Mishap probability

By using a matrix to quantify and prioritize the risks the intrinsic subjective nature of risk assessment is not reduced, but a consistent framework for evaluating risk is provided. Although different matrices may be used for various applications, any risk assessment tool should include the elements of hazard severity and mishap probability. The risk level defined in the matrix represents the degree of risk associated with a hazard by considering these two elements. 

Similarly, mishap probability can be expressed with numerical probabilities or word descriptions (Table 11-2). 

MISHAP PROBABILITY Frequent A Likely to occur frequently... 

This undesired event can result from personnel error, environmental conditions, design inadequacies, and procedural deficiencies. It can also relate to system, subsystem, or component failure or malfunction. System safety methods require acceptance of some level of mishap risk, determine mishap probability, establish severity threshold, and create appropriate documentation procedures (Table 1.25). 

Five examples of risk assessment matrices follow. First, an adaptation is shown in Table 7 of the Mishap risk categories and mishap acceptance levels as in the working draft of MIL-STD-882E, the Department of Defense Standard Practice For System Safety. A comment in Appendix A of 882E is pertinent here A mishap assessment matrix allows classification by mishap severity and mishap probability and assists in managing the decision-making to achieve the necessary risk elimination or reduction to an acceptable level. ... 

System safety typically applies the qualitative risk characterization method because for a large system with many hazards, it can become cost-prohibitive to quantitatively model, analyze, and predict the risk of each and every hazard. In addition, low risk hazards do not require the refinement provided by quantitative analysis. It may be necessary to conduct a quantitative analysis only on a select few high consequence hazards. Experience over the years has proven that qualitative methods are very effective, and in most cases provide decision-making capability comparable to quantitative analysis. Qualitative risk characterization provides a very practical and effective approach when cost and time are concerns, and/or when there is very little supporting data available. The key to developing a qualitative risk characterization approach is by carefully defining severity and mishap probability categories. 

Method 1 shows an example of quantitative risk characterization by probability and severity cost. In this example, the mishap probability and cost of a hazard must be determined. The hazard risk point is then plotted on the graph and is determined as acceptable or unacceptable based on where it falls in the predefined acceptance region. In this method, mishap cost represents a method for measuring mishap severity. The major concern with this method is that there is little gradation in the acceptance level. 

The MIL-STD approach is to make decisions regarding resolution of identified hazards based on the assessment of the risk involved. It requires the identification of the risk category by combining the mishap severity with the mishap probability. In this case, mishap is the same as an accident. ... 

MIL-STD-882D (para A.4.4.3.2.2) states that Mishap probability is the probability that a mishap will occur during the plaimed hfe expectancy of the system. It can be described in potential occurrences per unit time, events, population, items, or activity . Suggested mishap probability levels are shown in Table B.14. 

Severity category Mishap probability Catastrophic Critical Marginal Negligible... 

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