From: Douglas Bates <dmbates_at_gmail.com>

Date: Sat 28 Jan 2006 - 03:06:00 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Jan 28 03:19:52 2006

Date: Sat 28 Jan 2006 - 03:06:00 EST

On 1/27/06, gabriela escati peñaloza <gescati@yahoo.com.ar> wrote:

> Hi, I' m having a hard time understanding the computation of degrees of freedom

So do I and I'm one of the authors of the package :-)

> when runing nlme() on the following model:

*>
**> > formula(my data.gd)
**> dLt ~ Lt | ID
**>
**> TasavB<- function(Lt, Linf, K) (K*(Linf-Lt))
**>
**> my model.nlme <- nlme (dLt ~ TasavB(Lt, Linf, K),
**> data = my data.gd,
**> fixed = list(Linf ~ 1, K ~ 1),
**> start = list(fixed = c(70, 0.4)),
**> na.action= na.include, naPattern = ~!is.na(dLt))
**>
**> > summary(my model.nlme)
**> Nonlinear mixed-effects model fit by maximum likelihood
**> Model: dLt ~ TasavB(Lt, Linf, K)
**> Data: my data.gd
**> AIC BIC logLik
**> 13015.63 13051.57 -6501.814
**> Random effects:
**> Formula: list(Linf ~ 1 , K ~ 1 )
**> Level: ID
**> Structure: General positive-definite
**> StdDev Corr
**> Linf 7.3625291 Linf
**> K 0.0845886 -0.656
**> Residual 1.6967358
**> Fixed effects: list(Linf + K ~ 1)
**> Value Std.Error DF t-value p-value
**> Linf 69.32748 0.4187314 402 165.5655 <.0001
**> K 0.31424 0.0047690 2549 65.8917 <.0001
**> Standardized Within-Group Residuals:
**> Min Q1 Med Q3 Max
**> -3.98674 -0.5338083 -0.02783649 0.5261591 4.750609
**> Number of Observations: 2952
**> Number of Groups: 403
**> >
**>
**> Why are the DF of Linf and K different? I would apreciate if you could point me to a reference
*

The algorithm is described in Pinheiro and Bates (2000) "Mixed-effects Models in S and S-PLUS" published by Springer. See section 2.4.2

I would point out that there is effectively no difference between a t-distribution with 402 df and a t-distribution with 2549 df so the actual number of degrees of freedom is irrelevant in this case. All you need to know is that it is "large".

I will defer to any of the "degrees of freedom police" who post to this list to give you an explanation of why there should be different degrees of freedom. I have been studying mixed-effects models for nearly 15 years and I still don't understand.

> Note: I working with Splus 6.1. for Windows

Technically this email list is for questions about R. There is another list, s-news@biostat.wustl.edu, for questions about S-PLUS.

*>
**>
*

> Lic. Gabriela Escati Peñaloza

*> Biología y Manejo de Recursos Acuáticos
**> Centro Nacional Patagónico(CENPAT).
**> CONICET
**> Bvd. Brown s/nº.
**> (U9120ACV)Pto. Madryn
**> Chubut
**> Argentina
**>
**> Tel: 54-2965/451301/451024/451375/45401 (Int:277)
**> Fax: 54-29657451543
**>
**> ---------------------------------
**> 1GB gratis, Antivirus y Antispam
**> Correo Yahoo!, el mejor correo web del mundo
**> Abrí tu cuenta aquí
**> [[alternative HTML version deleted]]
**>
**>
**>
**> ______________________________________________
**> R-help@stat.math.ethz.ch mailing list
**> https://stat.ethz.ch/mailman/listinfo/r-help
**> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
**>
**>
*

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Jan 28 03:19:52 2006

*
This archive was generated by hypermail 2.1.8
: Fri 03 Mar 2006 - 03:42:14 EST
*