[R] More questions about gam and plot

[Tor A Strand]

When I use a "gam" command
c <- gam(depvar~var1+var2+s(var3)+s(var4)........)

Followed by a plot command
plot(c,....)   

The standard errors cross in the middle of the graph, (This variable is
almost linearly related with the outcome variable) why?

When I use the "persp" command after the gam fit, it does not graph the
smooth terms but usually two of the other terms in the model, why?

Dr. Tor A Strand   
Centre for International Health
Haukeland Hospital
University of Bergen
5021 Bergen    
Norway     
Phone: (country prefix 47)
Residence:56 51 10 88, office: 55 97 49 80,
fax: 55 97 49 79, cellular:  90 97 10 86