First - thanks to all who contributed to writing the drc package in R!
I have a four parameter logistic LL.4 model that fits my data well. I've figured out how to use
predict(model1, newdata=data.frame(367))
gives me an output:
Prediction
0.4884074
to find the y value at the inflection point. Yay!
Now...I would also like the slope of the curve at x=200. So I have seen examples of this elsewhere which (for my case) would look like:
predict(model1, newdata=data.frame(200), deriv=1)
but I still get:
Prediction
0.4884074
Any help?! Thanks!