Control Charts - Points TOUCHING Limits = Stable?

M

misguided_will

#1
hiall

i have a control chart that has data points that TOUCH but do not breach the control limits.

by touching, not breacing.. is the process considered stable or unstable?

thanks
 
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Miner

Forum Moderator
Staff member
Admin
#3
Re: Control Chart: points TOUCHING limits = stable?

It is important to remember that Shewhart developed the control chart for the economic control of processes. He recommended 3 standard deviation limits as the best economic balance between false alarms (a signal that the process is out of control, but is not) and failure to alarm when the process is actually out-of-control.

Therefore, it is legitimate for you to define this economically in your procedures. What action should you economically take to this point. Ignore it or react? Define it accordingly in your procedures.

Note: Use the same judgement on the extended rules (trends, runs, etc.). Each rule has an individual error rate (false alarm rate) of approximately 0.3%-0.4%. The more rules that you use the greater the overall false alarm rate. If you use all of the extended rules, the overall false alarm rate will be approximately 3%. That is, 3 of every 100 subgroups will be out-of-control by random chance alone.

I recommend that you decide which extended rules add value and use only those rules. Base this on your process. If your process does not trend out of control, or drift slowly out-of-control creating runs, do not use these rules. Again, this is an economic decision.
 

Stijloor

Staff member
Super Moderator
#4
Re: Control Chart: points TOUCHING limits = stable?

It is important to remember that Shewhart developed the control chart for the economic control of processes. He recommended 3 standard deviation limits as the best economic balance between false alarms (a signal that the process is out of control, but is not) and failure to alarm when the process is actually out-of-control.

Therefore, it is legitimate for you to define this economically in your procedures. What action should you economically take to this point. Ignore it or react? Define it accordingly in your procedures.

Note: Use the same judgement on the extended rules (trends, runs, etc.). Each rule has an individual error rate (false alarm rate) of approximately 0.3%-0.4%. The more rules that you use the greater the overall false alarm rate. If you use all of the extended rules, the overall false alarm rate will be approximately 3%. That is, 3 of every 100 subgroups will be out-of-control by random chance alone.

I recommend that you decide which extended rules add value and use only those rules. Base this on your process. If your process does not trend out of control, or drift slowly out-of-control creating runs, do not use these rules. Again, this is an economic decision.
Miner,

Excellent response! :applause:

Keep in mind that the OP claims to be a student.

sorry if i'm a little vague...totally newbie..just a colleage student.
He may not in a position to make "economically-based decisions" about when to adjust the process or when to leave it alone.

He, in this case, should consider various hypothetical decision-making scenarios...

FWIW, just my :2cents:

Stijloor.
 

Miner

Forum Moderator
Staff member
Admin
#5
Re: Control Chart: points TOUCHING limits = stable?

I am guessing that this was from a post in a different thread as I don't see it in this thread, but good catch.

In that case, I would treat this similar to the criteria for a hypothesis test where the null hypothesis is not rejected unless p < alpha. By this reasoning, the null hypothesis is that the process is in control and you do not reject the null unless p < alpha, so a point must be beyond the limit to reject the null.
 

Stijloor

Staff member
Super Moderator
#6
Re: Control Chart: points TOUCHING limits = stable?

I am guessing that this was from a post in a different thread as I don't see it in this thread, but good catch.
That's right. I read his other posts. He is working on a "home work" assignment.

In that case, I would treat this similar to the criteria for a hypothesis test where the null hypothesis is not rejected unless p < alpha. By this reasoning, the null hypothesis is that the process is in control and you do not reject the null unless p < alpha, so a point must be beyond the limit to reject the null.
Stijloor.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#7
Re: Control Chart: points TOUCHING limits = stable?

I've settled on the interpretation that the point must be outside of the limit to count. Generally in a variables chart it would be rare that a point would exactly be on a limit. On a c-chart, the most common problem is when the average works out to be a perfect square, such as 1.0, 4.0, 9.0; and the three, two, and one standard deviation lines are whole numbers.
 
P

pagnonig

#8
Re: Control Chart: points TOUCHING limits = stable?

I've settled on the interpretation that the point must be outside of the limit to count. Generally in a variables chart it would be rare that a point would exactly be on a limit. On a c-chart, the most common problem is when the average works out to be a perfect square, such as 1.0, 4.0, 9.0; and the three, two, and one standard deviation lines are whole numbers.
I have a variable sample size p-chart where 2 points are exactly equal to the respective UCL.

In order to discriminate about noise ans signal I've decided to plot a normalized Z chart and both points are above +3s limit... so, maybe some special cause here...

Do you think this can be a good approach?
Regards.
 
G

Geoff Withnell

#9
Re: Control Chart: points TOUCHING limits = stable?

I have a variable sample size p-chart where 2 points are exactly equal to the respective UCL.

In order to discriminate about noise ans signal I've decided to plot a normalized Z chart and both points are above +3s limit... so, maybe some special cause here...

Do you think this can be a good approach?
Regards.
The way i would look at it is that points at the limit should, if the process is stable, be fairly infrequent. If you have two in a short period of time, it seems worth investigating.

Geoff Withnell
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
Re: Control Chart: points TOUCHING limits = stable?

I have a variable sample size p-chart where 2 points are exactly equal to the respective UCL.
Two things:
1. Is there a rate at which the process approached the control limit, or did it jump there? Do any of the Western Electric rules apply?

2. I guess my biggest question is, what if it is out of control? What is you response to the out of control action? If it is that close, is it effective to begin the response now?

Are you overthinking this? It depends....:cool:
 
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