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Risk is an expression of possible loss over a specific period of time or number of operational cycles. It may be indicated by the probability of an accident times the damage in dollars, lives,and / or operating units.

Hazard Probability and Severity are measurable and, when combined, give us risk.

Total risk is the sum of identified and unidentified risks.

Identified risk is that risk which has been determined through various analysis
techniques. The first task of system safety is to identify, within practical limitations, all possible risks. This step precedes determine the significance of the risk (severity) and the likelihood of its occurrence (hazard probability). The time and costs of analysis efforts, the quality of the safety program, and the state of technology impact the number of risks identified.

Unidentified risk is the risk not yet identified. Some unidentified risks are subsequently identified when a mishap occurs. Some risk is never known.

Unacceptable risk is that risk which cannot be tolerated by the managing activity. It is a subset of identified risk that must be eliminated or controlled.

Acceptable risk is the part of identified risk that is allowed to persist without further engineering or management action. Making this decision is a difficult yet necessary responsibility of the managing activity. This decision is made with full knowledge that it is the user who is exposed to this risk.

Residual risk is the risk left over after system safety efforts have been fully employed. It is not necessarily the same as acceptable risk. Residual risk is the sum of acceptable risk and unidentified risk. This is the total risk passed on to the user.

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