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

From: Richard Chandler <rchandler_at_forwild.umass.edu>
Date: Sun 22 Jan 2006 - 09:28:27 EST

Please ignore my last message, I've realized that Peter's first reply was all I needed...thanks.

Richard

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

> Sorry that was a typo when I said 'resposnse'... I meant predictor.
> I
> want to fit lm(y ~ log(x)) and plot the line with confidence
> intervals on a log="x" plot so that I can see the real units of x
> rather than the log(x) units. I can't get the real line using
> predict.lm() without removing the log() from the formula. Thanks
> again.
>
> Quoting Peter Dalgaard <p.dalgaard@biostat.ku.dk>:
>
> > Richard Chandler <rchandler@forwild.umass.edu> writes:
> >
> > > 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.
> > >
> > > 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.
> > >
> >
> > First decide what you really want. I see log() hopping all over
> > the
> > place. Is it on the response or the predictor? Do you want a
> > straight
> > line on an x-logged plot or an x-logged plot of a straight line?
> > Do
> > you intend to fit y~x or y~log(x) ?
> >
> >
> >
> > > 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
> > >
> >
> > --
> > 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

-- 
Richard Chandler, M.S. candidate
Department of Natural Resources Conservation
UMass Amherst
(413)545-1237

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Received on Sun Jan 22 09:50:57 2006

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