[R] problem with predict()

Liaw, Andy andy_liaw at merck.com
Fri Jun 21 21:35:09 CEST 2002

> From: Czerminski, Ryszard [mailto:ryszard at arqule.com]
> --- first problem 
> If I store 'simulated' data in data frames:
> # train.data <- data.frame(matrix(rnorm(164*119), nrow = 164))
> # test.data <- data.frame(matrix(rnorm(35*119), nrow = 35))
> it still works the same way i.e. the code below works fine
> for simulated data and fails for 'real' data the only
> difference being in actual numeric values stored in data
> structures of the same shape and type.
> Any suggestions why this happens ?

As Prof. Ripley hinted, check the names of the data frames.  For your "real"
data, do 


to see how they are different.
> --- second problem
> > As Andy Liaw pointed out, xr is a matrix.  Take a look at 
> the names of
> > train.  Hint: they do not contain `x'.
> Following your hint I am guessing that the fact that names do 
> not contain
> 'x'
> explains why lm(y~., train) form works and lm(y~x, train) fails
> and "lm(y~., train)" means roughly: correlate column "y" to 
> all other colums
> Where I can find more detail specification of this syntax ?
> In help(lm) I find this paragraph:
>      Models for `lm' are specified symbolically.  A typical model has
>      the form `response ~ terms' where `response' is the (numeric)...
> which does not quite cover this case.

Reading the `An Introduction to R' manual, especially parts of Chapter 5.
(The `.' shorthand is explained in section 11.5.)


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