Results: Computation times
ADMB-RE: 0.58 seconds.
nlme: 1.6 seconds under S-Plus 6.1.
A growth curve model was fitted to the "orange tree" data by Pinheiro & Bates (2000, Ch.8.2)
as an illustration of the R (S-Plus) routine nlme.
The logistic growth curve is given as
y = f1
/(1 + exp[-(t-f2)/f3])
] + e,
where y is the response and t is the age of the three.
The regression parameters to be estimated are: f1, f2
and f3, and e is the residual error term. A random effect u is added to the parameter
f1. A full description of the model can be found here:
Comparison with nlme
Parameter estimates are shown in the following table.
The difference between the estimates obtained with ADMB-RE and nlme
is small. The difference is caused by the fact that the two approaches 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).