[R] Regression lines

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Jan 12 11:01:47 CET 2007

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?

Where did you tell it 'x' was the abscissa and 'y' the ordinate?
(Nowhere: R is lacking a mind_read() function!)  From the help page:

   reg is a regression object with a coef method. If this returns a vector
   of   length 1 then the value is taken to be the slope of a line
   through the origin, otherwise, the first 2 values are taken to be
   the intercept and slope.

There are some changes in R-devel, but not to recognize names of 

> 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))
> }
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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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