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Prediction using GAM

2 messages · Kerry Bush, Brian Ripley

Kerry Bush
# 
Recently I was using GAM and couldn't help noticing
the following incoherence in prediction:
1 
0.8017407
predict(gam.object,data.frame(x=gam.data$x[1],z=gam.data$z[1]))
        1 
0.1668452 

I would expect that using two types of predict
arguments should give me the same results.
When I used this to predict a new data set then it
seems OK:
predict(gam.object,data.frame(x=gam.newdata$x[1],z=gam.newdata$z[1]))
        1 
0.4832136
1 
0.4832136 

Could anybody explain the strange behavior of
predict.gam function?

Thanks,
Kai
Brian Ripley
# 
R has *two* gam() functions in contributed packages 'mgcv' and 'gam'. 
Which is this?

Please see the posting guide and provide a reproducible example.

If this is package 'gam', prediction difficulties of this sort for the S 
version are discussed in the White Book, MASS and elsewhere (but I recall 
reading that they did not apply to the R version).
On Wed, 23 Mar 2005, Kerry Bush wrote:

            

      
      
      
    

        
It is unusual to use data() on your own objects, but we cannot reproduce 
what you did without data.