Excerpt from the report Summary: Total farm power available to farmers increased from about 40 million horsepower in 1920 to 385 million in 1962. Tractor horsepower (maximum belt) increased from 5,000,000 to 162 million during this same period. The demand for additional new tractors evolved from many individual farmers maintaining or adding to their stock of tractors. In this report, a single-equation regression model is used to explain aggregate tractor horsepower purchases for the United States between 1920 and 1962. Tractor horsepower purchases are expressed as a function of economic, technological, and personal preference variables. They include tractor horsepower on hand, crop production, ratio of tractor prices to prices received for products sold, size of new tractors purchased, age of tractors, and number of farms.