Design Of Experiments, Response Surface, Spline, Box Behnken, Central Composite Design.
to fit a curve like this. The lack of fit becomes clear after the EC50 — the Top, Hill, and EC50 are well estimated, but the Bottom is just a random guess. So, by not having included the inflection dosage (dose at 50% response [EC50]), the model has trouble estimating both the EC50 and the bottom