Introduction: Just as the aviation system is complex and interrelated, so is aviation safety. Aviation safety involves design of aircraft and airports, training of ground personnel and flight crew members' maintenance of aircraft, airfields, en route and terminal area navigation and communication facilities definition and implementation of Federal Aviation Regulations (FARs)l air traffic control procedures and much more. Ultimately, every part of aviation has a safety aspect. No other transportation mode has its safety record so rigorously scrutinized. In part this is due to the general societal (and media) fascination with infrequent large disasters in part because U.S. legislators have a personal interest in air safety, as they rely upon aircraft for their seasonal commutes to Washington, and in part because people in the industry are aware that their paychecks ultimately depend on their customers' perception that travel by air is as safe as possible. (Various airlines still conduct aircraft familiarity classes for travelers who have a fear of flying, although as the younger generation of Americans gains experience with airlines, this particular phobia should become less prevalent.) Aside from the industry's self-enforcement attempts, the Federal government tries to assure safety of the traveling public through regulation. The National Transportation Safety Board (NISB) investigates all major air carrier accidents and subsequently makes safety recommendations to the Federal Aviation Administration (FAA) - which the FAA may or may not choose to accept. One of the long lasting standoffs in aviation safety is between the NPSB (backed by Congressional committees), whose sole concern is safety,and the FAA, which must also take the economics of safety regulations into account-unless it wishes to run into a buzzsaw of industry reaction every time it changes (or issues) a FAR. On the international side, the International Civil Aviation Organization (ICAO) issues technical rules affecting aviation safety, although such decisions as its upcoming ruling on twinjet aircraft over-water flights may be tinged with economic considerations as well. But for safety regulations, whether external or internal to the aerospace industry, to make any sense, they must be grounded, to some degree, in reality, i.e. they must be backed up by some technical, statistical, or economic factors which people can address on their own merits. The more quantitative the supporting data are for rule justifications or changes, the greater the likelihood is that the regulations will be successfully promulgated and accepted by industry. Thus aviation safety analysis came into existence. Most broadly stated, the purpose of safety analysis is to improve safety. The spectrum of analysis ranges from the investigative to the predictive. At one end of the spectrum is the after-the-fact investigation of accidents and a search for causes at the other end is the attempt to seek out likely causes (or, more typically, combination of causes) of system failure before the system is put into operation. However, the great quandary of aviation system analysis is the lack of sufficient data to make probabilistic statements - even while the goal of this analysis is the elimination of the very accidents that provide the data. Practitioners of classical statistics, who have grown up considering probability as the likely outcome of an event based on a large number of repeated trials, face a mental hurdle when asked to accept the concept that an event which has never taken place can nevertheless be assigned a 0.95 probability of success. This is essentially the dichotomy between the investigative and the predictive ends of safety analysis - one is based on few accidents (but real accidents nonetheless), the other is based on more subjective probabilities of system (and subsystem) failures.