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MathGroup Archive 2004

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how to graphically fill the tails of a normal distribution

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50012] how to graphically fill the tails of a normal distribution
  • From: Todd Allen <genesplicer28 at yahoo.com>
  • Date: Wed, 11 Aug 2004 05:53:11 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

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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