Hello,
 
I am attempting to use nlme to model the response of 19 groups.  I am able to get a reasonable fit of all of the groups to the observed data using an exponential decay model (a*exp-x*b).  The problem is that when I plot the fitted values to residuals, it demonstrates a pattern of increasing variance.
 
I attempt to model this variance using
 
Sig3.nlme <- update(Sig2.nlme, weights = varConstPower(power=0.1),
                   verbose = TRUE)
 
I get the message that 'step halving factor reduced below minimum in PNLS step'.  Some of the curves are actually flat lines where there is a value of 0 for the response at every observation within the group.  My thought is that, because this corresponds to 0 variance, it is causing problems for the pnls.  Don't know that this is the correct interpretation.
 
Is there anyway to model this type of data using nlme or do I need to find another option?
 
Thanks for any help.
	[[alternative HTML version deleted]]

