Multivariant charting and Normalization Methods

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    Terry DeLoach

    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



    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.


    Robert Butler

      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
    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.


    Carl H

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

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