How do we perform Quality Inspection
How to inspect goods ?
There are generally two inspection methodology :
- By inspecting everything Aka inspection 100% : in this case we inspect all the pieces one by one
- By performing a sampling : in this case we pick randomly a representative number of product which statistically represent the full batch
Each of them have advantages and drawback.
1./ Inspecting 100% of the batch
Inspect 100% of the good is the safest way to perform quality control because in this case each single piece is inspected and sorting for defective is operated.
- Normally this operation should already be performed by manufacturer during the Final Quality Control (FQC) phase. However as objectively performing quality control while being manufacturer is not necessarily compatible (it means in this case than as manufacturer you may produce some waste , hence doesn’t optimize your cost), we are quite often requested to perform a second 100% inspection operation to avoid the sieve effect which may happen during manufacturing.
In this case the 100% inspection can either be performed in our own inspection operation area or directly at manufacturer place.
The 100% inspection is usually performed when goods value can afford to add an extra cost on the top of product price. It is well adapted for high added value goods such as luxury item.
Advantage : safe, have a ultra reliable figure about defective rate, all defectives are eliminated
Drawback : can be long and potentially costly
2./ Inspecting by performing a sampling
When inspecting by sampling, we follow the international standard ISO 2859 also called MIL STD 105E or ANSI ASQC Z1.4, which is a statistical methodology for quality inspection purpose using the AQL (Acceptance Quality Level)
We select randomly a representative number of pieces X among the whole batch of product of quantity Y. This number X is supposed to represent the whole batch Y. It means that the quality of this Y full batch will be estimated and deducted from the quality of the X inspected pieces.
Sampling inspection methodology is used because most of time it would be costly to inspect full batch.
How it works ?
By considering a batch Y of product and considering of the level of inspection which should be used the table 1 indicate how many piers should subsequently be inspected.
Then some gauge/threshold are established as acceptable quality level. It short it means that base on X pieces inspects if we fin Z defectives and that Z value is higher than what the gauge is accepting, then the whole batch Y will be considered as defective.
Explaining the inspection level
This determinate the accuracy which we use to inspect the goods. For example if the inspection is supposed mostly be a visual inspection with focusing at a macro scale an appropriate inspection level would be 2. If we want to inspect considering a high level of details at the micro scale then the inspection level 1 is adequate (you can notice that subsequently the number of inspected pieces X will be diminished). If finally we want to inspect while maximizing the number of inspected product the inspection level 3 can be used.
Inspection level S1-2-3-4 are special inspection level used mostly for testing phase.
Explaining the gauges
Gauge or threshold indicate when, or at which percentage of defective found over the selected sample, the batch is declared proper or not.
We classify defectives in three categories ;
- Critical: product Is considered as dangerous
- Major : product is considers as unsellable
- Minor : product is considers as imperfect but sellable
Buyer or importers is usually deciding which are the acceptable threshold for production to be declared as proper or not. Most of them use a traditional scheme of
0.1 for critical
2.5 for major
4 for minor
In consequence once sampling is done and defect found reveal a certain number of defective over the sampling per classification, if at least one of the threshold is overwhelmed then the production is considered as improper. On the opposite if all the percentage of defect found are below the threshold then the production is considered as proper.
Understanding with a practical case
Example: you are importing 7500 pen for gift and are considering an inspection level 2, the table 1 for 7500 and level 2 indicate the letter L. Now in the table 2 L indicate the quantity to inspect and being representative of 7500 as being 200.
In consequence, according to the international standard 200 pieces will be inspected.
Now looking at the gauge, there are three level which are used for critical, major and minor defects. Here the buyers has decided
0.1/2.5/4 which in table 2 and for 200 pieces lead to some threshold of :
- For 0.1 : 0 acceptable critical defect over 200
- For 2.5 : 10 acceptable major defect over 200
- For 4: 14 acceptable minor defect over 200
In a result, if the rate of defective product are within those range , then the batch is declared within AQL and “pass”, if one of the threshold is overwhelmed then the batch is declared out of the AQL and “failed”.