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The graphic was created by User:Schutz for Wikipedia on 15 August 2007, using the R statistical project. The program that generated and exported the graphic is given below; the postscript file was converted to SVG with the pstoedit command and the layout was slightly modified using Inkscape before upload.

The example is taken from w:en:Simpson's paradox#Description of the paradox.

 success1 <- c( 60,  1)
 total1   <- c(100, 10)
 success2 <- c(  9, 30)
 total2   <- c( 10,100)
 
 ratios1 <- success1/total1
 ratios2 <- success2/total2
 
 postscript("simpson.eps", paper="special", height=4, width=6, horizontal=F)
 
 par(mfrow=c(2,1))
 par(las=1)
 par(yaxs="i")
 par(xpd=NA)
 par(cex=1.3)
 
 par(mar=c(2.1, 2.6, 0.7, 0.6))
 plot.new()
 plot.window(xlim=range(ratios1, ratios2)+c(-1,1)*0.05*diff(range(ratios1,ratios2)),
            ylim=c(0,max(total1, total2)))
 
 width  <- (par("usr")[2]-par("usr")[1])/15
 height <- (par("usr")[4]-par("usr")[3])/5
 
 rect( ratios1[1]-width/2, 0, ratios1[1]+width/2, total1[1], col="red")
 rect( ratios1[2]-width/2, 0, ratios1[2]+width/2, total1[2], col="blue")
 segments ( min(ratios1)-width/2, 0, max(ratios1)+width/2, 0, lwd=2)
 ratiototal1 <- sum(success1)/sum(total1)
 
 polygon(c(ratiototal1, ratiototal1-width/2, ratiototal1+width/2),
         c(0, -height, -height), col="black")
 
 axis(2)
 axis(1, at=c(ratios1))
 
 plot.new()
 plot.window(xlim=range(ratios1, ratios2)+c(-1,1)*0.05*diff(range(ratios1,ratios2)),
             ylim=c(0,max(total1, total2)))
 
 rect( ratios2[1]-width/2, 0, ratios2[1]+width/2, total2[1], col="red")
 rect( ratios2[2]-width/2, 0, ratios2[2]+width/2, total2[2], col="blue")
 segments ( min(ratios2)-width/2, 0, max(ratios2)+width/2, 0, lwd=2)
 
 ratiototal2 <- sum(success2)/sum(total2)
 
 polygon(c(ratiototal2, ratiototal2-width/2, ratiototal2+width/2),
         c(0, -height, -height), col="black")
 
 axis(2)
 axis(1, at=c(ratios2))
 
 dev.off()

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