[R] 4-parameter logistic model

Christian Mora crmora at unity.ncsu.edu
Sat Feb 22 03:37:03 CET 2003

Dear R users

I'm a new user of R and I have a basic question about the 4-parameter
logistic model. According to the information from Pinheiro & Bates the model

y(x)=theta1+(theta2-theta1)/(1+exp((theta3-x)/theta4)) ==

from the graph in page 518 of the book of the same authors (mixed models in
S) theta 1 corresponds to the horizontal asymptote as x goes to infinity and
theta2 the horizontal asymptote as x goes to -infinity. When I use the
function SSfpl(input,A,B,xmid,scal), I'm not sure why the value of A is the
lower of the two asymptotes if according to the original function A should
be equal to theta1 (upper asymptote).. or maybe I'm wrong.

I'll appreciate any comment on this.

Best Regards


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