[R] A question on modeling brain growth using GAM

[Leon Lee]

Hi, R experts

I am new to R & GAM toolbox and would like to get inputs from you all on my
models. The question I have is as follows:
I have 30 subjects with each subject being scanned from one to three times
in the first year of life. The brain volume from each scan was measured.
The scan time was randomly distributed from birth to 1 year.
Each subject has different gestational age ranging from 38 to 41 weeks
Each subject has chronological age from birth to 1 year old
Each subject has gender category.
Now, I want to look at how predictors, such as subject's chronological age,
gestational age and gender will explain the changes in brain volume. I also
want to include interactions between gender and age, gestational and
chronological age. Random effects are also included in the model to account
for subject variability. My model looks like the follows:

gam=gam(brainVolume~ s(age) + ti(age, gestationalAge) + gestationalAge +
sex + s(age, by=sex) +  s(subjIndexF, bs="re"), method="REML", data=mydata)

Are there any obvious mistakes in the model? Any suggestions will be
greatly appreciated!

L

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