Hi Bob, you fitted a log-logistic model specifying the model function LL.4().
If you want to fit a Boltzmann or logistic model then you can use the model function L.4() in place of LL.4(): drc.preTBS.2 <- drm( Response ~ Dose, data = mydata, fct= L.4( names = c( "Slope", "Lower Limit", "Upper Limit", "ED50") ) ) summary(drc.preTBS.2) The slope is get from the above fit is -13.7329718. The "slope" in GraphPad is then obtained using the following transformation: -1 / (-13.7329718) = 0.07281745 which is close to the number you report. So, I would say that the two fits (obtained using GraphPad and using "drc"/R) are the same. Depending on the software used, different parameterizations of the logistic model are used. So even though the parameters unequivocally have the same names in various software programmes they need not necessarily be identical! Christian ______________________________________________ [email protected] 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.
