#	Thursday  Sept 29, 2002

#	Chapter 2

eg.1 = function(n1=100,n2=100) {

   set.seed(123); par(err=-1); xn = rnorm(5000)
   xy1 = matrix(xn[1:(2*n1)],n1,2)
   xy2 = matrix(4+xn[1001:(1000+2*n2)],n2,2); xl = range(xn,xn+4)
   plot(xy1,pch="0",xlim=xl,ylim=xl,main=paste("sample sizes",n1,n2))
     points(xy2[,1],xy2[,2],pch="1",col=3)

   y = c(rep(0,n1),rep(1,n2)); X = rbind(xy1,xy2)
   b = lsfit(X,y)$coef
      myab = function(c,b,x,y,ch){ abline((c-b[1])/b[3] ,-b[2]/b[3]); text(x,y,ch,cex=2) }
   myab(0,b,-2,3,"yhat=0"); myab(.5,b,0,5.5,"yhat=1/2"); myab(1,b,2.75,6.5,"yhat=1")
}

eg.1(); eg.1(500,100); eg.1(1000,100)  # usa()