Hi, my english is not the best, i hope its comprehensible. Can i ask here in german too? my problem: I have plottet a curve with weibull1.4. I want to define a value on the y-axis and get the value of the x-axis for the plotted curve. This is my command: library(drc) effect<-c(0,22.84,40.74,54.63,62.04,59.26) conc<-c(0,90,150,260,440,750) drm(effect~conc,fct=W1.4()) model<-drm(effect~conc,fct=W1.4()) summary(model) plot(model) ED(model,c(50),interval='delta') With: "ED(model,c(50),interval='delta')" i get a value, but it is not on the fitted curve. What to do? Thanks Fabian -- View this message in context: http://r.789695.n4.nabble.com/drc-package-tp4676924.html Sent from the R help mailing list archive at Nabble.com.