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 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. Best Regards CM
"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?
Christian - I acknowledge Prof. Bates' answer and his superior knowledge. But for the first equation you have typed below, theta1 is clearly the asymptote as x goes to minus infinity, and theta2 is the asymptote as x goes to plus infinity. The equation you have typed and the excerpt you have quoted from the book clearly disagree. I don't know anything about the function SSfpl(), inparticular whether it agrees with the book or not. - tom blackwell - u michigan medical school - ann arbor - On Fri, 21 Feb 2003, Christian Mora wrote:> 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 > 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. > > Best Regards > > CM > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > http://www.stat.math.ethz.ch/mailman/listinfo/r-help >