[R] Regression lines

[Roger Bivand]

On Fri, 12 Jan 2007, Tom Backer Johnsen wrote:

> My simpleminded understanding of simple regression is that when 
> plotting regression lines for x on y and y on x in the same plot, the 
> lines should cross each other at the respective means.  But, given the 
> R function below, abline (lm(y~x)) works fine, but abline (lm(x~y)) 
> does not.  Why?
> 
> function () {
> attach (attitude)
> x <- rating
> y <- learning
> detach (attitude)
> plot (x, y)
> abline(v=mean(x))
> abline(h=mean(y))
> abline (lm(y~x))
> abline (lm(x~y))
> }

The axes are getting reversed:

xylm <- lm(x~y)
newdata <- data.frame(y=0:80)
lines(predict(xylm, newdata), newdata$y, col="blue")

gets them back.

Roger


> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no