problem with nls starting values
thanks for your reply I agree that an lm model would fit just as well, however the expectation from a mechanistic point of view would be a non-linear relationship. Also when I "simulate" data as in y_val<-115-118*exp(-0.12*(seq(1,100)+rnorm(100,0,0.8))) x_val<-seq(1:100) plot(y_val~x_val) summary(mod1<-nls(y_val~a-b*exp(-c*x_val),start=list(a=115,b=118,c=0.12))) I do not get a convergence. I obviously must be doing something wrong. Thanks for the help Benedikt Am 27.09.2012 20:00, schrieb Bert Gunter:
My guess: You probably are overfitting your data. A straight line does about as well as anything except for the 3 high leverage points, which the minimization is probably having trouble with. -- Bert On Thu, Sep 27, 2012 at 10:43 AM, Benedikt Gehr <benedikt.gehr at ieu.uzh.ch> wrote:
quantiles<-c(seq(.05,.95,0.05)) slopes<-c( 0.000000e+00, 1.622074e-04 , 3.103918e-03 , 2.169135e-03 , 9.585523e-04 ,1.412327e-03 , 4.288103e-05, -1.351171e-04 , 2.885810e-04 ,-4.574773e-04 , -2.368968e-03, -3.104634e-03, -5.833970e-03, -6.011945e-03, -7.737697e-03 , -8.203058e-03, -7.809603e-03, -6.623985e-03, -9.414477e-03) plot(slopes~quantiles)
Benedikt Gehr Ph.D. Student Institute of Evolutionary Biology and Environmental Studies University of Zurich Winterthurerstrasse 190 CH-8057 Zurich Office 13 J 36b Phone: +41 (0)44 635 49 72 http://www.ieu.uzh.ch/staff/phd/gehr.html