SPC Sigmas

This topic contains 4 replies, has 2 voices, and was last updated by  John Wick 1 year ago.

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• #55931

John Wick
Participant

So im reviewing the SPC rules (Nelson rules) and Im trying to figure it out how can i calculate the sigmas (1,2,3) can someone help ? Can you share the formula please.

Please see attached document so you can understand what im talking about

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#202212

Chris Seider
Participant

Have you searched this site or used google? Lots of calculations out there for limits and as long as you can divide by 3….should answer your question.

https://www.moresteam.com/toolbox/statistical-process-control-spc.cfm

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#202214

John Wick
Participant

I know how to calculate the UCL and LCL,

so if im getting this correctly

lets say i have an
X (average); 7.967
USL;9.3518
LSL ; 6.5822

so…

USL – Xbar 3 ? thats how i get every sigma?
9.3518-7.976 = 1.3848
then divided by 3 is = 0.4616

then 1σ is = 8.4286
2σ = 8.8902
3σ = 9.3518

Am i right?

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#202215

Chris Seider
Participant

You mean UCL and LCL? if so….yes but follow the formulas to calculate the limits.

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#202216

John Wick
Participant

yeah sorry UCL and LCL
Correct! , yeah the UCL and LCL were calculated with formulas.

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