Linear Regression plot with normal curves for error (sideways)


Vinh Nguyen


May 12, 2009

[caption id="attachment\_755" align="aligncenter" width="480" caption="Linear Regression: Normal Errors"][/caption]

I remember my first encounter to the classical linear regression model with normally distributed errors by Dan Gillen in Stat 120c in spring 2005 at UCI. I mean, I've done linear regression in high school science classes: you get a best fit line out of your data. In the stat class, I remember seeing data drawn on the board and a line going through the data. To illustrate the normally distributed errors, he drew sideway normal curves.

I liked this presentation of the classical model. How do convey this in R? I couldn't find a package for it (searched for an hour I think). I always search for packages because I always think others could do a better job at it than me and because I don't like to re-invent things, even though it requires less time for me to do it myself in many cases. Well, here it is, written by me in R:

sideways.dnorm <- function(where.x, where.y, values=seq(-4,4,.1), magnify=4, ...){
 ###... consists of mean, sd, and log passed to dnorm function
 dens <- dnorm(x=values, ...)
 x <- where.x + dens * magnify
 y <- where.y + values

temp <- sideways.dnorm(where.x=3, where.y=3, values=seq(-2,2,.1))

n <- 200
x <- runif(n, -8,8)
y <- 1 + .5*x + rnorm(n, sd=1)
fit <- lm(y~x)
plot(x,y, xlim=c(-10,10), ylim=c(-8, 10)
, main="Classical Linear Regression\nwith Gaussian errors"
##, main=expression(paste("Classical Linear Regression\nwith N(0,", sigma\^2, ") errors", sep=''))
, col="red"
abline(fit, lwd=3, col="blue")
where.normal.x <- c(-4,0,4)
for(i in 1:length(where.normal.x)){
 where.x <- where.normal.x[i]
 where.y <- predict(fit, newdata=data.frame(x=where.x))
 xy <- sideways.dnorm(where.x=where.x, where.y=where.y, magnify=4)
 abline(h=where.y, lty=2)
 abline(v=where.x, lty=2)