Tractors in Spain: a Further Logistic Analysis

The three parameter logistic growth function is known to give a good fit to annual statistics of the number of tractors in Spain. A parameterisation directly yielding the upper asymptote or saturation level is advanced, and the variance-covariance matrix of the parameter estimates is computed. A generalisation of the logistic, which proved useful in studying the growth of the number of tractors in Great Britain is examined and gives appreciably different results.