Example that highlights issues with AQL testing.
AQL Chart | “The Ultimate Guide” has been produced by InTouch Quality, a leading Quality Control, Factory Audit, Product Inspection, Sourcing, Supply Chain Management, Pre-shipment Inspection, and Industrial Equipment Inspection Company based out of Shenzhen, China and . Chapter 1: Why QC Professionals Use AQL Sampling for Product Inspection. Professionals in the quality control field have been using AQL as the basis for acceptance sampling for decades.
An OC curve describes the probability of acceptance for various values of incoming quality. P a is the probability that the number of defectives in the sample is equal to or less than the acceptance number for the sampling plan. The hyper-geometric, binomial, and Poisson distributions describe the probability of acceptance for various situations. The Poisson distribution is the easiest to use when calculating probabilities.
The Poisson distribution can often be used as an approximation for the other distributions. The probability of exactly x defects [ P x ] in n samples is. Someone who is not familiar with the operating characteristic curves of AQL would probably think that passage of this AQL 0. Well this is not exactly true because from this operating curve OC it can be seen that the failure rate would have to be actually about 2.
AQL sampling often leads to activities that are associated with attempts to test quality into a product. AQL sampling can reject lots that are a result of common-cause process variability. When a process output is examined as AQL lots and a lot is rejected because of common-cause variability, customer quality does not improve. From this plan we can see how AQL sampling protects the producer. The failure rate at the acceptance number is 7. Usually a sample size is considered small relative to the population size if the sample is less than ten percent of the population size.
In this case, the population size is 75 and the sample size is 13; i. To make matters worse, these large sample sizes would be needed for each test lot. With AQL testing, sampling provides a decision-making process as to whether a lot is satisfactory or not relative to a specification; however, this is often a very difficult, if not impossible, task to accomplish.
When one is confronted with the desire to answer a question that is not realistically achievable, we should first step back to determine whether we are attempting to answer the wrong or at least not the best question. Sometimes we might be wasting much resource attempting to answer the wrong question with much accuracy; e. When we are examining an AQL sample lot of parts, population statements are being made about each lot.
However, in most situations, a lot could be considered a time series sample of parts produced from the process. With this type of thinking, our sampling can lead to a statement about the process that produces the lots of parts.
With this approach, our sample size is effectively larger since we would be including data in our decision-making process from previous sampled lots. Scoping the situation using this frame of reference has another advantage. If a process non-conformance rate is unsatisfactory, the statement is made about the process, not about an individual lot.
What would be my sampling quantity and acceptance number for each shipment? Sure Jon, thanks for your comment. You should not be looking at the lot: The sample size should be pcs. Please be specific on your explanation of choosing AQL level. We manufacture rivets at 50, per lot. The company has always inspected 50 parts per lot for final inspection and I feel it is not enough.
But still, I do not understand exactly how to choose the acceptable quality level and would like a detailed explanation. Any help will be much appreciated. Hi There Austri — Is the item that the end user receives a set of 6 pieces? You should not base the sample on boxes. You should base it on units. During inspection, found 2 pieces of minor defects on 1 box. Should we consider 2 or 1 defects? I am required to determine the Sample size and the accept and rejection levels in each case using the sampling plans Table I -sample size code letters and Table II-A Single sampl.
You are then required to answer the following questions: The AQL level needs to be determined by the product type. For example we inspect party favors at very loose levels, but also medical devices that inspected at very strict levels. There is no general level. Regarding your point about the critical sampling size, this does not make sense to me. There should be no reason why you would need to sample at such a high level.
In general, almost all consumer products the critical level is applied at 0, so if there is one critical defect the shipment fails. I find typical AQL levels are. Is there a rule of thumb that. It would be a much more feasible sample size. I am having a tough time understanding your question. If an inspection is performed at Level I AQL and there are several defects on not just one item, but several of the items various defects , do you combine the total with the inspection lot amount and get your percentage of how much it failed at?
Or do you total up all the same defects say, chip paint and get your total percentage from the Level I inspection lot? Hi Ashley, Thanks for your comment. I am not exactly sure what you are looking at but I will try to explain. The AQL levels go from 0 through 6. I will be happy to explain further if this does not answer your question.
If one is using an AQL of or above, the arrows indicate that the acceptance number is greater than the sample size. Could someone clarify that for me please? If you want to reduce the defects to below 2. That said, not every defect is noticed by or returned by a consumer.
So in most cases you can have a 2. This depends on how picky your customers are! Dear Andrew, Thank you for your reply. It is very helpful. What influence do they have on the goods acceptance? Thank you very much. We are looking to reduce our returns rates down to 2.
Can you advise what AQL level I should use? FYI, we only have very small sample sizes to inspect and the bulk of returns are on furniture… Anyone help? That means the pcs of one style is its own inspection. Generally, this is how we handle garments. I would like to ask 2 questions: Please note we are facing customer complains on our shipments.
When we reveiw QC isnpection report then the shipment was accepted as it just met the acceptable limit. Now we have decided to perform double AQL on our shipments. In this regard I would like to know the exact procedure of performing double AQL. Also it will be really nice if you can advice any other way in which we can make our inspection better. Inspection and AQL level can be difficult to choose.
Cost is of course always the issue, but it also depends on the AIMS of inspections will destruction of products involved for inspection purpose? Would you want to improve the quality in general?
Back to your concern regarding budget, to maximise your bargaining power, think long-term cooperation with only 1 to 2 compan y ies. This can also help you to build solid relationship with your servcie provider s and to ensure thorough understanding of your requirements — even for the same product, what matters to you as a defect may not bother another buyer. Except the Manday cost, do not forget other possible costs , such as OPE out-of-pocket expenses.
This can vary greatly depending on the servcie provider you choose and the location of your supplier. Hi Mike — good question. The best way to go about this if you are a buyer and want to have a better idea about how your inspection cost is calculated is to ask your 3rd party QC company how many pieces they can check per man-day. Inspection costs are generally calculated based on man-day, so this should give you the info you need to be clear about the costs.
In regards to choosing the right inspection level — this also has to be correlated to the total cost of the inspection service. Is there a way for the client to know how much money difference it would cost between each inspection level?
My idea is in the quotations can clients choose which inspection level based on their budget, risk tolerance for rejects, and other variables that vary from client to client?
Your email address will not be published. How Quality Control Chec The Risks of Relying on Do You Really Need to Pe Are you getting the best Sign up for China Manufacturing Updates! No Comments Feb 26, Christopher Law September 17, Matt December 14, Could you please help me understand DPHU. Andrew October 30, Frank Rizzo October 30, Tom October 4, Hi James, thank you very much!!
James September 17, For example, if you find that your orders consistently fail inspection due to minor defects, but you find the quality acceptable enough to release these shipments, you may consider on of the following actions: Tom September 15, James August 27, Burhan August 25, Hi , i am working in garments factory as a production manager , i want to improve my knowlede in quality so please send me the details of AQL levels system and what is 2.
Pete August 15, Hi Andrew, I am looking for a list of typical A Critical , B Major and C Minor defects for components such as plastic bottles, glass bottles, caps, pumps and labels.
Bruce Curry August 10, Gergely July 24, David July 18, Hi Gergely, It seems that you are inspecting major defects at 0. Gergely July 17, Hi, My question is that when we use different levels of AQL testing for different failure types, how do I treat the failures?
David June 28, Hi Doug, Your sample size should be 13 units. Doug Stricklin June 27, I have a lot size of 37, parts. I am required to do normal single inspection to S-1 AQL 1. Andrew June 20, Hi John, There is no chart that indicates this specifically. Another way to investigate this is open up the stitching and check the root cause.
John June 20, Andrew June 19, Hi John, We just count it as one; the most severe one. John June 19, Andrew June 16, Hi Chris, Good question. Chris June 15, My question is how do you best employ the AQL system to these two different standards? Steve June 14, Jenny June 6, Then when do we accept or reject a lot. David May 18, Dodo May 17, This blog is so informative. My company is selling bookends.
We have outsourced the production to our supplier. Thank you for your time. David May 11, Hi PNG, Thank you for your questions. PNG May 10, This option is usually preferable because both the pool and spa water will be filtered and sanitized. Aqua Rite s that is installed in the system. Refer to the diagram below for typical relay wiring. VSP pump control and be on when the output is on. When the output is off, the relay will be off.
Note that when the relay is off power off to the VSP , the Aqua Plus will not display errors or diagnostics for the pump. See diagram on page 10 for the location of valve connectors. Heater Control The Aqua Plus provides a set of low voltage dry contacts that can be connected to most gas heaters or heat pumps with Tighten nut 24V control circuits. Refer to the diagram below for a generic connection. Connect 1 to 1, 2 to 2, etc. The wires to the Aqua Plus must be separated from any line voltage wires.
Failure to follow these instructions 4. Page of 18 Go. Aqua Plus Programming Flow Chart.
Workmanship, Finish, and Appearance B
Jon August 25, If five or more references are cited, type them in a separate list of references at the end of the manuscript, following annexes and appendixes, if any.