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.
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