Graphing Sigma Lines - DOE / MiniTab

smryan

Perspective.
#1
Hello!:D

I am in the midst of a great DOE / MiniTab class. Proff can't answer this, can anyone here? We've just done some R charts & C charts showing the Shewhart rule violations (cool!). Since these are related to the 1,2,3 sigmas values would there be some quick easy way to add the sigma lines to the graphs?

Thank you in advance!:bigwave:
 
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Statistical Steven

Statistician
Staff member
Super Moderator
#2
Re: Graphing sigma lines

You can use a "Zone Chart" for continuous data. Not sure why you want the 1S and 2S limits on your chart though.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
Re: Graphing sigma lines

just calculate the difference between the average (center line) and the UCL (or LCL if not zero) and divide by 3. this is the distance between the average and the + "1sigma" line and the 1 sigma line and the + 2 sigma line etc.

I'm a JMP user but I think Minitab is similar in that you can add lines if you just know what the number is...similar to adding a specification or goal line.

:topic:
Please use these lines only as intended: as zones for the application of all of the various control rules. eg 2 out of 3 points in zone 3 or beyond. (zone 3 is the area bewteen the "2 sigma line and Control Limit(3 sigma line).

Please also understand that these so-called sigma limits are only approximations they are NOT exact limits...nor were they intended to be. SPC is about being generally correct not exactly correct.

Sometimes the instructors for these software programs are more theoretical than practical and may never have actually used the tools they teach in real life. They just know the theory and how to 'do it' in minitab...

it bothers me that your instructor didn't know how to do this very simple thing?
 
A

Allattar

#4
Re: Graphing sigma lines

In a control chart all you need to do is go to the control chart options from the dialogue box.

Say a C-chart the button in the C-chart dialogue is labled C-chart options.
Select the tab at the top saying S-Limits. Then where it says multiples of the standard deviation put 1 2 3.

You control chart now has the 1 2 3 sigma lines on it.
 

smryan

Perspective.
#5
Re: Graphing sigma lines

just calculate the difference between the average (center line) and the UCL (or LCL if not zero) and divide by 3. this is the distance between the average and the + "1sigma" line and the 1 sigma line and the + 2 sigma line etc.
...

it bothers me that your instructor didn't know how to do this very simple thing?


Thanks - but I do know how to calculate it, and how to use it. The question was specifically how to get minitab to display it (answered by Allatar nicely, :thanks:). As for the instructor - it was a DOE class and he was wonderful. If he has never wanted to put the s limits on his graphs (as apparently Statistical Steven has also not) there is no reason he would have that "how to" at his fingertips. I am a visual person and I think the s limits are useful and informative on these graphs. I am happy to know the "how to" now.

Thanks all!:D
 
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