# Need help in quality control

Six Sigma – iSixSigma Forums Old Forums General Need help in quality control

Viewing 7 posts - 1 through 7 (of 7 total)
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• #31508

Parameswaran
Participant

Here is the problem:We had given a contract to a vendor to make CDROM sets (1 set consists of 2 CDs) of our training module. He delivered 7000 sets (total 14,000 CDs). These came in small boxes of 25 sets (50 CDs); eight such boxes are placed inside a bigger box (total 400 CDs). Therefore in all there are 35 large boxes and 35 X 8 = 280 small boxes. When we started to examine the CDs we found the several CDs did not work. Each CD takes 2hrs and a set takes 4 hrs to examine for errors and the errors appear at random. So it is impossible to verify each CD or set for errors. How should I determine a method to reject a lot of either the small box or large box whichever is more appropriate? Should I go in for a simple percentage of bad CDs out os small sample (how large should that be?) and extrapolate to the entire lot.Please remember that even a sample size of 100 sets (200 CDs) would take 400 hrs for verification. Also I concede that there is no way I can segregate good and bad CDs and I can only refuse payment for rejects and ask for free replacement whenever a bad CD is detected.Other details: I have no idea of population mean or std deviation of bad CDs.Please  help ASAPKrish

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#83118

Michael Schlueter
Participant

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#83132

Mstewart
Participant

Send them all back and let your supplier deal with it !!!!

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#83144

Participant

This might seem simplistic, but I would recommend returning to manufacturer to do this whole evaluation.

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#83149

KT
Participant

Krish,
Theoritically speaking, it is an excellent problem that you are facing.  Practically speaking, I could imagine the nightmare!  You acutally have a number of solutions paths.  My first suggestion is for you and whomever is helping you with this problem, to sit down and put together a decision tree.  You can start by the two extereme decisions:  Accept as is OR Reject the whole batch.  You can then perform the cost/benefit (rough calculations) for each decision.  One of the respondents gave you a very good model to follow.  In reality you are asking yourself: “what is the risk with this solution path.”
You can also consider the inbetween decisions; i.e., as you suggested, perform some “quality” checks yourself.  For each decision, you will have the cost, resources, etc – then you can make the appropriate decision.  One suggestion:  look at your contract – you might be stuck with a certain decision due to verbage in your contract.  That is, your contract might be written such that you may only return the “defective” CDs and not reject the batch.  Using the constraints you have (contract) and the potential risk/benefit, make your decision.
Now in regards to your sampling for “defects” in the CDs.  I don’t know how much you know about your supplier’s process;  mainly, is there a logical sequence to their packaging (first CDs in the first boxes, etc.).  That might help with your sampling plan.  If the answer is no, then you might want to consider the “Acceptance Sampling” method.  In the acceptance sampling, you will accept or reject the lot based on limited observations.  What you can not get is the actual quality level of the lot.  It is fast, but as anything that is quick, it provides you with little knowledge about your product.  Again, this works if you can reject the lot and send it back to the supplier.
The other method is to perform sampling to determine the quality level of your product with a certain confidence interval.  There have been numerous postings on calculating the confidence intervals.
I wish you success.

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#83174

John H.
Participant

Krish
As a first step, you should narrow the scope of the problem and the vendor must work with you on this. Some sample questions: Is there any marking traceability(CD Lot code, dates, times) that traces the CD’s packaging etc.. to the vendor’s production runs, disk burners, operators..?Was the Master ok or was it damaged?What type of Quality Specifications were listed in the contract and did the vendor follow Best Industry Practices? What is the acceptable defect level in the industry? Is there a better* way of checking the CD’s ? A superior supplier could guarantee CD critical defects in the PPM range and a rejection based on what you found would be obvious in this case.
In theory,I can appreciate the benefits of a risk cost benefits analysis. However, in the real world, elegant risk reduction mathematical models can often fail with disasterous financial consequences(example: Derivatives Trading). In lieu of the bad press that American Industry is getting with repect to financial reporting honesty, I don’t think that it is a good idea to pass off a quality problem to a customer because it is profitable to do so.
This is a difficult problem.
Good Luck
John H.
*Perhaps the NIST, DOD or other Government Technical Agency could assist with this.

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#83177

Dr. Steve W.
Participant

of defective rate (1%, 5% etc.) is acceptable first. With that established, you can compare it with your confidence interval for the proportion of defetive parts based on the out come of your test. The confident level dictates how many you need to sample. This is a traditional inspection issue and any well trained statistician can help you.

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