David Thank you for your reply. I apologize if I posted in the wrong forum, as I really couldn't decide which forum is the best place for my question and I saw similar questions asked before in this forum.
I agree that a sample of ~30 subjects (70 scans in total), the model can be too complicated. Based on that, I did the following: (1) ignored the gender effect, as we have less females than males. (2) corrected chronological age based on their gestational age, that is, we subtracted an infant's chronological age by 2 weeks, if the infant's gestational age is 38 weeks instead of 40weeks. When I ran the model with corrected age, gestational age and their interactions modeled, I found the main effect of gestational age and the interaction between the two are gone. So, my final model will look something like this: gamObj=gam(brainVolume~ s(correctedAge) + s(subjIndexF, bs="re"), method="REML", data=mydata) Does this look more reasonable? Yes, I am relatively new to the mixed model. We originally applied functional data analysis (PACE) on the data, but want to see the results using a different approach. Also, I couldn't find the Mixed Models list, do you mind sending me a link? Thank you! Longchuan On Tue, Mar 28, 2017 at 4:28 PM, David Winsemius <dwinsem...@comcast.net> wrote: > > > On Mar 28, 2017, at 9:32 AM, Leon Lee <bhamlio...@gmail.com> wrote: > > > > 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! > > I'm not seeing mistakes in the syntax but I would question whether 30 > subjects is sufficient to adequately support estimates in a a model of this > complexity. I would also think that the 's(age)' and 'sex' terms would get > aliased out in a model with "+ s(age, by=sex)". Most R regression functions > handle removal of over-parametrization automatically. > > You also have a variable number of measurements per subject. I am unable > to comment on the effort to account for the implicit and variably measured > correlation and auto-correlation of values within subjects using a "smooth" > on subjIndexF, since that is not an approach I was familiar with. But I am > getting concerned whether you are also new to statistical modeling in > addition to your use of R and GAM being "new to you"? > > (Perhaps Simon or one of the mixed-effects experts can correct the gaps in > my understanding of how to model repeated measures in the context of small > numbers of subjects and irregular emasurements.) > > Please read the Posting Guide and the pages of candidate mailing lists. > Rhelp is not really the place to go when you need statistical advice. I'm > not sure if this is really in the center of concerns that get discussed on > the Mixed Models list, but to my eyes it would be a better fit there. > > -- > David. > > > > L > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide http://www.R-project.org/ > posting-guide.html > > and provide commented, minimal, self-contained, reproducible code. > > David Winsemius > Alameda, CA, USA > > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.