Determining the Best Mathematical Model for Predicting the Repair and Maintenance Costs of the Existing Tractors in Misagh Sabz Agribusiness Co. Ilam Province

This research was conducted in order to determine a relationship between accumulated hours and R&M costs, for 27 conventional tractors in the Misagh Sabz Agri-Industrial. Company that were divided into two models including Massy Ferguson 285 and Universal 650. Required data including the R&M costs and hours of annual usage over the fifteen year period from 1991 to 2005 was collected. By performing regression analysis of the data, mathematical prediction models were estimated for both models of tractors separately and these estimations were performed again for all of the tractors. In the first period of life (about 2000 hours) power model as y = 0.024x1.600 and for the rest of its life quadratic model in the form of y = 0.005x2 – 0220x + 5.659 showed the best fitted models. Results revealed that accumulated R&M costs in percent of purchase price were higher in Universal 650 than the Massy Ferguson 285. To verify the results obtained from this study, a comparison was made between the model presented in the current study and some other national and international studies. Finally, it was shown that accumulated R&M costs as a percent of purchase price were nearly less than that of other previous studies. The reasons are discussed in this paper.