Risk Severity

The harm expected should the hazardous event occur, (i.e., loss, consequence, adverse outcome, damage, fatality, system loss, degradation, loss of function, injury) considering the risk associated with the hazardous event under evaluation.

Severity ranges should be sized so that events within each category are of comparable severity.

Equating the severity of event and conditions, which can cause one fatality with those, which

can cause 100 or 1,000 does not make sense. The potential problems associated with sizing of

the severity ranges grow as the size of the system grows. Program managers need to be

provided with risk information that has the fidelity to distinguish the hazardous events that meet

general criteria.

Severity range thresholds for each severity category should be comparable when considering

personal, system, or facility losses. For example, events or conditions that could cause the loss

of an entire aircraft or facility would be categorized by MIL-STD-882 as catastrophic. Loss of

a single crewman, mechanic, or passenger would also fall in the catastrophic category. Severe

injuries, such as total loss of sight of a mechanic, and system damage of several million dollars

are not normally considered to have equal value, even though both are in the critical category.

If the RHI ranking criteria use risk as a function of severity and probability, quantitative scales

or qualitative scales based on quantitative logic should be used. If the concept that the expected

losses (or risk) associated with a hazardous event or condition may be estimated by multiplying

the expected severity of the accident by the probability of the accident, then some sort of

quantitative basis is necessary. Failure to provide a quantitative basis for the scales can cause

significant confusion and dissipation of safety resources when an arbitrary risk ranking scale is

used.

Develop the severity values using order of magnitude ranges. There are several advantages to

separating severity categories by orders of magnitude ranges: They include:

• Limiting the likelihood of misuse of the analysis.
  
• Avoiding meaningless hair-splitting arguments.
  
• Simplifying severity assessment during PHAs without impacting usefulness.
  

Quantify the threshold values for the probability ranges. Quantification reduces confusion

associated with strictly qualitative definitions. Although it is impossible to quantify the ranges

in 882(C) due to its extremely broad application, developing quantified probability ranges for

specific systems is a relatively easy task to accomplish.

The probability of occurrence should refer to the probability of an accident/consequence as

opposed to the probability of an individual hazard/basic event occurring. The typical accident

sequence is much more complicated than a single line of erect dominos where tipping the first

domino (hazard) triggers a clearly predictable reaction.

Develop the probability values using order of magnitude ranges