[R-sig-eco] Offsets in Poisson or Neg. Bin regression

[Dixon, Philip M [STAT]]

Matias,

The situation when you most want to use an offset is when FECUND differs across individuals.  If FECUND is a constant for all observations, you could ignore it if you chose to.  If you did that, its constant effect gets rolled into the intercept.

When it's not constant, the logic is:

log(mu_i / F_i) = your model   <==> Log(mu_i) = your model + log(F_i)
where mu_i is the mean count and F_i is the fecundity, both for individual i.  

When F_i is a constant = F, the logic is:
log(mu_i / F) = your model   <==> Log(mu_i) = B0 + rest of your model + log(F)  <==> B0 + log(F)  + rest of your model 
where B0 + log(F) is the new "intercept"

Best wishes,
Philip Dixon