Additional Information
Model: y = f(x;beta) + e  =  (beta(1)) / (1 + exp[beta(2) - beta(3)x])^1/beta(4) + e

This model and data are an example of fitting sigmoidal growth curves taken from Ratkowsky (1983). The response variable is the dry weight of onion bulbs and tops, and the predictor variable is growing time.