However, CPLEX provides alternate settings of system parameters to solve the problem more efficiently. The most effective system parameter utilizes SOS constraints (5) when branching. In addition, a SOS variable (i.e. a price structure variable) is given a higher priory when branching if it contains fare classes with higher values. Devex pricing is activated to determine the entering variable in the simplex algorithm. The best objective value is considered in selecting the node when backtracking (best-bound search), and the variable having the largest fractional part is selected for branching (maxi¬mum infeasibility rule). It is noted that CPLEX was allowed to generate clique cuts, but they were automatically discarded by branch-and-bound routines. The same system parameter settings are also found to be the most efficient in solving Model II and Model III.
8.1. Computational comparisons of models I, II and III
The size of models being considered in this article depends on the following factors: (1) number of flight legs; (2) capacity of each flight leg; (3) number of O-D itineraries; (4) number of fare classes in each O-D itinerary; (5) CRS capacity; and (6) mean and standard deviation of demand in each O-D itinerary and fare class. Factors (1), (2), (3), (4), and (6) affect number of seat variables (i.e. Xijk variables) although factors (3), (4) and (5) influence price structure variables (i.e. Wim variables). As expected, each factor has a different relative effect on both the solvability of the problem and the associated computational requirements.
Six randomly generated pricing and seat allocation problems with different model instances (number of flights, number of O-D itineraries, number of fare classes in each price structure, and CRS capacity) were considered to study the capability of the computerized decision-support methodology. These problem instances were created based on hypothetical networks of flights. The capacity of each flight leg is set equal to 50 for each problem. Problems are named such that first two digits (after EX) designate number of flight legs, digits from 3 to 5 denote number of O-D itineraries, digit 6 represents number of fare classes in each price structure (that is, price structure capacity), and the last digit denotes the number of fare classes that the CRS can accommodate. For example, EX0200342 indicates that the problem considers 2 flight legs, 3 O-D itineraries, 4 fare classes in each price structure, and a maximum of 2 fare classes in the CRS. In addition to problem EX0200342, the following problems are tested: EX0200384, EX0802042, EX0802084, EX2212842 and EX2212874.
Table 1 summarizes further relevant information for Models I, II, and III (after seat elimination procedures, but before the PSVE procedure, are applied). Column 3 gives the number of inequality (