maintenance, depreciation and aircrew for 44%, servicing expense for 29% (traffic 11%, passenger 11% and aircraft 7%), 14% for reservations and sales and 13% for overheads (administration 6% and advertising 2%). An average US major Boeing 757-200 flies stages 11.3 block hours per day and costs $2,550 per block hour: $923 of ownership, $590 of maintenance, $548 of fuel and $489 of crew; or $13.34 per 186 seats per block hour. For a Boeing 737-500, a low-cost carrier like Southwest have lower operating costs at $1,526 than a full service one like United at $2,974, and higher productivity with 399,746 ASM per day against 264,284, resulting in a unit cost of $cts/ASM against $cts/ASM. McKinsey observes that "newer technology, larger aircraft, and increasingly efficient operations continually drive down the cost of running an airline", from nearly 40 US cents per ASK at the beginning of the jet age, to just above 10 cents since 2000. Those improvements were passed onto the customer due to high competition: fares have been falling throughout the history of airlines. Revenue Airlines assign prices to their services in an attempt to maximize profitability. The pricing of airline tickets has become increasingly complicated over the years and is now largely determined by computerized yield management systems. Because of the complications in scheduling flights and maintaining profitability, airlines have many loopholes that can be used by the knowledgeable traveler. Many of these airfare secrets are becoming more and more known to the general public, so airlines are forced to make constant adjustments. Most airlines use differentiated pricing, a form of price discrimination, to sell air services at varying prices simultaneously to different segments. Factors influencing the price include the days remaining until departure, the booked load factor, the forecast of total demand by price point, competitive pricing in force, and variations by day of week of departure and by time of day. Carriers often accomplish this by dividing each cabin of the aircraft (first, business and economy) into a number of travel classes for pricing purposes. A complicating factor is that of origin-destination control ("O&D control"). Someone purchasing a ticket from Melbourne to Sydney (as an example) for A$200 is competing with someone else who wants to fly Melbourne to Los Angeles through Sydney on the same flight, and who is willing to pay A$1400. Should the airline prefer the $1400 passenger, or the $200 passenger plus