In its Safety Oversight Manual, document 9735, ICAO defines safety as "A condition in which the risk of harm or damage is limited to an acceptable level". (Reference 1) The determination of aviation safety is primarily left up to ICAO, at least on an international level. ICAO achieves this by using a safety risk assessment matrix. (Reference 2) This matrix is a table in which one factor is the risk probability of an event, and the other is the severity of said event. In cases of frequent probability occurrences any severity greater than major, the risk is determined to be unacceptable, and immediate action must be taken to avoid the event or otherwise mitigate the likelihood of its occurrence at once. In the case of events which are improbable, any severity that is major or greater is deemed tolerable or mitigatable. ICAO determines the safety margins to be used in aviation, and passes this down to each of its contracting states, who are then responsible for the determination of aviation safety on the national level. In Australia, the Civil Aviation Safety Authority is the governing body that upholds ICAO's requirements. Each contracting state has a similar body; in America, the FAA; Transport Safety Board of Canada in Canada, and so on. These bodies determine the minimum safety requirements in their states, in most cases striving to improve these minimums to the highest possible standards; in keeping with ICAO's goal to ever-improving safety.