When to create a new Control Chart? Quick question

K

kwok007

#1
Hi All,

I am a newbie to this forum and a beginner to SPC too.

Recently I am working on a very basic project plotting xbar-r chart on some production measurements. Based on the data from the 1st week, I was able to find the out-of-control (OOC) points and the root cause (human, machine, material, measurement, environment...). After adjusting the process, I received the data during the 2nd week.

My question is: should I plot 2 weeks data together (after removing the OOC point from the 1st week) to find the new control limits or I should plot the data from 2nd week separately from the 1st week's data?

I tend to prefer the latter, since I think something has been adjusted to change the process (maybe improved the process) from the beginning of 2nd week.

If so, what if the data in 2nd week are in control, then what should I do to the 3rd week's data? will the chart from 2nd week serve purely as a monitor to 3rd week's data as a Phase II chart? or I should calculate the control limits based on the 3rd week's data again?

Thanks very much in advance!

Kwok007
 
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Steve Prevette

Deming Disciple
Staff member
Super Moderator
#2
Re: Quick question: When to create a new control chart

Sounds like you are off to a good start. If you believe you have removed the source of the out of control points for the first week, yes, try a set of average and control limits for the first week without the out of control points. I don't know how much data you have, but if the first week was a dozen or less points, bring in the second week and see how it looks. If this brings you close to 25 points, then use this baseline as a prediction for future performance, and to tell if future changes have an effect on the process. You don't want to keep recalculating the control limits, or they in effect become moving averages and you won't be able to detect if the process is really changing, or just has random noise.

Now, if you do generate a significant improvement, then you can calculate a new set of average and control limits following that shift. Just remember something I was told - a baseline is innocent until proven guilty (that is - when a trend signal develops).
 
K

kwok007

#3
Re: Quick question: When to create a new control chart

Hi Steve,

Thanks for your help very much. It helped a lot. I read of other posts of yours too. They are great resources for this forum. I am glad that I found elsmar.com:)

So basically whenever I adjusted something in a process, I need to revised the control limits, right?

Another question: what can I do if the process mean is still off target after the process was adjusted a couple of times?

Thanks again for your help!
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#4
Re: Quick question: When to create a new control chart

So basically whenever I adjusted something in a process, I need to revised the control limits, right?
No, wrong. You would only revise the control limits IF the process results prove that the adjustment changed the process data. Not every adjustment is effective, and sometimes there is a time delay from when you believe the adjustment was made until it has an effect.

Another question: what can I do if the process mean is still off target after the process was adjusted a couple of times?

Keep on adjusting. Learn what you can from previous adjustments. Did they nudge the mean towards target? Do more of that. Did they get worse? Don't do that. Was there no change? Why not?

Keep in mind that if you can make changes on a small scale (such as if you have five processing lines, change only one) and usually the response is faster, and the risk is less if the change makes things worse.
 
K

kwok007

#5
Re: Quick question: When to create a new control chart

Got it! Thanks Steve for your help very much!
 
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