From: Richard Chandler <rchandler_at_forwild.umass.edu>

Date: Sat 21 Jan 2006 - 09:30:18 EST

Date: Sat 21 Jan 2006 - 09:30:18 EST

Thanks for the reply though I don't think your suggestion worked. I
have found a way to get the correct line though it is not
convenient. Here is a better example:

x <- 1:100

y <- 1:100

plot(y ~ x, log="x")

#The only way I can get the correct line is to drop the log():
abline(lm(y ~ x), untf=T, lwd=2) #or

lines(x, predict(lm(y ~ x)), col=2)

#Neither of these work

abline(lm(y ~ log10(x))) #or

abline(lm(y ~ log10(x)), untf=T)

What I really would like to do is plot fitted lines and 95% confidence intervals using predict.lm, as in shown in the example, but when the predictor is log transformed and log="x". I can't figure out how to do this without removing the log() from the response part of the formula and this isn't helpful because I'm generally trying to give predict() a fitted object rather than a lm() formula. I still think I'm probably missing something simple but are there any other suggestions? Thanks.

Richard

Quoting Peter Dalgaard <p.dalgaard@biostat.ku.dk>:

> Richard Chandler <rchandler@forwild.umass.edu> writes:

*>
**> > Hello,
**> >
**> > I'm trying to plot a fitted lm() line on a plot when the one
**> > explanatory variable is log transformed and log="x". I get
**> different
**> > lines using abline and predict.lm().
**> >
**> > #Example
**> > x <- 1:100
**> > y <- rnorm(100)
**> > plot(y ~ x, log="x")
**> > abline(lm(y ~ log(x)))
**> > lines(x, predict(lm(y ~ log(x))), lwd=2)
**> >
**> > I'm sure I'm missing something but could someone tell me which
**> line is
**> > correct? Thanks.
**>
**> Base 10 is what you're missing.
**>
**> The latter form is agnostic with respect to base, the former is
**> not
**> (since the fitted values are the same, but regression coefficients
**> differ). So you need to know to use abline(lm(y ~ log10(x))).
**>
**> You don't really notice which kind of log is being used until you
**> look
**> at par(usr) for a plot with logged axes.
**>
**> --
**> O__ ---- Peter Dalgaard ุster Farimagsgade 5,
**> Entr.B
**> c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
**> (*) \(*) -- University of Copenhagen Denmark Ph: (+45)
**> 35327918
**> ~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk) FAX: (+45)
**> 35327907
**>
*

-- Richard Chandler, M.S. candidate Department of Natural Resources Conservation UMass Amherst (413)545-1237 ----- End forwarded message ----- -- Richard Chandler, M.S. candidate Department of Natural Resources Conservation UMass Amherst (413)545-1237 ______________________________________________ R-help@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.htmlReceived on Sat Jan 21 09:40:25 2006

*
This archive was generated by hypermail 2.1.8
: Sat 21 Jan 2006 - 14:11:27 EST
*