[R] Interactions in GAMMs

[Gavin Simpson]

On Wed, 2010-07-21 at 16:17 +0100, Karen Moore wrote:
> Hi,
> 
> I've an issue adding an interaction to a GAMM:
> 
> My model was of form:
> 
> gamm1 <- gamm(TOTSR ~ fROT + s(PH) + s(LOI) + s(ASP) + s(SQRT_ELEV) + CANCOV
> + s(SQRT_TOTCWD) + s(WELLF) + s(WELLN) + s(OLDWDLD) + s(DISTWOOD) +
> s(Annprec) + s(OLDWDLD:DISTWOOD) + (1|fSITE),  family = poisson, data =
> BIOFOR2)
> 
> with interaction of s(OLDWDLD:DISTWOOD).
> 
> However I got this error message below but don't know what it means? (my
> data is composed of info for 150 plots)

An interaction is formed in a GAMM using a smoother in two or more
dimensions, such as s(OLDWDLD, DISTWOOD) or te(OLDWDLD, DISTWOOD), the
latter allows for different basis function within the two smoothers and
for different inherent scales for the two variables.

IIRC the latter allows you to produce strictly nested models for
likelihood ratio testing:

te(OLDWDLD) + te(DISTWOOD)

is nested within

te(OLDWDLD, DISTWOOD)

HTH

G

> 
> #I Warning messages:
> #2: In OLDWDLD:DISTWOOD :
>  # numerical expression has 150 elements: only the first used
> #3: In OLDWDLD:DISTWOOD :
> #  numerical expression has 150 elements: only the first used
> 
> Can anyone offer advice on how to include this interaction in GAMM model?
> Thanks
> 
> Karen Moore
> 
> 	[[alternative HTML version deleted]]
> 
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