[R] compare observed and fitted GAM values

[David Winsemius]

On Aug 21, 2009, at 5:03 PM, Lucía Rueda wrote:

> Hi,
>
> I am comparing the observed and fitted values of my GAM model, which  
> includes the explanatory variables: longitude, depth, ssh, year and  
> month. When I compare observed and fitted values for longitude,  
> depth and ssh it works. But when I try to do it for month and year  
> (which are as factors in the GAM model) it doesn't work. My observed  
> and fitted values are exactly the same..

So then,  you are complaining because it "works" but _unexpectedly_  
well.

> How is that possible?  Thanks

If you have a balanced design in Year and it is the only covariate,  
then that is precisely what should happen with any regression method:

 > newdat <- data.frame(val=rnorm(40), cat=factor(1:4))
 > aggregate(newdat$val, list(newdat$cat), mean)
   Group.1          x
1       1  0.4972092
2       2 -0.1042936
3       3  0.1549305
4       4 -0.1500513
 > lm(val~cat, newdat)

Call:
lm(formula = val ~ cat, data = newdat)

Coefficients:
(Intercept)         cat2         cat3         cat4
      0.4972      -0.6015      -0.3423      -0.6473

 > 0.4972092 + coef(lm(val~cat, newdat))[2:4]
       cat2       cat3       cat4
-0.1042937  0.1549304 -0.1500514

> snipped output from OP


-- 
David Winsemius, MD
Heritage Laboratories
West Hartford, CT