[R] Interactions in GAMMs

[Vito Muggeo (UniPa)]

Hi Karen,
I think you should decide what you mean for "interaction". s(x:y) is 
meaningless

If you want to fit a surface you should use s(x,y).

If you want to fit a varying coefficient model (interaction between a 
linear and a smooth term) you should use the argument by in s().

The help files of the mgcv package by S. Wood  are quite clear and they 
also include a lot of examples.

Hope this helps you

vito



Karen Moore ha scritto:
> 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)
> 
> #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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-- 
====================================
Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
Università di Palermo
viale delle Scienze, edificio 13
90128 Palermo - ITALY
tel: 091 23895240
fax: 091 485726/485612
http://dssm.unipa.it/vmuggeo