# Re: [R] abline() or predict.lm() when log="x"

From: Richard Chandler <rchandler_at_forwild.umass.edu>
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 Øster 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

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