Is there an optimal amount of preventive maintenance? What caution should be exercised before calculating this optimal amount?

Too little preventive maintenance causes breakdown costs to rise sharply, adding more to cost than is saved by less preventive maintenance; too much preventive maintenance reduces breakdowns, but by an amount insufficient to offset the added cost of preventive maintenance. Operations managers should assure that all costs of breakdowns have been properly included in the calculations. There is a history of not including indirect and subjective breakdown cost elements, which leads to performing too little preventive maintenance.