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.
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F. R. Oliver (1981) studied this question.