[R-sig-ME] question on glme
Nicholas Mitsakakis
n.mitsakakis at theta.utoronto.ca
Wed Jan 31 05:21:34 CET 2018
Hello,
I was wondering if I could get some guidance/advice in using the glme
function for a problem of mine:
I have some continuous positive data C_ij representing accumulated
quantities, where i denotes subject and j a state. The assumption here is
that C_ij were accumulated at a constant rate within each state j, but
these rates were different across states. We have the times t_ij where
subjects spent in each state. We also have some other covariates, some time
dependent, some not. The interest here is to model the mean rate
conditional on state, as well as values of the other covariates.
I was thinking of fitting a mixed model, with the following characteristics:
- time values t_ij can be used as offset
- the model will be of the Gamma family with log link, for
convenience for the offset (log of times will be used)
- the correlation between C_ij across j values within i will be
expressed with one random effect expressing the random intercept per
subject i
- states will be categorical variable (choosing one of them as
baseline)
- what I am not sure about is if I also need to use time as weights.
I want to model the fact that C_ij for longer times should have higher
“weight”, since the “rate” is estimated over a longer period of
accumulation.
Any comment/suggestion would be greatly appreciated.
Thanks,
Nicholas
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