Hello,

I am trying to fit a nlme (non linear mixed effect). I am using  the SelfStart function SSlogis. However the  data in my hand  contains  few observations per subject  (4 or less), so the nlsList doesn't work...  In this case I should fixe initial parameter estimates. I  remark that  values of initial estimates have a greater effect on the model fit (i.e. loglikelihood,  AIC and also on the significance of some parameter). Below the fit of the same model for different values of initial  parameters:


1- Initial parameters = c(3, 20, -3.732865)

 >summary(fm.nlme)
Nonlinear mixed-effects model fit by maximum likelihood
  Model: MTL ~ SSlogis(age, Asym, xmid, scal) 
  Data: data
       AIC      BIC   logLik
  7.996858 33.22420 2.001571

Random effects:
 Formula: list(Asym ~ 1, xmid ~ 1)
 Level: optid
 Structure: Diagonal
             Asym             xmid                      Residual
StdDev: 0.2577518      0.004429942           0.1616483

Fixed effects: list(Asym ~ 1, xmid ~ 1, scal ~ 1) 
         Value Std.Error  DF  t-value p-value
Asym  1.414976  0.029927 282 47.28124  <.0001
xmid 23.828353  9.517606 282  2.50361  0.0129
scal -5.108539  4.047323 282 -1.26220  0.2079
 Correlation: 
     Asym   xmid  
xmid  0.489       
scal -0.642 -0.953

Standardized Within-Group Residuals:
        Min          Q1         Med          Q3         Max 
-3.09765727 -0.45088811  0.00807415  0.53313907  2.45621965 

Number of Observations: 495
Number of Groups: 211  


2- Initial parameters: c(1.339120  8.559620 -0.296747)

> summary(fm.nlme)
Nonlinear mixed-effects model fit by maximum likelihood
  Model: MTL ~ SSlogis(age, Asym, xmid, scal) 
 Data: dataMedian 
       AIC      BIC    logLik
  39.82897 65.05632 -13.91449

Random effects:
 Formula: list(Asym ~ 1, xmid ~ 1)
 Level: optid
 Structure: Diagonal
            Asym     xmid Residual
StdDev: 0.239688 3.209776 0.162114

Fixed effects: list(Asym ~ 1, xmid ~ 1, scal ~ 1) 
         Value Std.Error  DF  t-value p-value
Asym  1.423446 0.0193297 282 73.64024  <.0001
xmid 11.894065 0.7795040 282 15.25850  <.0001
scal -1.082031 0.3130138 282 -3.45682   6e-04
 Correlation: 
     Asym   xmid  
xmid  0.001       
scal -0.196 -0.513

Standardized Within-Group Residuals:
         Min           Q1          Med           Q3          Max 
-3.122723638 -0.451811989  0.001009602  0.540504726  2.398619776 

Number of Observations: 495
Number of Groups: 211 


I think its a problem of local optimum......

Help!!!!

Many thanks,

Abderrahim


Dr Abderrahim Oulhaj
Oxford University
Department of Pharmacology
Mansfield Road
Oxford OX1 3QT
Tel:  +44 1865 224098
Fax: +44 1865 224099
Email: abderrahim.oulhaj@pharmacology.oxford.ac.uk


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