# Control Limits

Six Sigma – iSixSigma Forums Old Forums General Control Limits

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• #47880

Mike Archer
Participant

Hello – I am in search for a good explanation on why control limits are always 3 sigma.  I have read that anything outside of 3 sigma is considered to be special cause variation.  Is that a correct statement?
Thanks,
Mike

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#160163

annon
Participant

Special cause is suggested in CC whenever a single point falls outside the CL or where multiple points follow a nonrandom pattern within the CL. The patterns depend on the type of chart (ie data) you are using and can results in a more sensitive interpretation of special cause.
As for why the 3 sigma limits are used, it is my understanding that it goes to the underlying assumption of normality and the corresponding Empirical Rule ( ie probability theory) , where app. 99.73% of your data will fall within plus or minus 3 sigmas of the mean.  Thus, you would expect only about 3 points out of 1000 to occur outside the control limits naturally, making the chart a highly reliable instrument (if done right) for determining where to ¨debug¨ a process of special cause.
Make sure you are using the right chart the right way…they can be a little tricky…also look into run charting when working with a small number of individual values.
And double check my explanation.  It has been awhile.
Good luck.

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#160178

Savage
Participant

“Why control limits are always 3 sigma?” Because setting the control limits at this distance works well in minimizing that chance for you to:
1. Looking for something when you should not be looking
and
2. Not looking for something when you should be looking.

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#160192

Jim Shelor
Participant

Mike,
Shewhart and Deming originally chose + or – 3 sigma for the control limits based on these limits being the most economical way to control a process.
Regards
Jim Shelor

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