Risk is a term used across governments and industries to classify the likelihood and outcome of events. Phrases such as "highly probable" or "catastrophic" may be sufficient classifications for many applications. However in cases where consequences could mean loss of human life or millions of dollars in assets, decision-makers look for numbers as a more solid basis on which to quantify risky decisions and the uncertainty of these decisions.

Thus arose the concept of Probabilistic Risk Assessment (PRA), which may also be called Quantitative Risk Assessment (QRA) or Probabilistic Safety Assessment (PSA). Historically, PRA has been applied in the nuclear, chemical, and aerospace industries. More recently, it has been emerging in construction, transportation, financing, and management planning.

PRA is a well-established technique for integrating various reliability modeling tools, such as Fault Tree, Event Tree, Reliability Block Diagram (RBD), and FMEA to numerically quantify risks. The PRA sets out to determine what hazardous scenarios can occur, what is the likelihood they can occur, and what are the consequences given they occur. It uses statistical reliability data for basic events to answer these questions.

The first step of a Probabilistic Risk Assessment is to identify an undesired top event, such as "loss of life" or "loss of mission," and trace out all the hazards that could lead to this event. This is usually conducted through the use of event trees, in which the hazards become the initiating events. For the initiating events and all subsequent intermediate events, fault trees are developed. At the lowest level, the basic events of the fault trees are assigned probabilities. These probabilities are propagated up the logic to reach a probability (and uncertainty) of the undesired top event.

PRA is a key tool in safety management. Software to implement PRA is necessary for any large-scale system due to the large, complex logic chains that must be analyzed. Using Relex Fault Tree and Event Tree, the analyst can allow the software to do the logic calculations automatically and can concentrate on the safety engineering. Software links between faulttree top events and the events in event tree can be easily associated. Thus Relex proves to be a valuable safety software tool for any PRA program.