The problem is that scale() returns a matrix, even if only a vector is
supplied. Thus the regression model actually has a matrix (scaled xx) as
the predictor. See:
> str(Data1)
`data.frame': 10 obs. of 2 variables:
$ xx: num [1:10, 1] -1.486 -1.156 -0.826 -0.495 -0.165 ...
..- attr(*, "scaled:center")= num 5.5
..- attr(*, "scaled:scale")= num 3.03
$ yy: num 2.1 3.1 4.1 5.1 6.1 7.1 8.1 9.1 10.1 11.1
and note that xx is not `num [1:10]', but `num [1:10, 1]'. If you give a
single column matrix as xx to predict(), it would work:
> predict(reg1, data.frame(xx=I(matrix(2:4, ncol=1))), se.fit=T)
$fit
1 2 3
12.65530 15.68295 18.71060
$se.fit
1 2 3
4.582710e-16 6.513913e-16 8.510747e-16
$df
[1] 8
$residual.scale
[1] 6.210772e-16
HTH,
Andy
> From: Anon.
>
> Moi!
>
> A student here has been getting a bit irritated with some
> side effects
> of scale() (OS is Windows XP, the behaviour occurs in R
> 2.0.0, but not
> 1.7.1). The problem is that she scales a variable in a data
> frame, then
> does a regression, and tries to get some predictions for some
> new data.
> However, at this point she gets an error (see the example
> below). This
> seems to be because the scaled variable in the new data frame
> does not
> have the center and scale attributes, but the one in the old
> data frame
> does.
>
> The work-around is to put the scaled variable intro a new data frame,
> which again won't have the attributes. But it seems odd to me that
> whether a scale()'d variable has attributes depends on where it's
> placed. I presume that this is because I'm not understanding
> something
> about the way R is working, rather than it being a bug. Would anyone
> care to enlighten me?
>
> > Data1=data.frame(xx=1:10, yy=2.1:12)
> > Data1$xx=scale(Data1$xx)
> >
> > reg1=lm(yy~xx, data=Data1)
> > New=data.frame(xx=2:4)
> > b=predict(reg1, New, se.fit=T)
> Error: variable 'xx' was fitted with nmatrix.1 but numeric
> was supplied
> >
> > New=data.frame(xx=scale(2:4, center=5.5, scale=3.02))
> > b=predict(reg1, New, se.fit=T)
> Error: variable 'xx' was fitted with nmatrix.1 but numeric
> was supplied
> >
>
> Bob
>
> --
> Bob O'Hara
> Department of Mathematics and Statistics
> P.O. Box 68 (Gustaf Hällströmin katu 2b)
> FIN-00014 University of Helsinki
> Finland
>
> Telephone: +358-9-191 51479
> Mobile: +358 50 599 0540
> Fax: +358-9-191 51400
> WWW: http://www.RNI.Helsinki.FI/~boh/
> Journal of Negative Results - EEB: www.jnr-eeb.org
>
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