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MathGroup Archive 2004

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RE: how to graphically fill the tails of a normal distribution

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50022] RE: [mg50012] how to graphically fill the tails of a normal distribution
  • From: "David Park" <djmp at earthlink.net>
  • Date: Thu, 12 Aug 2004 05:43:28 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Todd,

Needs["Statistics`ContinuousDistributions`"]
Needs["Graphics`FilledPlot`"]
Needs["Graphics`Graphics`"]
Needs["Graphics`Colors`"]

npdf[x_] = PDF[NormalDistribution[0, 1], x];

DisplayTogether[
    FilledPlot[{0, npdf[x]}, {x, -4, -2}, Fills -> Red],
    FilledPlot[{0, npdf[x]}, {x, 2, 4}, Fills -> Red],
    FilledPlot[npdf[x], {x, -2, 2}, Fills -> LightBlue],
    Epilog -> {Line[{{-2, 0}, {-2, npdf[-2]}}],
        Line[{{2, 0}, {2, npdf[2]}}]},
    PlotRange -> All,
    Frame -> True,
    Axes -> None,
    PlotLabel -> "Normal Distribution with Filled Tails",
    Background -> Linen,
    ImageSize -> 450];

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/


From: Todd Allen [mailto:genesplicer28 at yahoo.com]
To: mathgroup at smc.vnet.net


Hello all,

      Hope you don't mind a question from a biologist trying to learn
Mathematica.  I have been teaching myself the syntax of Mathematica to
analyze microarray data (i.e, a technology to simultaneously monitor the
expression levels of thousands of genes).  I am interested in producing a
plot for a presentation in which the tails of a normal distribution are
colored red to illustrate a concept.  I can get the plot to work with a
single tail, but I can't get the syntax to work for both tails of the
disribution.  Here is my code:

This works for a single tail, with the following code (necessary packages
already loaded):
ndist=NormalDistribution[0,1]

npdf=PDF[ndist,x]

FilledPlot[{npdf,If[x³2,0,npdf]},{x,-4,4},Fills®RGBColor[1,0,0]]

However when I try the following with the Or operator (at least I think Or
operator):

FilledPlot[{npdf,If[x³2.0þx£-2.0,0,npdf]},{x,-4,4},Fills®RGBColor[1,0,0]]

I get a series of error messages:

Plot:plnr:If[x³2.0þx£-2.0,0,npdf] is not a machine-size real number at x=-4

This error repeats itself with different x values and then a normal curve is
drawn with neither tail colored in.



What am I missing here?

Any insight is much appreciated, thanks!
Todd



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