Estimating and predicting repair and maintenance costs of tractors in any mechanized farm would help the owner or manager to get up-to-date information on overall costs, enabling him to control balance of income and costs .It would also help to determine the useful life of tractors as well as appropriate time for replacement. In order to obtain a suitable mathematical model and an accurate estimation of repair and maintenance costs for farm tractors, a case study was carried out on three current types of tractors in the Karoon Agro-Industry Co. , in north of Khozestan . The tractors were: MF-285 The data for which were collected from complete record files, 168 units. JD-3350 tractors data for which were collected from 20 existing ones, JD-4955 tractors selected from a total number of 25 tractors. The main data were obtained from the annual repair and maintenance costs (based on 1999 prices), as well as hours of annual use of tractors. The accumulated figures of the so called items were calculated in different years of lives of tractors . The regression analysis for calculated data was carried out using Least Square Method and a statistical computer software. The analyses were figured out for six different mathematical models (Linear, Reciprocal, Exponential, Multiplicative, Quadratic and Cubic). Results showed that the multiplicative equation was best fitted for predicting R & M costs for each separate model tractor. Using the same procedure ,it was found that the same model (multiplicative) was best fitted for all tractors. This model was compared with some other suggested models the results indicating that the predicted costs through the use of this model were of lower value than in the other models . The main reason for these differences is the low share of R & M costs as a percentage of purchase price of tractors (specially in early years of life).
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