[R] offset in gam

[Lucia Rueda]

Hi, we are modelling dusky grouper distribution using underwater visual
census. Our response variable is the abundance of groupers but we want to
see differences in habitat related to maturity because dusky groupers use
different habitats when they're juveniles versus adults. So we're using the
% of maturity.

ie 1 individual that is 50cm TL gets aprox 0.5 (50% chances to be mature and
50% to be inmature) and so on. If we have let's say another grouper that is
40 cm TL it will have a probability for example 0.7 to be inmature so for
our transect we will have a total of 0.7 +0.5= 1.2 for 2 individuals. 

We're using the Poisson distribution because our data is not overdispersed
and it's not normal. But we don't know what offset we should include. Would
it be the area of the transect? all transects have the same area, 90 m2. Or
should it be the total abundance of groupers per transect? in the example
offset=2

Thanks a lot in adavance

Lucia

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