[R] predict.lm() question

[Rolf Turner]

You called lm() with a predictor named ``D$X'' and called predict.lm()
with a predictor name ``X''.

Simplest remedy:  Use

	fit <- lm(Y ~ X, data = D)

Remark:  Not a good idea to use ``D'' as the name of your data frame  
(``D'' is
the name of a function --- derivative).  Likewise don't use ``lm'' as  
the name
of an object (result of fitting a model) --- ``lm'' is the name of a  
function,
as well you know!

No immediate harm will come, but there can be subtle consequences I  
believe,
and anyhow it's confusing.

		cheers,

			Rolf Turner

On 8/04/2008, at 9:57 AM, Chip Barnaby wrote:

> Dear R-people ...
>
> I'm a new user.  I can't get predict.lm() to produce predictions for
> new independent data.  There are some messages in archived help about
> this problem, but I still don't see my error after reviewing
> those.  I understand that the new independent data must have the same
> name(s) as used when the model was made.
>
> In the example below, predict.lm produces the predictions for the
> original (model input) data plus a warning message.  What I want is
> predictions for alternative data (in data frame DX in the example).
>
> Thanks,
> Chip Barnaby
>
>> D<-data.frame( X=seq(1:10))
>> D$Y<-D$X+rnorm( 10)
>> D
>      X          Y
> 1   1  0.3811634
> 2   2  1.8770049
> 3   3  3.5253376
> 4   4  3.1851957
> 5   5  3.8088813
> 6   6  5.7333074
> 7   7  7.4896623
> 8   8  7.9394056
> 9   9  8.6683570
> 10 10 10.7480675
>> lm<-lm( D$Y~D$X)
>> summary( lm)
>
> Call:
> lm(formula = D$Y ~ D$X)
>
> Residuals:
>       Min       1Q   Median       3Q      Max
> -0.98812 -0.36354 -0.09808  0.48154  0.88288
>
> Coefficients:
>              Estimate Std. Error t value Pr(>|t|)
> (Intercept) -0.58935    0.41680  -1.414    0.195
> D$X          1.07727    0.06717  16.037 2.29e-07 ***
> ---
> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
> Residual standard error: 0.6101 on 8 degrees of freedom
> Multiple R-Squared: 0.9698,     Adjusted R-squared: 0.9661
> F-statistic: 257.2 on 1 and 8 DF,  p-value: 2.293e-07
>
>> DX<-data.frame( X=seq( 5.5, 11.5))
>> DX
>       X
> 1  5.5
> 2  6.5
> 3  7.5
> 4  8.5
> 5  9.5
> 6 10.5
> 7 11.5
>> predict.lm( lm, DX)
>           1          2          3          4          5           
> 6          7
>   0.4879174  1.5651887  2.6424600  3.7197313  4.7970026  5.8742739   
> 6.9515453
>           8          9         10
>   8.0288166  9.1060879 10.1833592
> Warning message:
> 'newdata' had 7 rows but variable(s) found have 10 rows
>>
>
> ---------------------------------------------------------
> Chip Barnaby                   cbarnaby at wrightsoft.com
> Vice President of Research
> Wrightsoft Corp.               781-862-8719 x118 voice
> 131 Hartwell Ave               781-861-2058 fax
> Lexington, MA 02421         www.wrightsoft.com
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting- 
> guide.html
> and provide commented, minimal, self-contained, reproducible code.


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