Tractors in Spain: A Logistic Analysis

This paper presents an attempt to make reliable projections of the stock of agricultural tractors in Spain. The approach to be followed is that of fitting a logistic trend to the historical data for the period 1951-1976. A set of possible trend curves is presented and their properties discussed; reasons for choosing the logistic trend are given. Preliminary estimates for the parameters of the logistic are obtained by means of the relatively unsophisticated three point method. Alternative heteroscedasticity assumptions in the data are explored with the help of a non-linear regression. Final estimates are obtained by means of a non-linear optimization algorithm. The influence of economic variables and government policies is traced in the residuals.