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Reaction to Out of Control SPC Charts

beaser3

Involved In Discussions
#1
Hello everyone,

I am hoping someone can help answer a question regarding out of control SPC charts. We have multiple charts that have acceptable CPK levels and the histograms show a tight groupig around mean. There will be 1 or 2 points that fall outside of the control limits on the average chart making the chart "out of control". My question is should we really be reacting to these points or should we accept the process as stable? Is it fair "in a cusotmers eyes" not to react to every point? I've attached one chart as an example. Any input would be greatly appreciated.
 

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Craig H.

#2
Hi, beaser!

Looking at your data, I have to ask if this is a process where you have, say, 3 cavities in a mold, and sample 1 is from cavity 1, sample 2 from cavity 2, etc.? A quick look at the data makes me think that you may have 3 different, distinct populations here. If so, you should be tracking the 3 independently.
 

Jim Wynne

Staff member
Admin
#3
Hello everyone,

I am hoping someone can help answer a question regarding out of control SPC charts. We have multiple charts that have acceptable CPK levels and the histograms show a tight groupig around mean. There will be 1 or 2 points that fall outside of the control limits on the average chart making the chart "out of control". My question is should we really be reacting to these points or should we accept the process as stable? Is it fair "in a cusotmers eyes" not to react to every point? I've attached one chart as an example. Any input would be greatly appreciated.
You can't "accept the process as stable" so long as statistical stability hasn't been achieved (nor should you be calculating Cpk). There are single measurements in the 1st, 8th and 15th subgroups that are causing trouble; you need to find out why. Perhaps it's measurement error. There is also a run of seven points below the mean at the beginning of the range chart ends with the anomalous eighth subgroup mentioned above; it appears that something happened between subgroups 7 and 8 to cause it. On the range chart, 16 of 25 points are below the mean, which is also odd.

If you're going to do statistical analysis, you have to listen to what the numbers are trying to tell you, and investigate. You have the luxury of having everything on the subject chart being within the tolerance range, but there's stuff going on that begs for investigation and correction. If you're just going to wave it off and move on, there's no reason to be doing SPC to begin with.
 

beaser3

Involved In Discussions
#4
The 3 samples that you see are all from the same cavity. This is a 2 cavity mold and each cavity is set up seperately. The samples are pulled consecutively.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
you are correct to question the validity of out of control points when everything is in spec. However, the answer is not to ignore it. The answer in this case appears to be either measurement error or a poor choice for subgrouping. 3 sequential pieces off of a single cavity are likely to be 'exactly' the same. however as time passes (different presses, different setups, different raw material lots, utility fluctuations. etc.) you can get sample to sample variation that is normal adn in your case small enough to not matter to making bad parts.

I would recommend that you investigate the sources of variation as indicated by the chart. Then (most probably) if you find that these sources are naturally occuring change your subgoup scheme to encompass parts that aren't consecutive but are representative of these changes.

research rational subgrouping to help determine how best to do this.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#6
Hello everyone,

I am hoping someone can help answer a question regarding out of control SPC charts. We have multiple charts that have acceptable CPK levels and the histograms show a tight groupig around mean. There will be 1 or 2 points that fall outside of the control limits on the average chart making the chart "out of control". My question is should we really be reacting to these points or should we accept the process as stable? Is it fair "in a cusotmers eyes" not to react to every point? I've attached one chart as an example. Any input would be greatly appreciated.
If you aren't going to act upon the results of the SPC - why are you doing SPC in the first place?

As has already been pointed out, Cpk has no meaning if the SPC isn't stable and predictable. Best recommendation is to start investigating the signals you are seeing, before the customer starts demanding you fix things. Note - I'm not saying you have to do a full shut-down, but there should be some investigation and understanding of what is happening, and appropriate corrective actions taken.

I'd say it is unfair to the customer to not deal with out of control points.
 
R

ralphsulser

#7
I used to see similar results due to shift change "machine molestation"
It didn't matter that the product had been running fine all day, when the 2nd shift operator came in he just had to make his own adjustments.
We had one case where we had to put a cover on the controls and padlock it shut.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#8
I used to see similar results due to shift change "machine molestation"
It didn't matter that the product had been running fine all day, when the 2nd shift operator came in he just had to make his own adjustments.
We had one case where we had to put a cover on the controls and padlock it shut.
I've also heard the story of a company who created a dial on the machine that was connected to nothing, just to allow people to "set their own adjustments". Apparently arguments broke out between shifts as to the best setting for the dial . . .
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#9
If you aren't going to act upon the results of the SPC - why are you doing SPC in the first place?

As has already been pointed out, Cpk has no meaning if the SPC isn't stable and predictable. Best recommendation is to start investigating the signals you are seeing, before the customer starts demanding you fix things. Note - I'm not saying you have to do a full shut-down, but there should be some investigation and understanding of what is happening, and appropriate corrective actions taken.

I'd say it is unfair to the customer to not deal with out of control points.
I agree with both you and Jim that out of control points should never be ignored. But looking at the histogram clearly indicates that this process isn't close to producing out of specs parts. So the temptation to ignore is great and understandable. The key is what to look for. teh molestation comment is a great one. and my experience with this type of process is that his subgrouping scheme is also a great candidate for the source of variation - not that his process is out of control.
 
Q

qualityboi

#10
We have SPC reaction mechanisms built into our SPC system. If a data point goes out of control or out of spec an engineer or tech gets page automatically and that person reviews the data and attaches a comment to the data point, even if it just states "appears to be an outlyer, will monitor the next lot". It at least shows evidence that someone reviewed the data real time. During customer audits we get complimented on this procedure. Once in a great while it actually contributes in mitigating a larger issue.
 
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