Dear all,

I'm currently doing some GAM analysis and would like to include 2-way  
interactions between my continuous, explanatory variables. I know that  
for this, I need to include 2-dimensional smoothers but I'm not sure  
how to include them.
For linear models I learned that I should always include the variables  
that are in the interaction term as main terms, i.e. y = b0 + b1*x +  
b2*z + b3*xz (usually everybody cites Underwood, 1997)

How is it with GAMs if I want to include a 2-dimensional smoother for  
interactions? In some papers I see the same approach, i.e. y = b0 +  
s1(x) + s2(z) + s3(x,z). But in some papers I see only models like  
this: y = a + s(x,z), i.e. only the 2-dimensional smoother. Is there a  
general rule with GAMs?

Also, some of my smoothers have in the models p-values between 0.01  
and 0.05. Since p-values in this range should not be trusted it is  
always advised to do bootstrapping to get better p-values. Does  
anybody know how to do that?

My model looks like this:

GAM1 <- gam(cop  ~ s(Temp) + s(Sal) + s(Chl a), data = daten.sub)


Thanks for your help and a happy new year!!

Saskia Otto



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