First - thanks to all who contributed to writing the drc package!
I have a four parameter logistic LL.4 model that fits my data well. Yay! I can't find documentation detailing usage of the model though! :(
I want to find the intersection between two lines: 1 - the tangent to the point of inflection and 2 the upper asymptote.
To get the line tangent to the point of inflection: The model spits out the independent value at the POI, but now I need to 1) use the model to get the value of the dependent variable at the POI and 2) plug the POI into the first derivative function. This will give me y, m and x so I can get b (y = mx + b).
To get the intersection: I can find X where this line crosses the line of the upper limit (upper asymptote) UA = mx + b.
I don't get how to use the resulting functions from drm to get y and m in this case.
Thanks for any help!