[R] gnls - Step halving....

[Christian Mora]

Hi all,

I'm working with a dataset from 10 treatments, each
treatment with 30 subjects, each subject measured 5
times. The plot of the dataset suggests that a
3-parameter logistic could be a reasonable function to
describe the data. When I try to fit the model using
gnls I got the message 'Step halving factor reduced
below minimum in NLS step'. I´m using as the initial
values of the parameters those obtained from the nls
fit. Is a problem of the initial estimates of the
parameters that I get the error or could be something
else?

The code for the nls fit was:

options(contrasts=c("contr.helmert","contr.poly")) 
VA1.lis<-nlsList(DRAM~SSlogis(MED,phi1,phi2,phi3)|TRAT,
data=VA1,na.action=na.omit)

The code for the gnls fit was (using a 'difference
parameterization' like SAS):

options(contrasts=c("contr.SAS","contr.poly")) 
VA1.gnls<-gnls(DRAM~SSlogis(MED,phi1,phi2,phi3),
data=VA1,params=list(phi1~TRAT,phi2~TRAT,phi3~TRAT), 
start=c(
23.36209,  ****avg of phi1 for the 10 trts *********
-0.854979000000004,  ***** diff. between avg and nls
estimate of TRAT#2 ******
.....and so on

I´d appreciate any comment

Thanks
CM