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