Multivariant charting and Normalization Methods

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• #30097

Terry DeLoach
Member

Novice Needs Help
I want to chart several different variables for a product or products in a chemical process on a single SPC chart. I read an article several years ago (1970 or so) about “normalizing” data to targets so that variable of different ranges can be charted on an SPC chart. Can any one help with 2-3 ways that this is typically done?

kind regards
Terry DeLoach

0
#78042

Ron
Member

Using Shewhart control charts precludes the need for normal data sets as proven time and time again ( ref Wheeler / Chambers Understanding Statistical Process Control  SPC Press).
There are quite a few folks out there that do not understand control charts and in many cases the blind lead the blind.
My favorite and the most powerful is the XBar and R chart. Go get that data and plot it.

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#78103

Robert Butler
Participant

If you have a program that will permit graph overlays or one that will permit you to plot multiple Y’s against a given X then you should be able to do Multivariate plotting.
I would recommend taking each response of interest and normalizing them to their respective +-3sigma limits.  If we call the -3sigma limit Ymin and the +3sigma limit Ymax then to normalize each Y to a -1 to 1 range you would compute the following:
y(norm) = (y-Yavg)/Ydiff
where Yavg = (Ymax+Ymin)/2
and
Ydiff = (Ymax – Ymin)/2
overlayed plots of the averages and ranges of the normalized Y’s will give you your multivariate plots.
The only problem I can see with graphs of this sort is that for more than 3 Y’s or so the chart will run the risk of being so busy that its utility as an analytical tool may be diminished.

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#78104

Carl H
Participant

To automate this, try using a Z-MR chart in MINITAB or other software that offers it.
Carl

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