Example of the quantitative version of w:Simpson's paradox.

The graphic was created by User:Schutz for Wikipedia on 14 June 2007, using the R statistical project. The program that generated and exported the graphic is given below; the uploaded version was generated slightly differently to workaround a bug during the PS to SVG conversion with pstoedit command. The layout was slightly modified using Inkscape before upload.

 
 x1 <- c(1,2,3,4)
 y1 <- x1 + 5
 
 x2 <- x1 + 7
 y2 <- x2 - 7
 
 x <- c(x1,x2)
 y <- c(y1,y2)
 
 postscript("simpson.eps", paper="special", width=4.5, height=3)
 
 par(las=1)
 par(mar=c(3,3,0.5,0.5))
 par(mgp=c(2,1,0))
 
 plot(x,y, cex=2, pch=21,
 col=rep(c("blue", "red"), each=4), bg=rep(c("lightblue", "pink"), each=4),
 xlim=range(x)+c(-2,2), ylim=range(y)+c(-2,2))
 abline(lm(y1 ~ x1), col="blue", lwd=2)
 abline(lm(y2 ~ x2), col="red", lwd=2)
 abline(lm(y ~ x), lwd=2, lty=2)
 
 dev.off()