| Results: Computation times |
ADMB-RE: 5 seconds.
nlme: 7
seconds
| |
|
Model descriptionPinheiro
& 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
ResultsThe 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).) |