From ripley@stats.ox.ac.uk  Sat Jan 12 07:49:39 2002
From: ripley@stats.ox.ac.uk (ripley@stats.ox.ac.uk)
Date: Sat, 12 Jan 2002 08:49:39 +0100 (MET)
Subject: [Rd] Bug in predict(newdata=x) with poly() (PR#1258)
Message-ID: <200201120749.IAA29489@pubhealth.ku.dk>

This is long-term known in S: there have been examples in V&R for years.
The S solution is predict.gam.

predict.poly is easy. The problem (as for bs and ns) is getting it used by
predict.lm.  Thomas Yee (I think) has made a solution available, but the
consensus was that it could be done more elegantly at a lower level.  We
already preserve information on contrasts and sets of levels: we need to
preserve similar information for poly() and spline terms.

I noticed this was still open last week, and put it on my (private) TODO
list.

On Sat, 12 Jan 2002 murdoch@stats.uwo.ca wrote:

> Bug in predict.lm & poly
>
> The predict function doesn't work when used with poly and newdata.
>
> For example, I'd expect the following code to work, and plot a fitted
> cubic to the nearly straight line:
>
>  x <- 1:10
>  y <- x + rnorm(10)/100
>  plot(x,y)
>  fit <- lm(y ~ poly(x,3))
>  newx <- seq(1,10,len=100)
>  lines(newx,predict(fit,newdata=data.frame(x=newx)))
>
> However, the plotted line is way off the data.  The problem is that
> poly(x,3) doesn't agree with poly(newx,3) at equal values of x, so
> predict applies the fitted coefficients to the wrong values.
>
> To fix this we need a predict.poly procedure analogous to the
> predict.ns() and predict.bs() procedures. This doesn't look easy to
> write.
>
> Duncan Murdoch
>
> --please do not edit the information below--
>
> Version:
>  platform = i386-pc-mingw32
>  arch = x86
>  os = Win32
>  system = x86, Win32
>  status =
>  major = 1
>  minor = 4.0
>  year = 2001
>  month = 12
>  day = 19
>  language = R
>
> Windows 98 4.10 (build 1998)
>
> Search Path:
>  .GlobalEnv, package:ctest, Autoloads, package:base
>
>
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-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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[Rd] Bug in predict(newdata=x) with poly() (PR#1258)
ripley@stats.ox.ac.uk