[R] two comments regarding predict.lm

[Ritter, Christian C SRTCL-CTGAS]

Here is the first one.

 It concerns the handling of multiple offsets.
	The following lines creates a list with 3 explanatory variables and
one response.
		> x<-seq(0,1,length=10);y<-sin(x);z<-cos(x);
w<-x+y+z+rnorm(x)
		> data<-list(x=x,y=y,z=z,w=w)

	A lm is fitted with one explanatory variable and two offsets. So
far, so good.
		> tt<-lm(w~x+offset(y)+offset(z),data=data)
	Then predict dies since it can only handle one offset term.
		> predict(tt,data)
		Error in eval(attr(tt, "variables")[[off.num + 1]], newdata)
: 
			attempt to select more than one element

	The following shows that it works with a single offset term.
		> tt<-lm(w~x+offset(y+z),data=data)
		> predict(tt,data)
		       1        2        3        4        5        6
7        8 
		1.011794 1.302522 1.579625 1.841981 2.088647 2.318876
2.532122 2.728049 
		       9       10 
		2.906535 3.067673
I would find it useful, if I could handle several offset terms.

Here is the second. It concerns again offsets. Deep in the belly of
predict.lm we find the lines:
	    predictor <- drop(X[, piv, drop = FALSE] %*% beta[piv])
	    if (!is.null(offset)) 
	        predictor <- predictor + offset
	X is the model matrix as obtained by 
	        X <- model.matrix(delete.response(tt), newdata, contrasts =
object$contrasts,            xlev = object$xlevels)
	and offset is a vector if there is an offset term. Now imagine that
the variables in newdata which make the offset contain some missing values
and that the variables in newdata which correspond to the model matrix dont.
In this case, predictor will be longer than offset and taking the sum
predictor + offset will make no sense. In the current version, this will
result in a warning of "different lengths". I would prefer if the result
were one of the following two:
		a) a stop with an error message (since the consequence of
ignoring the warning could be disastrous)
		b) that predictor and offset would be constructed of the
same (maximum) length, eventually padded by NA's where necessary
The second choice may make things easier to work with. 

Greetings,
		Chris

         _              
platform i386-pc-mingw32
arch     x86            
os       Win32          
system   x86, Win32     
status                  
major    1              
minor    4.1            
year     2002           
month    01             
day      30             
language R              
> 
> Kind Regards, 
> Christian Ritter, Statistical Services
> 
       Tel  +32 10 477349/ Fax +32 10 477219       
       email : Christian.C.Ritter at OPC.Shell.com

       Shell Coordination Centre
       Shell Research and Technology Centre 
       Avenue Jean Monnet 1
       B-1348 Ottignies Louvain-La-Neuve (Belgium)





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