Dear All,
I am modeling a longitudinal dataset with nlme in S-PLUS 6.2, and nlme
suggested no random effects, i.e., all random effect estimates are “tiny”,
and anova comparisons with nls fits have p-value>0.999. This seems strange
to me and I wonder whether anyone has similar experiences.
A few more details: I am fitting a simple model of the form a +
b*exp(-c*Time) on a dataset with 3,700+ subjects and on average 10+
observations per subject. Other than the large size, the data do not appear
particularly “unusual.” In particular, it is difficult to believe that no
random effects are present. I used nls() results as initial estimates
(perturbations did not change anything). Does this mean that nlme has
difficulty handling large data?
I searched the archive and did not find anything helpful. Eecause I do not
subscribe to s-news, I would appreciate any response be copied to me
directly.
Thanks in advance,
Chuanpu
-------------------------------------------------------------------
Chuanpu Hu, Ph.D.
Assistant Director, Biostatistics
sanofi-aventis
9 Great Valley Parkway
Malvern, PA 19355-1304
Tel: (610) 889-6774
Fax: (610) 889-6932
E-mail: chuanpu.hu@sanofi-aventis.com
-------------------------------------------------------------------
----------------------------------------------------------
Le présent message ainsi que ses éventuelles pièces jointes est
exclusivement destiné au(x) destinataire(s), personnes physiques ou
morales, qu'il désigne.
Il constitue de ce fait une correspondance à caractère privé et peut
contenir des informations confidentielles.
Si ce message vous est parvenu par erreur, nous vous remercions d'en aviser
immédiatement l'expéditeur par retour de courrier électronique puis de le
détruire, ainsi que ses éventuelles pièces jointes, sans en conserver de
copie.
This message, including any attachment, is intended for the use of the
individual or entity to which it is addressed.
It is therefore to be considered as a private correspondence which may
contain confidential information.
If you are not the intended recipient, please advise the sender immediately
by reply e.mail and delete this message and any attachment thereto without
retaining a copy.
----------------------------------------------------------
|