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]]