Hi Mike, the model you consider is a special case of the four-parameter logistic model where the lower and upper asymptotes are fixed at 0.5 and 1, respectively.
Therefore, this (dose-response) model can fitted using the R package 'drc': library(drc) xy.m <- drm(y~x, fct = L.4(fixed=c(NA,0.5, 1, NA)), type = "binomial") plot(x,y) lines(x[x.ord], fitted(fit)[x.ord], col = "red", lwd = 3) # psyphy fit plot(xy.m, add = TRUE, log = "", col = 3, type = "fit", lwd = 3) Christian ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
