e) Forecasting and inventory control. By having discrete fare classes, the yield management system of forecasting the demand and then allocating the right number of seats to each class in order to optimize the revenue. This activity is called inventory control, and it is usually implemented for all flights operating between any combinations of city-pairs of the network up to one year into the future. The approach is to forecast and protect enough seats for high yielding demand and then leave other seats progressively available for lower fares. The seat availabilities are set to obtain that particular mix of business in each class which will maximize the expected revenue. In short the yield management problem is to get the best mix from a portfolio of fares with different values risks attached. The problem is solved by optimization algorithms, which depend on forecasts of demand for each booking class. These forecasts are based on large databases recording the complete booking history of each booking class flight, day over several years