, inspection Level II will be used.
  • Special Levels S-1, S-2, S-3 and S-4 may be used where relatively small sample sizes are necessary or large sampling risks can be taken. Examples of this are inspections involving destructive or costly (time consuming) type inspection, or where large lots are involved, small sample sizes are desired and large risks can be tolerated such as repetitive processes (screw machine, stamping, bolting operations, etc.) performed by a quality supplier. Larger sample sizes are for inspection levels increasing from S-1 to S-4.

You can use the following table as a quick guide to decide how to select a proper inspection level.

CONSIDERATION I II III S1 S2 S3 S4
QTY. INSPECTED Half Normal Twice Small
INSPECTION COST Low High Low
DEFECT DISCRIMINATION Low High Low
RISK OF ACCEPTING DEFECTS Higher Low Higher
INSPECTION TYPE NON DESTRUCT. DESTRUCTIVE
PROCESS TYPE MANUAL AUTOMATIC

Table B. Guide to select between Normal and Special levels.

How many pcs/units shall be inspected?

According to ANSI/ASQ Z1.4- 2003, the quantity of products to be inspected (sample size) is defined by the inspection level to be utilized and the lot size (quantity of product) in question. Three general and four special inspection levels are provided.

Lot Size General Special
From To I II III S1 S2 S3 S4
2 to 8 A A B A A A A
9  to 15 A B C A A A A
16  to 25 B C D A A B B
26  to 50 C D E A B B C
51  to 90 C E F B B C C
91  to 150 D F G B B C D
151  to 280 E G H B C D E
281  to 500 F H J B C D E
501  to 1,200 G J K C C E F
1,201  to 3,200 H K L C D E G
3,201  to 10,000 J L M C D F G
 10,001  to 35K K M N C D F H
 35,001  to 150K L N P D E G J
150,001  to 500K M P Q D E G J
500,001  and over N Q R D E H K

Table A. Table to define the sample size according to the Inspection Levels.

After potential defects are identified & classified before the inspection, inspectors shall follow the inspection plan in the next table to start inspecting the sampled goods. Please note that the arrows indicate a change in the Sampling Plan, where the sample size and the Acceptance and Rejection numbers will change as well.

CODE SAMPLE
SIZE
0.065 0.10 0.15 0.25 0.40 0.65 1.00 1.50 2.50 4.00 6.50
Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej Acc Rej
A 2  ↓ 0 1
B 3 0 1
C 5 0 1
D 8 0 1
1 2
E 13 0 1
1 2 2 3
F 20 0 1
1 2 2 3 3 4
G 32 0 1
1 2 2 3 3 4 5 6
H 50 0 1
1 2 2 3 3 4 5 6 7 8
J 80 0 1
1 2 2 3 3 4 5 6 7 8 10 11
K 125 0 1
1 2 2 3 3 4 5 6 7 8 10 11 14 15
L 200 0 1
1 2 2 3 3 4 5 6 7 8 10 11 14 15 21 22
M 315
1 2 2 3 3 4 5 6 7 8 10 11 14 15 21 22  ↑
N 500 1 2 2 3 3 4 5 6 7 8 10 11 14 15 21 22  ↑
P 800 1 2 2 3 3 4 5 6 7 8 10 11 14 15 21 22  ↑
Q 1250 2 3 3 4 5 6 7 8 10 11 14 15 21 22  ↑
R 2000 3 4 5 6 7 8 10 11 14 15 21 22  ↑

Table C. See here the AQL Level table that displays from  0% to 6.5% defectived and  the Acceptance and Rejection rate to be considered during a regular inspection.

How defective products are counted or defined for the inspection?

According to ISO and the ASQ an Individual with Non-Conformances (defects) is called a Non-Conformant (defective) sample. In the inspection process, one defective sample is counted one for the most serious defect only no matter how many defects found in the said sample.

Product Defects considered for SBE inspections are usually classified within 3 categories:  “Critical”,  “Major” and  “Minor”:

Critical: likely to result in unsafe condition or contravene mandatory regulation or reject by import customs.

Major: reduces the usability/function and/or sale of the product or is an obvious appearance defect.

Minor: 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.

The above classification is not an Industry standard but a practical manner to classify defects in products. Clients may indicate an specific Sampling plan, same as  what defects are minor, major or critical according to the Acceptance Quality Level % they are willing to accept per type of defect.

The following AQLs levels are usually applied by SBE unless client suggest others:

CRITICAL DEFECTIVES

MAJOR DEFECTIVES

MINOR DEFECTIVES

Low or medium valued products

AQL 0% 

AQL 2.5 %

AQL 4.0 %

High valued products

AQL 0%

AQL 1.5 %

AQL 2.5 %

Table D. Proposed or common classification of defectives

Example

Let’s take a lot of 4,000 units to inspect, with an AQL of 2.5%.
1. According to Table C, under General Level II, the sample size “Code” is L.
2. According to Table A, the sample size corresponding to code “L” is therefore 200 units, and the corresponding acceptance number is 10.
In other words:
– If the number of defective units is more than 10, the lot has to be rejected.
– If the lot is accepted, it is commonly perceived that there is a maximum of 2.5% of defective units in the lot (with a failure risk percentage of less than 5%).

What to do when an inspection fails?

1. Do not panic, revise with calm and detail the inspection reports/alerts.

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 lossing 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, perform 100% inspection, 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, use deferred LCs to pay suppliers, monitor your products in early stages of production, etc).

For more suggestions or questions please post a comment!

One response on “AQL ANSI/ASQ Z1.4- 2003

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