Probability categories

Probability category scores Definite > 9 Probable 5-8 Possible 1-4 Doubtful < 0. 

The system safety requirements paragraph discusses program objectives, general design requirements, the system safety precedence (safety precedence sequence), and risk assessment descriptions of both severity and probability categories (Tables 3-1 and 3-2). 

Even though some NASA documents reference the use of risk assessment codes (RACs), risk assessment for many NASA efforts is based on hazard level or criticality category. If risk assessment codes are used, they tend to use the hazard severity and probability categories and matrices from MIL-STD-882B. The NASA hazard levels are... 

It is not possible to quantify all risks. In many cases, qualitative risk analysis is the best available. One can use a classification scheme such as that shown in Figure 34-1. Table 34-4 and Table 34-5 provide a similar scheme. An analyst assigns one of five probability categories from Table 34-4 and one of four severity categories from Table 34-5. This scheme leaves out quantities completely. Analysts need considerable judgment to apply the categories consistently. 

MIL-STD-882, first issued in 1969, is the grandfather of risk assessment matrices. All of the over 30 variations of matrices I have collected include the basics that came out of 882. They include event probability categories, severity of harm or damage ranges, and risk gradings. 

Probability Category Rating Frequency of Exposure Rating Mid- Score Catas- trophic Final ScOTe Critical Final Score Medium Final Score Minimal Final Score... 

Table 1. Probability categories and consequence categories for Life and health— Deaths . 

In a qualitative risk analysis, probability categories must be defined. One such definition (Brauer 1990, p. 530) includes the following categories ... 

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. 

The UK MOD base their acceptance of hazards on a risk classification scheme, which is based on the combination of the severity, probability and time of exposure for each particular hazard. For the purposes of the accident risk classification scheme, accidents are considered single events (Table B.9). These classifications can be combined to determine a hazard risk index (HRI), which is a numerical risk factor that can be used to prioritise the need for corrective action or resolution. The HRI matrix in Table B. 10 is an example showing how the hazard severity and the hazard probability categories combine to yield the HRI. 

---