Qualitative hazard probability ranking

Table V. Example of Qualitative Hazard Probability Ranking (Ref. 39)...

Ref. 39 suggests an initial qualitative hazards analysis early in systems design, with only general levels of hazard probabilities identified, in addition to severity categories. An example of such a qualitative ranking from Ref. 39 appears in Table V. 

Hazard Probability Levels are qualitatively ranked from A to E. An example description from the Federal Transportation Administration follows ... 

O, D and S are estimated by a team of experts who rank the considered hazard with respect to the three factors of the RPN, each in a range between 1 and 10. It is evident, that the term probability used in this context is not equivalent to the mathematical value defined in probability calculus. It is in fact a qualitative rank where 1 stands for the least harmful value, 10 for the worst. As a result, the RPN is computed as a numeric value between 1 and 1,000. In this metric, 1 stands for an event with no hazardous consequences which is always detected in advance but never occurs. 1,000 means a catastrophic event which happens regularly or continuously and is never detected before the catastrophe occurs. 

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