Results: Computation times 
ADMBRE: 5 seconds. nlme: 7
seconds
 

Model descriptionPinheiro
& Bates (2000, Ch. 6.4) fitted a socalled 'onecompartment open
model' to a dataset known as the 'phenopharbital data'. A patient is given
a dose D at time t_{d}, giving rise to a
phenopharbital concentration c_{t} at a later time
t:
c_{t} = D/V
exp[Cl/V(tt_{d})], where
V and Cl are parameters (the socalled 'Volume of
concentration' and the 'Clearance', respectively). Doses given at
different time points contribute additively to c_{t}.
Pinheiro
& Bates (2000) fitted a model with a linear predictor (and a
loglink) 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 ADMBRE and
nlme are rather similar. The small difference is caused by the
fact that the two methods use different approximations to the likelihood
function. (ADMBRE uses the Laplace approximation, and for nlme
the reader is referred to ( Pinheiro
& Bates, 2000, Ch. 7).) 