[R] offset in gam and spatial scale of variables

Lucia Rueda lucia.rueda at ba.ieo.es
Wed May 19 16:51:51 CEST 2010

Hi Joris,

We're using mgcv. 

We have data on abundance of groupers on line transects that have the same
legth. My coworker has selected a bunch of variables and he has calculated
them in terms of total area in different sizes of buffers around the
centroid of the transect. He has run gam models (quasipoisson, mgcv) for
each explanatory variable at each size of buffer. Then he has selected the
signifficant variables. Some variables explain a higher percentage of
deviance at different sizes of buffers. And now he wants to build a gam
model trying the different explanatory variables but using the values that
correspond to the size of the buffer where they explain a higher deviance,
so one variable might have the values of a smaller scale whereas other might
correspond to a higher buffer size (I don't know if I made myself clear). I
am wondering if this is correct. 

Also I don't know if he should include an offset in spite all the transects
have the same length. 

I'm in charge of looking at the spatial correlation once he builds the
model. I don't know much about it but I was thinking of doing a Moran test,
correlogram and variogram and then if there's spatial autocorrelation doing
gamm, sar or gee. 


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