R help - Jan 2007 - Regression lines

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))
}

Try this version of your function and then think about it

tst <- function () {
attach (attitude)
x <- rating
y <- learning
detach (attitude)
plot (x, y)
abline(v=mean(x))
abline(h=mean(y))
abline (lm(y~x))
cc <- coef(lm(x ~ y))
abline (-cc[1]/cc[2], 1/cc[2])
}

> 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))
> }
	[[alternative text/enriched version deleted]]

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

> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
-- 
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

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 
coefficients.

> 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))
> }
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
-- 
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

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?

Well, abline() in fact plots a line using the estimated coefficients for 
intercept and slope and assumes you have plotted LeftHandSideOfFormula 
against RightHandSideOfFormula.
If you did vice versa, abline() does not respect your mistake.

Uwe Ligges



> 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))
> }
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

This should do the trick:

mind_reader <- function() {
	ll <- letters[round(runif(6, 1, 26))]
	ff <- ll[1]
	for (ix in 2:length(ll)) {
		ff <- paste(ff, ll[ix], sep = "")
		}
	if (exists(ff)) {
		cat("The function that you were thinking of is")
		return(ff)
		} else
		{ cat("please update libopenmind to the patched
			or development version")
			}
}


Prof Brian Ripley wrote:
> R is lacking a mind_read() function!)
	[[alternative text/enriched version deleted]]