[R] degrees freedom in nlme

From: gabriela escati peñaloza <gescati_at_yahoo.com.ar>
Date: Fri 27 Jan 2006 - 09:42:21 EST


I'm having hard time understanding the computation of degrees of freedom 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.

Lic. Gabriela Escati Peñaloza
Biología y Manejo de Recursos Acuáticos
Centro Nacional Patagónico(CENPAT).
CONICET
Bvd. Brown s/nº.
(U9120ACV)Pto. Madryn
Chubut
Argentina

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