4PL model fitting

[Greg Tarpinian <sasprog474@yahoo.com>]

WinXP, Splus7.


All,

I have been using the SSfpl and SSlogis self-starting functions
in the nlme library to fit 4PL and restricted 4PL models.  I need
to adapt these routines to fit the alternative model

   f(x) = A + (B-A)/(1 + abs(x/EC50)^C)

My Question:  How do I obtain good starting values for this alternative
model?

(The pseudo-code found on pages 517 - 520 of "Mixed Effects Models in
S and Splus" is not applicable to my problem because it deals with
f(x)=A+(B-A)/(1+exp(EC50-x)/C)) which is not the same function.)


Many thanks,

      Greg

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