[R] 4-parameter logistic model

[Douglas Bates]

"Christian Mora" <crmora at unity.ncsu.edu> writes:

> 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
> is:
> 
> y(x)=theta1+(theta2-theta1)/(1+exp((theta3-x)/theta4)) ==
> y(x)=A+(B-A)/(1+exp((xmid-input)/scal))
> 
> 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.

Is scal negative in your call to SSfpl?