Quality Control Basic Concepts

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Quality control basic concepts

In this article I explain three fundamental concepts that every buyer should be familiarwith when it comes to quality inspections:

1. Inspection levels

2. The AQL

3. When to inspect?

After 10 minutes, you will be able to (1) understand the reliability of an inspectorsfindings and (2) take more informed decisions based on an inspection report.

If you have not started doing professional quality control, you will need to understand

these 3 concepts to make sure the inspection plan meets your needs.

1. Inspection levels

Why use random sampling?

Shipments often represent thousands of products. Checking100% of the quantity would

be long and expensive. A solution is to select samples at random and inspect them,

instead of checking the whole lot.

But how many samples to select? On the one hand, checking only a few pieces mightprevent the inspector from noticing quality issues; on the other hand, the objective is to

keep the inspection short by reducing the number of samples to check.

The relevant standards propose a standard severity, called normal level, which isdesigned to balance these two imperatives in the most efficient manner.

Within this normal severity, there are three general levels: I, II, and III. Level II is used

for more than 90% of inspections. For example, for an order of 8,000 products, only 200

samples are checked.

Militari Standard 105 was created by the US Department of Defense to control theirprocurements more efficiently. In 1994 they decided to rely on non-governmental

organizations to maintain this type of standard. The ANSI, ISO, and other institutes all

created their own standard, but in essence they are similar. The major third-party QCfirms use the same standards and the same statistical tables.

When to adopt a different level

Suppose you source a product from a factory that often ships substandard quality. You

know that the risk is higher than average. How to increase the discriminating powerof

the inspection? You can opt for the level III, and more samples will be checked.
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Similarly, if a supplier has consistently delivered acceptable products in the past and

keeps its organization unchanged, you can choose level I. As fewer samples have to be

checked, the inspection might take less time and be cheaper.

The relevant standards give no indication about when to switch inspection levels, so most

importers rely on their gut feeling.

The special levels

Inspectors frequently have to perform some special tests on the products they are

checking. In some cases the tests can only be performed on very few samples, for tworeasons:

1. They might take a long time (e.g. doing a full function test as per claims on the

retail box).

2. They end up in product destruction. (e.g. unstitching a jacket to check the lining

fabric).

For these situations only, the inspector can choose a special level.

So we have three general inspection levels, and four special levels. For a given order

quantity, each level gives a different number of samples to check. Lets see how it playsin two examples.

Example 1: you order 40,000 products

The number of samples to draw varies from 8 to 800. Depending on the level you choose,

the inspection might take only one inspector for one day, or up to 4 or 5 days of work.

Example 2: you order 3,000 products

Note: do not mix up inspection levels (I, II, III, S-1, S-2, S-3, S-4) and inspection

severity (reduced, normal, tightened). They are different concepts. The above examplesare based on a normal severity.

2. The AQL (Acceptance Quality Limit)
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In part 1, we explained the different inspection levels that can be used. Another basic

concept rings familiar to many importers, but is often not clearly understood: the AQL

(Acceptance Quality Limit).

There is no such thing as zero defect

First, as a buyer, you have to know what proportion of defects is tolerated on your

market. If you are in the aviation business, any defective part might cause a disaster, so

your tolerance will be very, very low. But you will have to accept a higher percentage ofdefects if you source consumer products that are assembled by hand in China or in India.

An objective limit is necessary

So, how many defects are too many? It is up to you, as a buyer, to make this decision.

There are two reasons why you should not leave this to the inspectors judgment:

1. When it comes to giving instructions to an inspector, you should never leave grayareasas they might open the door to corruption.

2. Your supplier should have clear criteria for acceptability, or they will see

rejections as unfair.

The AQL is the proportion of defects allowed by the buyer. It should be communicated tothe supplier in advance.

The three categories of defects

Some defects are much worse than others. Three categories are typically distinguished:

Critical defects might harm a user or cause a whole shipment to be blocked by the

customs.

Major defects are not accepted by most consumers, who decide not to buy the

product.

Minor defects also represent a departure from specifications, but some consumerswould still buy the product.

For most consumer products, critical defects are not allowed, and the AQL for major

defects and minor defects are 2.5% and 4.0% respectively.

Some important remarks:

A professional inspector will notice defects and evaluate their category byhimself. But it is better if the buyer himself describes the most frequent defects

and assigns categories to each one.

Defects can be on the product itself, on the labeling or on the packaging.

If one sample presents several defects, only the most severe one is counted.


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How to read the AQL tables

The master tables included in the relevant standards are commonly called AQL tables.

Lets take an example.

You buy 8,000 widgets from a factory, and you choose inspection level II. In the tablebelow (which is only valid for single sampling plans), you see that the corresponding

letter is L.

AQL table 1 (sample code letters):

Now lets turn to the next table (which is only appropriate for normal-severity

inspections). The letter L gives you the number of samples to draw at random: 200 pcs.

And what about the AQL? Lets say you follow the usual practice of tolerating 0% ofcritical defects, 2.5% of major defects, and 4.0% of minor defects. The maximum

acceptable number of defects is 10 major and 14 minor. In other words, the inspection is

failed if you find at least 1 critical defect and/or at least 11 major defects and/or at least15 minor defects.

AQL table 2:
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Notes:

These 2 tables are only useful for inspections under normal severity. See all the

tables for single sampling onAQL tables. You can use our calculator to get asampling plan for a QC inspection.

Additional notes, for accuracy

The number of defects is not the only cause for acceptance of refusal. The

products can be refused because they are not conform to the buyersspecifications, even though their workmanship is very good.

If you have two different products (made with different processes or in differentfactories), you should do two separate inspections. If you inspect them together,one product might be accepted even though it presents too many defects. Why?

Because the better workmanship of the other product might compensate for its

poor quality.

3. When to inspect?

The first two parts focused on the different inspection levels and on the AQL tables. So

you know how to set the number of samples to check and how many defects have to be

accepted. With these settings and your detailed product specifications, a QC inspector can

check your products and reach a conclusion (passed or failed).

But importers face one more question: when should the products be inspected? This is an

extremely important issue for buyers willing to secure their supply chain. Spending a few

hundreds of dollars to check and fix issues early can be an excellent investment; if mightsave you weeks of delay, shipments by air, and/or lower quality products that you have to

accept and deliver to your own customers.
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Four types of inspections

Lets picture the simplified model where one factory turns raw materials into finished

products. (If you also have to manage the quality of sub-suppliers products, the samemodel can be applied to them)

Pre-production inspection

This type of inspection is necessary if you want to check the raw materials or components

that will be used in production. Buying cheaper materials can increase a factorys marginconsiderably, so you should keep an eye on this risk. It can also be used to monitor the

processes followed by the operators.

During production inspection

This inspection allows you to get a good idea of average product quality, and to ask for

corrections if problems are found. It can take place as soon as the first finished productsget off the line, but these samples might not be representative of the whole order. So

usually an inspection during production is done after 10-30% of the products are finished.

Final (pre-shipment) inspection

Inspecting the goods after they are made and packed is the standard QC solution of mostimporters. The inspector can really check every detail, including counting the total

quantity and confirming the packaging. Final inspections are usually performed in a

hurry, just before shipment. To avoid creating delays, inspectors can usually start after all

products are finished and 80%+ of the shipment quantity is packed.

Loading supervision

In some cases, a buyer wants to make sure the factory ships the right products, in the

right quantity, and with the right loading plan.

Further reading about the different types of inspections:quality inspection services.
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4. Putting it all together

Example 1: 5,000 widgets from a new supplier

You have no information about the factory, so you should adopt level II.

The order quantity is comprised between 3,201 and 10,000pcs. In the first AQL table(higher in this document), you see the code letter is L.

And in the second AQL table you see that 200 samples have to be checked by the

inspector.

If you opt for the standard AQL limits (0 C. / 2.5% M. / 4.0% m.), the inspection is failedif at least one of these conditions comes true:

One or more critical defects are found

Eleven or more major defects are found Fifteen or more minor defects are found

And lets say you want the inspector to check all the product functions on a few samples.This test takes some time. You can choose special level S-2. You see this test will be

done on 8 samples.

Lets say the 200 samples can be checked by one person in one day, so a third-party QC

firm would quote you one man-day.

When to inspect? If this is a standard product and you have flexibility with timing, a final

(pre-shipment) inspection should be enough.

Example 2: 30,000 watches from unknown factory(ies)

You suspect that your supplier gave orders to several workshops to produce your goods,

but he wont tell you. And these items are rather valuable. A level-III inspection isprobably the most appropriate.

If we read the AQL tables as we did above, we have to inspect 500 samples, and the

maximum number of defects is: 0 critical, 21 major, 21 minor.

If a visual check on all samples and a function/accuracy check on a few pieces is enough,it probably takes two or three man-days.

When to inspect? If the supplier refuses to disclose the factory information, you have to

go for a final (pre-shipment) inspection.

Example 3: four different styles of garments from a good factory


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You know that this factorys workmanship is quite good. But you want an inspector to

check all the conformity elements, and in particular the measurements. Level I should be

enough.

You sell these products in boutiques at a high price, so you can only accept 1.5% major

defects (M) and 4.0% minor defects (m). Note: for garments, there are generally nocritical defects.

The fitting is quite important, especially for the brassiere and the brief, so 3 to 5 samplesshould be measured in each size.

There are 4 different types of products, so there has to be 4 inspections:

The total number of samples to check visually is 317 pieces. And 63 of these samples

also have to be measured. So it will probably take 2 inspectors (maybe 3, depending on

the products complexity) for one day.

When to inspect? The best is clearly during production, after enough finished products

have come off the lines. As the 4 products might not all be processed at exactly the same

time, it might be preferable to send an inspector at different times. And if an inspection isfailed, a re-inspection can probably be performed at the suppliers charge.

Here is a short version:

QA = all the activities that aim at ensuring a certain level of quality. It includes defining

what the requirement are + setting up a proper management system + QC.

QC = only the activities that consist of checking whether conformity is achieved ornot. In the context of an importer who needs to secure his product quality, QC means

checking if the specs are respected in production, and it translates into 2 types ofactivities: on-site inspections (statistical quality control) & laboratory testing (only on afew samples taken out of bulk production).

Here is the more detailed version:

1. Good QA takes time but is worth it
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For an importer buying finished products from low-cost Asia, QA includes the following

steps:

Making sure the design is manufacturing-friendly and will not cause defects

Auditing and approving factories based on a relevant checklist

Getting the manufacturers approval of the quality control plan Writing down detailed product specifications, and getting the manufacturers

approval Identifying failure points with production engineers, and removing them (back to

design)

Following pre-production sampling closely, and if possible launching a pilot run

Ensuring that lessons from sampling will be taken into account for production

Performing QC at one or several points of mass production

Following up with corrective/preventive action plans, when applicable

Re-engineering production processes (to reduce opportunities for errors)

Revising product specifications for the next production batch

2. QC is part of QA

It is important to understand that QA includes QC. It is not either QA or QC. Good QA

is dependent on QC, since the information from inspection is necessary to implementcorrective and preventive measures.

The old way: inspect bad quality out; The new way: build quality in.

- W. Edward Deming

3. QA is constantly mixed up with QC

QA and QC are often used interchangeably. QA sound smarter than QC, so QAis frequently substituted to QC. I have met with QA technicians who actually do

statistical quality control every day.

On the last trade show I attended, I noticed that I explained over and over again how we

perform random quality inspections. There must be a better way to pass this message.

One of my friends (and ex-colleague) in France is a graphic designer. He helped merender it visually, and here is the result:


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Is there still anything unclear about the general process? We tried to illustrate it from

10,000 feet high, without getting into any confusing detail

TRANSCRIPT:

FINAL RANDOM INSPECTIONS - How is quality controlled before shipment?

Most consumer goods exported from low-cost Asia to Europe and the USA are inspectedrandomly. For example, for an order of 8,000 pieces, only 200 samples are selected for

inspection. How can an inspector draw valid conclusions after checking some pieces at

random?
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Here is how it works.

1. COMMUNICATION OF REQUIREMENTS

The purchaser describes his product: specifications, dimensions, labeling, packaging

The more precise the information, the more you take advantage of the inspection. Ifpossible, an approved sample can be sent for the inspectors reference.

2. AN INSPECTOR GOES TO THE FACTORY

When? After all production is finished and packed, and about 2 days before the goods

leave the factory.

(In most cases, one inspector is enough)

3. QUANTITY VERIFICATION

The cartons are counted, to check if the whole quantity is presented. Then the inspector

randomly selects a few cartons, opens them, and checks their content.

4. RANDOM SELECTION OF SAMPLES

Some samples are taken from each of these cartons, totally randomly. The inspectorfollows industry-standard statistical rules to ensure his findings are valid.

5. COSMETIC AND WORKMANSHIP CHECK

These samples are checked thoroughly for visual defects. A defect is an imperfection onthe product (or its packaging). The statistical rules provide a maximum number for each

type of defect. If there are too many defects, the inspection is failed.

6. CONFORMITY VERIFICATION

The inspector also checks if the goods presented by the factory correspond to the

purchasers requirements. For example, the products might be too small, in the wrong

color, incorrectly labeled, or insufficiently protected.

7. TESTING IN THE FACTORY

The inspector performs some tests that are specific to the product, with the factorys

equipment. Tests vary according to the nature of the products. A few examples: checking

if there is current leakage on an electrical device; checking if a piece of furniture falls

over easily; checking if the export carton is strong enough.

8. REPORT PREPARATION


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The inspector issues a report that describes the situation and illustrates it with photos. It

documents his findings about presented quantity, visual defects, conformity to

requirements, and on-site tests.

Here is my report. You have the info you need to take a decision(accept or refuse the

shipment).

Yesterday I wrote about the danger of relying on Chinese manufacturers certificates. So,

if you import products that are subject to regulatory standards in your country, whatshould you do?

First, you should look for a supplier that already works with other importers in your

country. If possible, call a couple of reference customers (warning: it wont be easy to get

their contacts from your potential suppliers).

Second, you can contact a quality control firm to check what safety/regulatory standards

are applicable to your importing project.

Third, you can ask your suppliers if they have certifications from a international lab.

Then you can contact that lab, tell them the report number, and ask whether it islegitimate. It does not eliminate risks on your side, but it is better than nothing.

Fourth, you should tell your supplierfrom the beginningthat you will run lab tests on

their goods. Some of them will increase their prices, others will refuse your order. It is an

easy screening method.

Fifth, you are strongly advised to take the process in your own hands and to follow these

steps:

Send an inspector to pick up some real production samples in a random manner,

for on-site testing and/or for sending to a laboratory. It is important to use a

testing lab of YOUR choice, that YOU will pay, and that sends all the resultsdirectly to YOU. Depending on the risks to avoid, this step can take place once

the bulk materials/components are in the factory and/or when some totally

finished products are off the lines.

If the tests are failed, communicate with the lab to see if the goods are way

beyond what can be tolerated, or if only an insignificant part of the test protocol

triggered this general failure.

If the tests are failed for a valid reason, your supplier should pay for re-pickingrandom samples and for re-testing, and should follow the exact same procedure as

the first testing round. This is a procedure that should be defined in advance, in a

quality control plan.

Is this expensive? Yes it is, for small orders.

Is this too expensive for you to make enough margin? Then do not import directly.
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Remember, if you import potentially unsafe products, you (as the importer) carry the

same legal risks as if you were the manufacturer

Some importers have been buying from China for many years, and yet they have neverdone quality control in a professional manner. Thescience behind inspection protocols

seems complexnearly intimidating. Buyers dont know where to start, and they dontknow how their suppliers will react.

On the other hand, quality control is a necessity for most shipments. The constant searchfor cheaper suppliers, the bad habit ofsubcontractingto lower-grade factories, and the

high risk of communication mistakes, all make a strong case for systematic inspections.

So, how to start? What are the first steps? After helping a few importers to start doing

quality control, here are the first four steps I recommend.

1. Establish clear expectations

Some buyers choose a sample, negotiate a price, and then wait for delivery. This might

work for off-the-shelf (standard) items with low quality/safety constraints, but not for

most made-to-order products. And, think about it: on what basis will an inspectorapproveor rejecta production?

You should try to getperfect/golden samples (i.e. representative of what you expect to

get out of bulk production), but this is usually not enough. You also have to prepareor

confirm, if your supplier accepts to do ita list of specifications. And these specs willbecome the checkpoints for the inspector. See this useful infographic:How to prepare a

quality inspection checklist.

2. Dont focus on final inspections

Final random inspectionsare a good tool for approving all aspects of production (totalquantity, product specs, aesthetics, packaging). But they tend to put a lot pressure on

the supplier: what happens if serious non-conformities are found at that time? It is too

late. The risks for a factory that gets caught are pretty high: re-work of the goods, re-production, penalties, air freight, or even order cancellation

Instead of sending inspectors at the end (i.e. using them as policemen), try to send them

when the goods are in process. Issues can get caught and corrected early: this is not onlyan extra safety for the buyer, but also a helping hand for the factory. This is how youshould frame the discussion when you tell your suppliers about your QC intentions.

Early inspections (during production) have several positive side effects. They are a way

to ensure that production is taking place in the right factory. Samples can be picked up

randomly for lab testing. And it can prevent long shipment delays if the factory correctscourse immediately after quality issues are noticed.
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Dont get me wrong, Chinese suppliers will not welcome this idea warmly. Many of them

see QC inspectors as a nuisance. I have seen long-time suppliers of an importer (more

than five years) getting used to inspections But they would never admit that it is anecessity. Which leads me to the third step.

3. Inspections are not an option

You should be careful about the signals you send to your suppliers. Small things can go a

long way:

You should write Quality inspection required prior to shipment on your P/Os.

If youpay by letter of credit, you can require a passed inspection report from your

nominated QC provider.

When you develop new products, ask extra samples for the inspectors use.

Keep track of the final inspection date andthe shipment date, not just the

shipment date.

All this is quite standard, and thousands of importers follow these tips.

You still have the freedom not to book an inspection for a given shipment, or to do skip-lot inspections for the most reliable suppliers. But you are the one to take this decision,

not your suppliers. They should see inspectors as an extension of your organization. On

the other hand, you should make sure you work with professionals who will be respectedby factories.

4. Find the right balance between helping and arm-twisting

A buyer can play it tough, be easy on his suppliers, or find the right balance in

between.

The tough way: a focus on final inspections performed rigidly.

Suppliers have no choice: either they comply with the rules, or they are chargedpenalties

and/or re-inspection fees. Charge-backs are triggered by late changes in planning or non-

respect of quantity requirements, for example. The fees are charged by the inspection

firm to the importer, who re-invoices everything to the supplier.

It works well for large buyers who are adequately organized and who have the power tocharge penalties systematically. But small-and-medium-sized importers can seldom play

this game.

The easy way: in-line inspections and/or tailored final inspections.

As noted above, inspections during production dont create much adversarial tension, and

there is less timing pressure.
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Once production quality has been secured, final inspections can be a little less formal.

Why? Because it is less risky to loosen requirements about the proportion ofpresented

products.

You want to perform a product inspection?

You would like your procedure to be statistically valid?

Enter your information on this page and you'll get the info you need!

1. Find how many samples to draw at random

Total quantity of products: pieces (not sets o

What inspection level do you choose?

What kind of product is it?

2. Find how many defects are acceptable

Defects are usually split into three categories:

Critical defects are a non-compliance with applicable law and/or might hurt use

Major defects would cause a consumer/user to refuse buying the product.

Minor defects are a non-respect of specifications, but some consumers would n

AQL for critical defects

AQL for major defects

AQL for minor defects

What AQL do buyers often choose?

For most consumer products: 0 for critical, 2.5 for major, and 4.0 for minor.

For more valuable items: 0 for critical, 1.0 or 1.5 for major, and 2.5 or 4.0 for m
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You want to learn more about AQL and inspection levels?

Sampling plan calculations for qualityinspections [infographic]

by Renaud Anjoran on 14 February 2011

Most inspections only involve 1 or 2 references, and can be conducted in one man-day

(i.e. one inspector on site for one day). However, in certain situations, deciding on a

sample plan is very difficultsee my previous article about shipments including manyreferences.

This is one of the most difficult things to explain to my clients. Fortunately, my designermanaged to explain it all nicely in this illustration:
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Related resources:

To see the whole statistical tables, you can go to AQL tables.

To understand how to read these tables, you can read Get familiar with AQLtables.

To calculate the sampling plan and the AQL limits, you can use ourtool togenerate sampling plans.

How to prepare your QC inspection

report template

by Renaud Anjoran on 14 August 2011

A few months back, we drew an infographic to show how a random inspection works, but

we didnt get into all the specifics. I regularly get questions from readers who ask mehow to report the results.

There are many ways to prepare a QC inspection report. But here are a few guidelines toprepare a document that suits your needs.

A report is usually composed of the following parts:

Conformity to specifications

You probably want the inspector to look at the following aspects of the products:

Components

Assembly

General outlook, colors

Labeling

Packaging & packing

It can look like this (this example counts only 1 checkpoint, but you are encouraged to

list up to 10 or 15 points in each sub-part, to be inspected during the job):

What to report?
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In my mind, the inspector should only show the non-conformities if he was trained

properly and if you trust him. But thats up to you. Some inspection firms commonly use

freelancers that they dont really know, and they require reporting (and photos) foreverything. Up to you!

On-site tests & measurements

This is the place for the checks that take some time, and that are generally performed

only on a few samples:

The tests (to be clearly described, including the equipment to use and the required

result)

The measurements of the cartons and of the products (size and weight)

Here is an example of test:

And for the measurements:

Production status

This is where the presented quantity is noted.

Here is a common way of displaying this information:
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It is extremely important to know whether the inspector could count the quantity of

products (sometimes they are piled up in bulk), and whether all products were available

for sampling (sometimes they are under packing or repairing).

If the inspection takes place during production, you can add some extra questions to ask

the factory: how many lines are working on my products, when will you get to 50%finished, etc.

Visual defects on the product and the packing

The inspector will look for defects, will place them in the right categories

(critical/major/minor), will add the numbers up, and will compare them to the AQL

limits.

This is the most basic part of every QC report, and every inspector knows how to presentthis. Make sure you get photos, and a clear description of each defect (if its not obvious

on the photos, indicate the size of each defect and its position on the product).

Appendix: photos

This is the place where you list the photos that you want to see, whether there areproblems or not.

A good tip: describe how each photo should be taken (what angle). This way, you can

compare the photos across several inspection reports. Some buyers have noticed an

evolution in the manufacturing process based on this technique.

Is it helpful?

Any comments from QC pros?

Documents

Quality Control Basic Concepts