Hello all,
I am trying to fit some data using the NLINFIT of MATLAB and using SAS.
I am trying to fit the well know RICHARDS growth curve.
It looks like y=a*(1-b*exp(-c*t))^d where we want to estimate the
parameters a,b,c,and d. Here t is my input and is age squared while y
is the weight of some animals. I choose my initial parameter starting
values
and they do converge and I get a wonderful looking fit with awesome residual
plot. My question is this though. The estimate MATLAB finds for d
is .44 which of course causes my predicted y's to be complex numbers
when 1-b*exp(-c*t) is negative (very frequent in my case). What questions
should this bring up. Is it ok to simply use the real parts of the numbers.
That is what SAS did and when ploting the predicted curve to the original
data I must say it looks just fine. Let me know your thoughts on the use
of only the real parts of the complex values. Any advice and input will be
much appreciated. Thanks ,
Mike
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