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Results: Computation times
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ADMB-RE: 17 seconds.
nlme: 7 seconds
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Model description
Pinheiro & Bates (2000, Ch. 6.4) fitted
a so-called 'one-compartment open model' to a dataset known as the 'phenopharbital data'.
A patient is given a dose D at time td, giving rise to a phenopharbital concentration
ct at a later time t:
ct = D/V exp[-Cl/V(t-td)],
where V and Cl are parameters (the so-called 'Volume of concentration' and the 'Clearance', respectively).
Doses given at different time points contribute additively to ct.
Pinheiro & Bates (2000) fitted a model with
a linear predictor (and a log-link) for each of the paramere V and Cl. Each of the linear predictors
contained one covariate Wt and one random effect.
A full description of the model can be found here: pheno.pdf
Results
The results produced by ADMB-RE and nlme are rather similar.
The small difference is caused by the fact that the two methods use
different approximations to the likelihood function. (ADMB-RE uses the
Laplace approximation, and for nlme the reader is referred to (
Pinheiro & Bates, 2000, Ch. 7).)
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