Reducing the Frequency of a Production Line Test

Q

Qualityfan34

#1
Can anyone advice me what will be the best approach in order to determine if the frecuency of a test can be reduce?

Background: i have a test that is performed hourly but i pull out some data (about 3 month data) and in that period of time it has not been reported any failure. Based on this, i understand that an improvement on the frecuency can be perform but i want to use a proper approach in order to provide the info.

Thanks in advance,
 
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P

pearsonow

#3
Re: Reducing the frequency of a test

How much variation is there within the data - are the operators doing something to adjust the data and prevent bad parts?

If you are confident in the process, then you can justify a reduction in sample frequency to save time and money. If you have the authority to make this decision then simply document it in line with your Quality System and do it.
 
C

chemohd

#4
Re: Reducing the frequency of a test

perhap you can consider taking 1 year data & analyse it. what is the trend, variation, then your have strong evidence that you process is controllable.

hope it will help.
 
Q

Qualityfan34

#5
Re: Reducing the frequency of a test

The CPK is 5.31 and the StDev is 3.21. The operator are limited to perform the test only. With this data, are you guys think is enough to provide in a report (just attaching the CPK Graph)?
 
S

Sturmkind

#6
Re: Reducing the frequency of a test

I would think it would depend if the test is customer mandated. Also, I agree with chemohd that the data should be verified for a state of statistical control. If there are no trends, stratification, or other out-of-control conditions you may well be justified in cutting back on the frequency.
 
Q

Qualityfan34

#7
Re: Reducing the frequency of a test

Hi Sturmkind, thanks for your feedback. The test is not required by the customer, is just internal control. I have verified the trends (YTD) and no issues have been reported. At the moment i have the following data:

Complaints trend
NCR trends
CPK
STDEV

The question now is, am i missing any statistical fact that needs to be included in order to have a complete report to be provided to quality management?
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#8
Things I take into account are:

what is the severity of the failure resulting from a defective part? A very severe failure might require continued high frequency because we can't afford a miss should it happen. Remember that statistical control today doesn't guarantee control tomorrow...


What are the positive controls in place to maintain a stable and capable process in the future?

Are there other detection points prior to the Customer receiving a defective part?

As others have eluded to, a "Cpk graph" is probably not hte best choice. A run chart (all individual values if you use subgroups of n>1) vs spec limits is a better display of all of the data as shifts, trends and cycles are visible.
 
Q

Qualityfan34

#9
Hi Bev D, very appreciate for your valuable opinion. The severity of a failure of the product is major because the product performance will be compromised. However, the occurrence is very low and i don't want to eliminate the test, my suggestion is reducing the frecuency lets say from taking 10 samples every hour to 10 samples every two or three hours.

On the other hand, i'm agree with you that a run chart will make more sense instead of a CPK graph.

Hence, what additional approach you can recommend me to include on this evaluation?
 
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