[R] 4PL algorithm

From: Greg Tarpinian <sasprog474_at_yahoo.com>
Date: Thu 12 Oct 2006 - 23:36:42 GMT

WinXP, Splus7 and R2.3.1.


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,


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