Thanks a lot to Chuck Cleland who pointed at the
function model.matrix() to see the coefficients used for a particular
model.
Through this function the very easy (really obvious) answer to my question
emerged.
There are no real "contrasts" in this case. The occurring factor level
becomes "1" whereas all others become "0" in the equation.
In practice, this means that the value for the continuous variable in the
interaction term is simply multiplied by the coefficient for the occurring
factor level.
Thanks again to all,
Ana
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Original question:
Dear All
I am currently applying Generalized Linear Models for plant
species-environment modelling.
These models contain continuous variables, factors and an interaction term
between a factor and a continuous variable. Because the variables in this
interaction term only make sense as "interaction" they are modelled as
x1:x2 (as opposed to x1*x2)
with x1 = factor and x2 = continuous variable. This results in one
coefficient for each factor level.
My question is which contrasts are used for the factor in the interaction
term. Contrasts are well documented for factors, but I cannot find a
reference for a factor:continuous variable interaction.
Thanks,
Ana
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