In the real world, yearly machine costs fluctuate, therefore, some rule of thumb must be used to implement the theoretical replacement criterion. There are several alternative rules which may be suggested. First, the machine can be replaced at the theoretical optimum replacement interval. Second, the farmer may replace when some average of marginal costs exceeds the minimum average cost of the proposed replacement. Third, replacement may occur when marginal costs in any year reach a specified level. The size of repair cost required will be discussed later.