We have a production with a run rate of 1k per shift. Production manually assemble the product and have it tested electrically at test station. Before the product is send for packing, Production will 100% visually inspect for mechanical/cosmetic defect. QA inspector will do sampling buy-off electrical and mechanical/cosmetic.
The product does not come to QA in a constant rate. My question is how do I generate a correct sampling plan? Customer goal is 0.08dphu

I'd probably set up a sampling plan based on ANSI/ASQ Z1.4 since it sounds like you're doing attribute sampling. MIL-STD-105E is equivalent to that standard, and you can download it free: Free Military Standards (http://elsmar.com/pdf_files/Military%20Standards/).

Yes,

105-E is very good and there is also a good one in Juran. The important aspect, in my opinion, is that these guidelines and tables can be modified from their original content to work with any company specific situation.

The thing is not where it is from, but if it works.

Great guidelines

Al...

How to set or determine the AQL and which inspection level?

Originally posted by johnnybegood
How to set or determine the AQL and which inspection level?
From MIL-STD-105E page 2:

Use of a value of AQL for a certain defect or group of defects indicates that the sampling plan will accept the great majority of the lots or batches provided the process average level of percent defective (or defects per hundred units) in these lots or batches be no greater than the designated value of AQL.
So what you want to do is find the AQL closest to your customer's requested AQL (which is the same as DPHU) of 0.08. The options you have are 0.063 or 0.10. I'd probably play it safe and pick 0.063. You can look at the OC curves in the standard to see if it makes sense.

Normally inspection Level II is used. Other levels can be used depending on the sampling risk.

Let say my customer does not spell out the dpku goal. Now how do I set a sampling plan then? I really do not know where and how to start.


From MIL-STD-105E page 2:


So what you want to do is find the AQL closest to your customer's requested AQL (which is the same as DPHU) of 0.08. The options you have are 0.063 or 0.10. I'd probably play it safe and pick 0.063. You can look at the OC curves in the standard to see if it makes sense.

Normally inspection Level II is used. Other levels can be used depending on the sampling risk.

Food for thought;
If production is already doing the testing and visual inspection, what is the need for an additional QA inspection?

Food for thought;
If production is already doing the testing and visual inspection, what is the need for an additional QA inspection?


call it escapee.......Production may not be testing/visual inspection 100% eventhough they are suppose to do so. Visual inspection are manually human dependent thus defect may skip thru.

Food for thought;
If production is already doing the testing and visual inspection, what is the need for an additional QA inspection?

I agree with Sam, however certain customers (from a supposedly forward thinking organisation) still insist that we perform an indepent QA sample before releasing product!:bonk:

Need help on how to determine:
1) AQL
2) Producer’s risk
3) LTPD
4) Consumer’s risk


Supposing this is a new product.

Any takers on the helping with details or definitions?

I suggest these threads:
http://Elsmar.com/Forums/showthread.php?t=6037
http://Elsmar.com/Forums/showthread.php?t=5190
http://Elsmar.com/Forums/showthread.php?t=2022
and
http://Elsmar.com/Forums/showthread.php?t=1892

Mil-hdbk-53, if it is still available. Otherwise, I would suggest a trip to your local library.

Need help on how to determine:
1) AQL
2) Producer’s risk
3) LTPD
4) Consumer’s risk


Supposing this is a new product.

Johnny,

Complete answers to your questions go beyond the scope of answers from the Cove I think. Some of these questions can only be answered by your organization and the customer. What do you/they want/need? Please take the time to read MIL-STD-105 or ANSI Z1.4 and you should have many of your questions answered by doing so (definitions are in there). Then, if you have a specific question write back.

BTW -- Most electrical components/products companies I have worked with use "single normal" sampling plans, General Inspection Level II. This may or may not work for you.

johnnybegood,

A copy of MIL-STD-105 can be obtained at this link

http://stinet.dtic.mil/

CarolX

Mil-hdbk-53, if it is still available. Otherwise, I would suggest a trip to your local library.
Mil-hdbk-53 is available at CarolX's link.

Need help on how to determine:
1) AQL
2) Producer’s risk
3) LTPD
4) Consumer’s risk
Supposing this is a new product.

One very good source of information is the AQL Primer at:
www.samplingplans.com (http://www.samplingplans.com)

AQL - Acceptable Quality Level, normally contractually agreed between supplier and customer. Very helpful if the AQL is near to your process capability for defect rates.

Producers risk - The portion of batches above the OC curve where it bisects the AQL which might be rejected even though their proportion defective is within the AQL level.

LTPD - Lot Tolerance Percentage Defective, again probably contractually agreed. Set to a batch defect rate at which we want to be sure most batches will be rejected.

Consumers risk - The portion of batches below the OC curve where it bisects the LTPD which might be accepted even though their proportion defective is beyond the LTPD.

Do I need to know all the 4 information (AQL, alpha, LTPD, beta) to determine the sampling plan? I came across some example which only requires AQL and alpha. Pls. advise.

Nope, you only need the ones that apply to the table you are using.

If AQL=0.065, alpha = 0.05 refering to the chart, if my lot size is between 3201 to 10,000 the sample size required is 200. What if my lot size is 91 to 150, what would the sample size be? Is it 20 ?

Hi all
Can anybody tell me the dirrerence between MIL-STD-105E & ANSI/ASQC Z1.4-1993? I know the former was cancelled by the latter. What's the content of ANSI/ASQC Z1.4-1993? Does it only rename from the 105E for the reason of non-goverment background? What's the relationship between them?

Any inputs are appreciated!

Link

If AQL=0.065, alpha = 0.05 refering to the chart, if my lot size is between 3201 to 10,000 the sample size required is 200. What if my lot size is 91 to 150, what would the sample size be? Is it 20 ?

I don't have a copy of the std,but, If I remember correctly; Select the next sample size as indicated by the arrows in the column marked "0.065".

If AQL=0.065, alpha = 0.05 refering to the chart, if my lot size is between 3201 to 10,000 the sample size required is 200. What if my lot size is 91 to 150, what would the sample size be? Is it 20 ?

Since the arrow points down, follow the instruction "Use first sampling plan below arrow. If sample size equals, or exceeds, lot or batch size, do 100% inspection". So you would do 100% inspection for any quantity of 200 or less.

Hi Link,

From http://www.samplingplans.com/forum/_disc1sam/0000011f.htm


Re: About MIL-STD-105D, MIL-STD-105E, Z1.4
From: Stan Hilliard
Date: 30 May 2001
Time: 16:42:13


Comments
Hi Nia,

I think that Mil-Std-105E was cancelled because the US Congress passed a paperwork reduction act. Z1.4 for attributes is the same as Mil-Std-105E, except that it is no longer free. The philosophy behind it is the same. The sampling plans in it are the same.

Mil-Std-105E is same as Mil-Std-105D with an additional set of OC Curves for the case where switching rules are used. (Between normal, tightened, and reduced plans). A few words were changed -- like substituting "nonconforming" for "defective".

I think it is best to use the operating characteristic curve directly to design sampling plans rather than the cookbook approach of Z1.4. Z1.4 does not inform the casual user of the amount of protection that its plans offer. It is a "one-point" standard based on AQL. See my discussion of AQL at:

www.samplingplans.com/aqlprimer.htm

Stan Hilliard


I tink this answers your question.

:smokin:

Hi all
Can anybody tell me the dirrerence between MIL-STD-105E & ANSI/ASQC Z1.4-1993? I know the former was cancelled by the latter. What's the content of ANSI/ASQC Z1.4-1993? Does it only rename from the 105E for the reason of non-goverment background? What's the relationship between them?

Any inputs are appreciated!

Link
Here's what I found somewhere about six months ago when someone else asked me the same question. I can't remember the original source, but I found this in an email I sent.

"What are the differences between MIL-STD-105E and ANSI/ASQ-Z1.4?
ANSI/ASQ Z1.4 (1993) is nearly identical to Mil-Std-105E. There are no changes in the tables of sampling plans. The only change in the switching rules is that ANSI Z1.4 makes the use of the limit numbers for switching to reduced optional. In addition, ANSI/ASQ-Z1.4 contains additional OC curves called scheme OC curves that describe the protection provided by the switching procedure during periods of constant quality. Numerous changes where also made to the explanatory text but which do not affect any procedures."

Thank you all for the good answers.

In addition
What's the difference between mil-std-1916 and 105E?

And
Is there any MIL stardard related to part's reliability test?

TKS!

Reference to a post by Darius,
"I think that Mil-Std-105E was cancelled because the US Congress passed a paperwork reduction act. Z1.4 for attributes is the same as Mil-Std-105E, except that it is no longer free."

??? How could it be a paperwork reduction when all that is done is to shift it from one source to another?
The real reason is the three little words "no longer free".

Thank you all for the good answers.

In addition
What's the difference between mil-std-1916 and 105E?

And
Is there any MIL stardard related to part's reliability test?

TKS!
"Mil-Std-1916 Preferred Methods for Acceptance Sampling of products." Based on the process improvement methodology.

MilHdbk- 108, Sampling Procedures and Tbles for Life and Reliability Testing.

Mil-Std 781, Relibility Testing for Engineering Development, Qualification and Production.

These document should also contain other references.

Is there any MIL stardard related to part's reliability test?

TKS!

Mil-Std-781, Mil-Std-883, and/or Mil-Hdbk-217.


??? How could it be a paperwork reduction when all that is done is to shift it from one source to another?
The real reason is the three little words "no longer free".

Actually, many Mil-Specs and Federal Specs were eliminated and several converted to either commercial or other "government" documents as a part of the "Perry Initiative" (where Defense Sec. William Perry gave the directive to streamline military procurement by reducing the amount of "red tape/military-specific specifications" and "commercialized" the military procurement process).

Thank you all for the good answers.

In addition
What's the difference between mil-std-1916 and 105E?

And
Is there any MIL stardard related to part's reliability test?

TKS!

I believe 1916 focuses on SPC and AOZ (accept on zero decects) sampling plans. Below is an article from Quality Digest. I will also attach MIL-STD-1916 and its handbook if it is not too big.

Accept-on-Zero Sampling Plans
Improve Quality at the DOD


The world's
largest customer
now pushes
suppliers to
strive for
100-percent
conformance.

by Carmen Liuzza and Geza M. Pap

In 1986, the Department of Defense directed the removal of acceptable quality levels (AQLs) from all of its specifications. The implementation of this directive also involved the policy that, when lot acceptance is based on only a sample of the total lot quantity, all of the samples should be defect free for outright acceptance: That is, only "accept on zero" (AOZ) plans are to be used for attributes sampling.

AOZ plans are primarily intended for inspection of quality-type characteristics and not for establishing the product's performance. The AOZ requirement doesn't apply to destructive tests.

Several factors prompted the Defense Department to change to AOZ plans, but industry's example was clearly significant. The policy was set in an era where Harley-Davidson recaptured its eminence through process controls, Detroit fought its way back to profitability through "Quality is Job 1," and Motorola won the Baldrige Award and the bulk of the cellular phone business using total quality management and continuous improvement.

The message was quite clear: Customers have the right to expect quality. The Defense Department just happens to be the world's largest customer. After considering their dealings with industry, the department's decision makers realized that AQL-based specifications sent a message about their quality expectations that was contrary to their objectives.

Although the DOD's contracts had always specified in clear legal language that the department required all-conforming product, it immediately withdrew that "customer demand" by allowing nonconformities within the sample. In other words, the objective evidence that was supposed to prove that the lot met all requirements was permitted to include evidence to the contrary.

The notion of "permissible defect levels" was also contrary to the economics of the Defense Department's increasingly complex weapon systems. Relatively minor variances in $5 parts could cause the failure of $5 million systems. AQLs in specifications then compounded the problem with the inertia they created.

The success of an AQL-based production run is judged by the lot-acceptance rate. Both customer and vendor rest on their laurels if the lots pass. In fact, both sides became quite adept at explaining how failed lots are just statistical aberrations and should be bought on waivers. Every aspect of the AQL system favors inaction and avoidance of continuous improvement.

It became, and still is, the Defense Department's policy to do everything it can to encourage contractors to monitor, control and continuously improve their own processes; minimize variability; achieve high capability; and prevent rather than inspect for nonconformities. The Defense Department recognized that proper use of MIL-STD-105 certainly wasn't disabling these objectives; however, the department realized that it had allowed its suppliers to build an undesirable operational culture around the standard.

The new philosophy is that 100-percent conformance is the contractual and business goal, and that all processes should constantly aim for that goal. Furthermore--and this is the cornerstone concept of the Defense Department's use of AOZ plans--every nonconformance requires action. The old notion of "the lot passed with two defects, just pay the bill" had to be erased from the department's business process.

In practice, realizing that perfection is not always feasible, the Defense Department has said that it is willing to accept product lots whose quality levels are in the neighborhood of parts per million (PPM). This neighborhood is usually understood to be from 1 PPM to 100 PPM, depending on the importance of the characteristic.

This has been interpreted to mean that no nonconformances can be tolerated in any sampling plan used by the Defense Department because any nonconformance in such a sample could be considered inconsistent with the PPM quality goal.

The department is now selecting AOZ plans from MIL-STD-1916, which it developed to reflect its changed view of the business process. This standard actually starts with process control-based acceptance as the preferred method, and defines AOZ-type sampling plans as interim procedures until the vendor can move into the PPM range.

Countering AOZ misconceptions

Misconceptions abound about AOZ sampling plans, and they deserve to be heard and refuted. Keep in mind that AOZ plans don't change the statistics; they change the emphasis from throughput to prevention and encourage suppliers to strive continuously for fully conforming product. Some of the misconceptions include:

1. Zero nonconformances in a sample implies zero nonconformances in the lot.

This isn't true, because one doesn't imply the other. The purpose of requiring zero nonconformances is not to demonstrate that a lot's quality level is perfect or meets PPM goals. The purpose of AOZ sampling is either to accept product as long as no nonconformances are found, thus giving the contractor the benefit of the doubt, or to withhold acceptance of any lot when a sample contains a nonconformance, reasoning that the lot's quality is probably not at a PPM level.

2. AOZ sampling requires that the entire lot be perfect.

It might be unreasonable to expect an entire population to be flawless; however, AOZ sampling requires PPM quality levels, not perfection. The presence of a nonconforming item in a sample (especially if it's a small sample) reveals that the quality level isn't in the PPM range.

An important function of AOZ plans is to communicate to the supplier that the customer will not happily accept nonconforming product without redress. Compare this with a sampling plan of, for example, n = 100, accept-on-one: How could a supplier not get the impression that the customer would be perfectly satisfied to receive product that is 1-percent nonconforming?

Yet how many employees, during a 30-year career, would tolerate one error per 100 pay stubs regarding their social security withholding, vacation time or pension contributions? Wouldn't most employees expect errors like these to be followed by an immediate rework, corrective action and changes made to prevent their recurrence?

3. AOZ sampling plans are inferior because they are not as discriminating as non-AOZ plans.

It's true that non-AOZ plans are more likely to pass lots whose quality levels are near the AQL and more likely to fail lots whose quality levels are near the lot tolerance percent defective (LTPD). However, the AOZ plans presently used by the Defense Department shouldn't be viewed in the same way as AQL plans.

The Defense Department didn't employ AOZ plans to compete with non-AOZ plans. AOZ plans don't utilize an AQL, nor do they select by indexing the table with an AQL or an LTPD. AOZ plans simply say that, whatever sample size is chosen, no nonconformances will be allowed for outright acceptance, and the more important the characteristic is, the larger the sample should be to verify that there are virtually no nonconformances in the lot.

AOZ plans can be thought of as having associated AQLs, but this isn't the way the department uses these plans. Of course, an operating characteristic (OC) curve can be drawn for any given AOZ plan to describe the risks inherent in the plan, but the Defense Department is only secondarily interested in the attributes of an OC curve for a particular AOZ plan.

The Defense Department isn't looking for a particular appearance of the OC curve as it did when it indexed plans by their AQLs. Therefore, the department isn't looking to compare the discrimination ability of one OC curve to another, nor is it using AOZ plans to economize on sample size.

4. Use of AOZ plans for some contracts will result in excessive lot rejection.

In MIL-STD-1916, failure to pass an AOZ plan doesn't necessarily imply lot rejection, but it does mean that lot acceptance will be withheld and that the contractor will be required to take several actions listed in the standard.

Lots with nonconformances in the sample are candidates for a normal part of business, namely, negotiations between the consumer and the supplier based on waiving the contractual requirements for concessions that have balanced value to both. These negotiations, frequently based on evaluations of the subject lot regarding its "fitness for use," drive decisions to reject, screen or rework the lot. MIL-STD-1916 and MIL-HDBK-1916 describe these disposition procedures in more detail.

During negotiations either prior to signing the contract or after a lot fails to meet the AOZ plan, both the customer and the supplier should take into account the reality that nonconformances can occur for various reasons: occasional random failures do occur in a sample even when the process is operating on a low PPM level, the process may have degraded, the state of the art for this process may not be high enough, or drawing limits may be too tight for the capability of the current equipment or process.

Process control

The Defense Department's goal is for its contractors to control their processes so well, and with such a high capability, that lot acceptance via AOZ plans becomes unnecessary.

Keep in mind that the normal contract requires totally conforming product. Samples are just one form of objective evidence. Compared with the information given by other techniques, like SPC charts or error-proofing, samples make for a fairly weak set of evidence. That's why MIL-STD-1916 establishes very clearly that AOZ is not the department's final word on product acceptance.

However, AOZ plans will be the criterion for final lot acceptance as long as contractors have processes that aren't mature enough to generate better objective evidence. How long it takes to make the necessary improvements is the supplier's own choice.

When AOZ plans stand alone, higher quality levels are achieved only because of the supplier's fear of excessive lot rejection. The big picture is that process controls are needed to stabilize, monitor and improve processes.

The intent of MIL-STD-1916 and its accompanying MIL-HDBK-1916 is to give process control primary importance--AOZ plans are secondary.

About the authors

Carmen Liuzza, of U.S. Army TACOM-ARDEC at Picatinny Arsenal, has been a mathematical statistician in quality assurance for 33 years. He has recently worked as an SPC consultant and teacher. For the past two years, Liuzza has chaired an NDIA committee, which has just completed a new MIL-HDBK-1916, a guidance document to MIL-STD-1916. Among his current projects is the international standardization of sampling and SPC documents.

Geza M. Pap is the team leader for the Product Data Team of the Quality Engineering Directorate, with 30 years civil service experience in quality assurance at Pictinny Arsenal. He led the Army Materiel Command team that developed the concepts for elimination of AQLs from Army specifications. His ISO 9000 background involves contractor performance certification and Army certification process based on ISO 9001, and the co-chairmanship of the Department of Defense Working Group for implementation of the Q90 series.

Last try...

Okay -- I guess I did not wait long enough before. My fault. Sorry!

Ref to Mike's post,
"About the authors"

And they owe it all to Phillip Crosby who initiated the Zero Defects concept/methodology, despite the laughter and ridicule from his peers.

If AQL=0.065, alpha = 0.05 refering to the chart, if my lot size is between 3201 to 10,000 the sample size required is 200. What if my lot size is 91 to 150, what would the sample size be? Is it 20 ?

Try this on-line calculator:
http://iew3.technion.ac.il/sqconline/milstd105.html

I just purchased the latest mil std 105 E from ASQC, I was disapointed to see they took out the referance to variable sampling plans. I would like to use a statistical basis for choosing the number of pcs to be sampled at final
inspection based on the product's history. This is for tier 1 automotive. Might I be on the right track with mild std 114?

I'd probably set up a sampling plan based on ANSI/ASQ Z1.4 since it sounds like you're doing attribute sampling. MIL-STD-105E is equivalent to that standard, and you can download it free here (http://www.variation.com/techlib/standard.html).
Hello Folks,

I just want to confirm from the gurus this statement. I need to implement a plan today, and I already have a copy of 105E. I haven't check availability of Z1.4 for delivery into my hands.

Thanks in advance.....you folks are the best!!!

Carol

:confused: What statement are you wanting confirmed? If you mean that 105E is equivalent to ANSI/ASQ Z1.4, here's a link that explains the differences:
http://www.variation.com/FAQs.html#AS4
What are the differences between MIL-STD-105E and ANSI/ASQ-Z1.4?
ANSI/ASQ Z1.4 (1993) is nearly identical to Mil-Std-105E. There are no changes in the tables of sampling plans. The only change in the switching rules is that ANSI Z1.4 makes the use of the limit numbers for switching to reduced optional. In addition, ANSI/ASQ-Z1.4 contains additional OC curves called scheme OC curves that describe the protection provided by the switching procedure during periods of constant quality. Numerous changes where also made to the explanatory text but which do not affect any procedures.

Thanks Howste...just what I needed to know...Your the best!!! Carol

Hello Folks,

I just want to confirm from the gurus this statement. I need to implement a plan today, and I already have a copy of Mil Std 105E. I haven't check availability of Z1.4 for delivery into my hands.

Thanks in advance.....you folks are the best!!!

Carol
Carol:

It is the same. See the attached CANCELLATION notice (105N1.pdf).

I know this is an old question, but I wanted to add a few comments.

My question is how do I generate a correct sampling plan? Customer goal is 0.08dphu

I can think of at least three ways to interpet this goal, and this interpretation has a big affect on the sampling you do:
1) The customer will accept no more than 0.08 dphu and will reject most lots that are any worse than 0.08 dphu.
2) The customer will accept an average of 0.08 dphu and will lots as long as the average doesn't get above about 0.08 dphu.
3) The customer will accept 0.08 dphu and will accept most lots that are no worse than 0.08 dphu.

The sampling plan used by a customer would vary greatly depending on which situations was the case. The required quality required from the producer also varies widely.
In case 1, you might need to aim for 0.04 dphu in order to avoid getting too many rejected lots back from the customer.
In case 2, you would aim for 0.08 (or a little less) dphu in order to avoid getting too many rejected lots back from the customer.
In case 3, you might get away with 0.12 dphu and still have most of the lots accepted.

The ASQ Z1.4 (MIL-STD-105E) tables are generally set up for situation 3. I.e, when you sample, you will accept most (say 90% to 99%) of lots with the specified AQL.

As a producer doing sampling, you need to decide your goal. Do you want to ensure that nearly all lots meet or accede the customer requirement? Or do you simple want to ensure that the customer's sampling plan won't reject too many shipments?


Do I need to know all the 4 information (AQL, alpha, LTPD, beta) to determine the sampling plan? I came across some example which only requires AQL and alpha. Pls. advise.

Yes and no. With the ASQ Z1.4 tables, you only need to specify AQL, because the four values are assumed to have reasonable values if you follow the plan. This upside is that you don't need to worry about the details, because the authors of the plan did that for you.

The downside is that you can't easily know what the actual numbers are. For example, you might think that the tables would be set up so that the alpha values would be something like tightened = alpha = 0.1, normal = 0.05, reduced = 0.01, but that isn't the case. There is a considerable variation in alpha depending on the AQL and the lot size. There is even less uniformity in the LTPD & beta. (Personally, I'd like to see these tables scrapped and replaced by a "standard" piece of software. You would input the AQL, LTPD, alpha, & beta, and it would output the sampling plan.

If you are motivated, you can look at the tables & OC curves at the back of the book to determine what the specific values are for each plan.

If you are really motivated (or buy the right software) you can specify all four values and generate your own plan to exactly meet your specs.


Tim F

Hello all,
I've found the following information should it be of interest to anyone,
2. ANSI/ASQ Z1.4-2003 and ANSI/ASQ Z1.9-2003
ANSI/ASQ Z1.4-2003 (Sampling Procedures and Tables for Inspection by Attributes) and ANSI/ASQ Z1.9-2003 (Sampling Procedures and Tables for Inspection by Variables for Percent Nonconforming) have been published.

ANSI/ASQ Z1.4-2003 is an acceptance sampling system to be used with switching rules on a continuing stream of lots for a specified Acceptance Quality Limit .It provides tightened, normal, and reduced plans to be applied for attributes inspection for percent nonconforming or nonconformities per 100 units.

ANSI/ASQ Z1.4-2003 replaces ANSI/ASQC Z1.4-1993 and is available through ASQ for a member price of $88.00. Go to: http://qualitypress.asq.org/perl/catalog.cgi?item=T004 .

ANSI/ASQ Z1.9-2003 is an acceptance sampling system to be used on a continuing stream of lots for a specified Acceptance Quality Limit. It provides tightened, normal, and reduced plans to be used on measurements which are normally distributed. Variation may be measured by sample standard deviation, sample range, or known standard deviation. It is applicable only when the normality of the measurements is assured.

ANSI/ASQ Z1.9-2003 replaces ANSI/ASQC Z1.9-1993 and is available through ASQ for a member price of $76.00. Go to: http://qualitypress.asq.org/perl/catalog.cgi?item=T009 .


also the following sites may be of some help
http://www.ma.mbe.doe.gov/a-76/FINALQASP31403.pdf pages 10 to 14
http://www.gisdevelopment.net/proceedings/gita/2001/dfd/techgi010b.shtml
http://www.tesco-advent.com/pdf/Statistical-electric.pdf pages 14 to 19

Can someone do the math fast:
I am inspecting lots of 1200 parts, using an AQL of 1.00 at c=0, how does that relate to Six Sigma? To reverse this:
If I want my supplier to be at 3 or 4 Sigma level, what AQL should I use?

Six Sigma really can't be inspected in. Is your lot size 1200 or sample size 1200?

If your sample size is 1200 (not lot size), and you accept on 0 you will accept lots that are 0.004% defective 95% of the time (AQL) and you will reject lots that are 0.192% defective 90% of the time (LTPD). Your probability of accepting a 3 sigma quality lot is 0.000%. Your proobability of accepting a 4 sigma quality lot is 0.057%. Your probability of accepting a 5 sigma quality lot is 75.606%. And your probability of accepting a 6 sigma quality lot is 99.593%.

If your lot size is 1200 and you are shooting for 3 sigma to 4 sigma quality a sampling plan of n=57, c=1 would work. That would set your LTPD to 66800 dPPM (3 sigma) and AQL to 6210 dPPM (4 sigma). If you want to see what your OC Curve would look like you can plot these numbers in a scatter diagram:
Pd(x) Pa(y)
0.1% 99.8%
0.5% 96.7%
1.0% 88.9%
1.5% 78.9%
2.0% 68.4%
2.5% 58.1%
3.0% 48.7%
3.5% 40.3%
4.0% 32.9%
4.5% 26.7%
5.0% 21.5%
5.5% 17.2%
6.0% 13.6%
6.5% 10.8%
7.0% 8.5%
7.5% 6.6%
8.0% 5.1%
8.5% 4.0%
9.0% 3.1%
9.5% 2.4%
10.0% 1.8%

Assuming I did my math correctly. :o

I see that this post was started in 2001 but it brings up some inportant points:

1. How much/many defects can your customer stand? Really a basic question but has implications, namely that a certain customer base will stand a lot more defects where as others Military/automotive/aerospace/nuclear...etc can stand very little if at all. Based on what your customer base requires then you need to determine your risks and costs of rejected a good lot. There is more in this than I can go into here...but I think you get the idea.

2. And this most important to consider - At any AQL or AOQL or LTPD and with proper sampling, you are guaranteeing that you will send out bad product. This is built into the any plan. A sample plan is based on probability as one poster has shown mathmatically. Can you and your customer afford to have ANY reject? If not then you need to make your process robust or relatively impervious to outside influence, next, insure that the raw material coming in is defect free (not imposible - except for rogue units, etc). Of course, you also will have some rogue product but once discovered can be anticipated (usually they occure in cycles).

And last (for this post) is that we (quality professionals) are in the middle of a 6-sigma quality cycle where we talk about DPPM as less then one and Ppk's greater then 2 (accounting for long term shift of 1.5 sigma).

So how much risk can you and you customer stand?

Along with this does anyone have a DOC or XLS form they would be willing to share as an example/useful tool for those of us trying to better establish this system in our own facilities? By the way thanks for the many helpful link this was a very useful thread for me.