Adjusted Control Limits - Usage of Control Charts

G

Ghemzie

#1
Hi SPC experts!

I have problem with the usage of control charts, control limits seems to strict. Is there a way, computation or factor that can be added to adjust the control limits? Sometimes it seems action is not necessary on the detection of outside points in control limits specially if there is no specification requirement.

Hope to hear any idea.

Thanks
 
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harry

Super Moderator
#2
Re: Adjusted Control Limits

Discussions of this nature can be made much easier if there is a set of common data. If you do, please attached it here - our regular Covers can quickly go through and then give you relevant advice.
 
Last edited:

Jim Wynne

Staff member
Admin
#3
Hi SPC experts!

I have problem with the usage of control charts, control limits seems to strict. Is there a way, computation or factor that can be added to adjust the control limits? Sometimes it seems action is not necessary on the detection of outside points in control limits specially if there is no specification requirement.

Hope to hear any idea.

Thanks
What's the process? What type of chart? What type of sampling/subgroups? You could be working with something other than a normal distribution, which cause the type of thing you're seeing.
 

Miner

Forum Moderator
Staff member
Admin
#4
In addition to the previous comments, I have seen this caused by improper sub-grouping and inadequate gage resolution.
 
G

Ghemzie

#5
We used to monitor the frequency characteristics of a certain electronics part by Xbar R charts, there is no specification requirements for this, but the control chart shows some points out of the control limits.

We thought that since there is no specification given, then actions is not necessary. But we need to stablish some sort of limit that will trigger us for an action, not too strict as that of control limits.

We're thinking of adding let say 2standev from the control limit as "trigger" limit, or is there any possible way?

Thanks in advance
 

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Miner

Forum Moderator
Staff member
Admin
#6
The gage resolution appears to be fine. This is probably a sub-grouping issue, so I have a few questions:

  1. What was the basis for choosing a sub-group size of 10? The control limits are tighter than those for a sub-group of 5.
  2. What do these 10 parts represent in the process? When the range of the subgroups consists entirely of measurement variation, and there is virtually zero product variation in the subgroup, the control limits will be too tight for the process. Your distribution of averages appears to be random variation with no apparent special causes.
  3. What was the sub-group sampling frequency? What can change on a regular basis in the process between subgroups.
 

Jim Wynne

Staff member
Admin
#7
The gage resolution appears to be fine. This is probably a sub-grouping issue, so I have a few questions:

  1. What was the basis for choosing a sub-group size of 10? The control limits are tighter than those for a sub-group of 5.
  2. What do these 10 parts represent in the process? When the range of the subgroups consists entirely of measurement variation, and there is virtually zero product variation in the subgroup, the control limits will be too tight for the process. Your distribution of averages appears to be random variation with no apparent special causes.
  3. What was the sub-group sampling frequency? What can change on a regular basis in the process between subgroups.
The charts have the familiar look of ex post facto data.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
  1. What do these 10 parts represent in the process? When the range of the subgroups consists entirely of measurement variation, and there is virtually zero product variation in the subgroup, the control limits will be too tight for the process. Your distribution of averages appears to be random variation with no apparent special causes.
Good point, this looks suspiciously similar to the error one gets on web processes when they calculate the range across the web rather than along the web (not that it is what they are measuring - just a similar effect)
 
G

Ghemzie

#10
Thank you for all your responses.
Sorry but i can not get a clear answer to my question. Is there a way to adjust the control limit or any assumptions that can be applied to loosen the control limit?

Let say for example i have calculated UCL of 40.5 and std dev of 0.3, to adjust the limit i will add 40.5 + 3*0.3 to have 41.4 new limits.

:(
 
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