Poisson Models 319
The fitted model states that the intensity of gold deposits declines by a factor e
−0.1038
= 0.9014 for
every add itional kilometre of d istance from the n earest fault, a nd for a given distance, the intensity
is e
2.754
= 15.71 times higher inside the greenstone outcrop than it is outside the greenstone.
Similarly we could build a n additive model with two factor covariates, such as
> ppm(gor
~
vege + styp, data=gex)
which states that the log intensity of gorilla nesting sites is the sum of a n effect due to vegeta-
tion/cover type and an effect due to slope type. Equivalently the intensity is a vegetation effect
multiplied by a slope effect.
Note that the mode l bei
~
grad + I(grad^2) discussed in Section 9. 3.2 is also an additive
model. ...