Hi,
I want to use dredge to test several gam submodels including interactions.
I tried to find a way in order to keep models with interaction only if the
single variables occurring in the interaction are also included.
i.e.: for
y~s(x0)+s(x1)+ti(x0, x1)
I want to keep
y ~ s(x0)
y ~ s(x1)
y ~ s(x0) + s(x1)
y ~ s(x0) + s(x1) + ti(x0,x1)
and I want to remove
y ~ s(x0) + ti(x0,x1)
y ~ s(x1) + ti(x0,x1)
y ~ ti(x0,x1)
I know that I should use the "subset" option of the dredge function.
However, I can not find the correct matrix / expression to obtain what I
need !
Here a small example.
################
# Create some data (use mgcv example)
library(mgcv)
set.seed(2)
dat <- gamSim(1,n=400,dist="normal",scale=2)
# Create the global gam model
# Here a model with interaction. Note the use of ti()
bt <- gam(y~s(x0)+s(x1)+s(x2)+s(x3)+ti(x1,x2), data=dat,method="ML")
# Use dredge to test sub-models
library(MuMIn)
print(modstab <- dredge(bt))
# Here the 11th model include the interaction but do not include the single
variables x1 and x2
# ... I want to avoid that kind of model.
get.models(modstab, subset = 11)
################
Any help would be appreciated !
Arnaud
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