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Help on Treatment of Points Outside of Control Limits

P

pagnonig

#1
Dear all,

I would like to share with you a maybe trivial doubt about the treatment of points outside control limits.

Let's consider the following situation: an I-mR chart is drawn from some individual point data (50 values).
I would like to know if the process is stable and if I can extend the control limits in time to be used on the shop-floor by the operator who is manually filling the measurement results on a handwritten control chart.

I proceed with the analysis of the first 50 values taken from the current process:

Step 1 - check the mR chart

I check the moving range chart for points outside the upper control limit.
If one or more points are below this level, a special cause investigation is triggered.
Let's say the special cause is understood and removed. Here comes the first question.

Question N.1: Do I have to remove the point from the moving range average calculation and compute the new average and then the new limits for both the mR and the Individual chart? If yes, do I have to repeat Step 1 until a stable mR chart is achieved and all special causes are addressed?


Step 2 - Check the Individual chart

I check the Individual chart for points outside the upper control limit.
If one or more points are below this level, a special cause investigation is triggered.
Let's say the special cause is understood and removed... which of the following choice is more appropriate:


Choice N.1: Recalculate all the moving ranges without considering the Individual value out of the control limits on the I chart. Compute the new mR average and the new limits for both the mR chart and the I chart. Look for new points outside control limits on both the mR chart and the I chart and repeat steps 1 and 2.



Choice N.2: Recalculate the Individual average without considering the point outside control limits, therefore expecting only a shift in the average.


Step 3 - Extending Control limits

Once special causes are removed, I would consider the new control limits of both I chart and mR chart as the reference for the shopfloor documentation used by the operators (maybe collecting further 50 data in order to confirm the stability of the process and the validity of the new limits). They would manually record additional data from the process and check for stability using the given limits.

Question N.2: Does it make sense to ask the shopfloor operator to record both the I value and the mR value, therefore filling both the I and the mR control chart? Or in your experience would you recommend to use only the individual chart?


...I think that the same approach could be adopted for Xbar-R charts.

Thanks for any comment on this.
Kind regards.

Giuseppe
 
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Stijloor

Staff member
Super Moderator
#2
Dear all,

I would like to share with you a maybe trivial doubt about the treatment of points outside control limits.

Let's consider the following situation: an I-mR chart is drawn from some individual point data (50 values).
I would like to know if the process is stable and if I can extend the control limits in time to be used on the shop-floor by the operator who is manually filling the measurement results on a handwritten control chart.

I proceed with the analysis of the first 50 values taken from the current process:

Step 1 - check the mR chart

I check the moving range chart for points outside the upper control limit.
If one or more points are below this level, a special cause investigation is triggered.
Let's say the special cause is understood and removed. Here comes the first question.

Question N.1: Do I have to remove the point from the moving range average calculation and compute the new average and then the new limits for both the mR and the Individual chart? If yes, do I have to repeat Step 1 until a stable mR chart is achieved and all special causes are addressed?


Step 2 - Check the Individual chart

I check the Individual chart for points outside the upper control limit.
If one or more points are below this level, a special cause investigation is triggered.
Let's say the special cause is understood and removed... which of the following choice is more appropriate:


Choice N.1: Recalculate all the moving ranges without considering the Individual value out of the control limits on the I chart. Compute the new mR average and the new limits for both the mR chart and the I chart. Look for new points outside control limits on both the mR chart and the I chart and repeat steps 1 and 2.



Choice N.2: Recalculate the Individual average without considering the point outside control limits, therefore expecting only a shift in the average.


Step 3 - Extending Control limits

Once special causes are removed, I would consider the new control limits of both I chart and mR chart as the reference for the shopfloor documentation used by the operators (maybe collecting further 50 data in order to confirm the stability of the process and the validity of the new limits). They would manually record additional data from the process and check for stability using the given limits.

Question N.2: Does it make sense to ask the shopfloor operator to record both the I value and the mR value, therefore filling both the I and the mR control chart? Or in your experience would you recommend to use only the individual chart?

...I think that the same approach could be adopted for Xbar-R charts.

Thanks for any comment on this.
Kind regards.

Giuseppe
Does anyone have comments for Giuseppe?

Thank you!

Stijloor.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#3
Let's say the special cause is understood and removed. Here comes the first question.

Question N.1: Do I have to remove the point from the moving range average calculation and compute the new average and then the new limits for both the mR and the Individual chart? If yes, do I have to repeat Step 1 until a stable mR chart is achieved and all special causes are addressed?
If this is the capability run, and you can identify and remove the special cause, then, yes, calculate the control limits without it. The control limits should describe the expected data for a process in control. I do not believe it would be necessary to repeat, unless you feel the data about the out of control condition was unduly affected by the special cause.

Step 2 - Check the Individual chart...
Same applies here

Question N.2: Does it make sense to ask the shop floor operator to record both the I value and the mR value, therefore filling both the I and the mR control chart? Or in your experience would you recommend to use only the individual chart?
Both - each holds its own value. To detect control you need to know "location" (individual value) and "spread" (range) - either can be a clue to impending special cause - and they may show it independently, depending on the cause.
 
P

pagnonig

#4
Thanks Bobdoering for the answer!

About I chart. With "the same applies here" you mean I should recalculate both mR and I charts limits excluding out of control points detected in the I chart, therefore applying my Choice N.1. Isn't it?
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
Thanks Bobdoering for the answer!

About I chart. With "the same applies here" you mean I should recalculate both mR and I charts limits excluding out of control points detected in the I chart, therefore applying my Choice N.1. Isn't it?
Yes. Control limits from data of the process in control. If you can't remove the cause, then you have to keep them in...they are a part of your process.
 
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