USING QRA RESULTS

When managers are faced with the necessity of using QRA results on an absolute basis, they must respect the potentially large uncertainties associated with the numbers and use prudent and conservative interpretations of these results for their decisions. Absolute risk estimates in... 

When used judiciously, the advantages of QRA can outweigh the associated problems and costs. Companies that prudently commission QRAs and conscientiously act on the resulting recommendations are better off for two reasons (1) they have a better base of information to make decisions and (2) their judicious use of QRA technology represents another demonstration of responsible concern for the health and safety of workers and the public. However, companies should resist the indiscriminate use of QRA as a means to solve all problems since this strategy could waste safety improvement resources, diverting attention from other essential safety activities. Once executives are able to interpret and use QRA results, they will appreciate that the quality of their decisions rests largely on their ability to understand the salient analysis assumptions and the limitations of the results. 

Once the decision has been made to use QRA, the next step is to execute it effectively. Chapter 3 describes the process of setting up an individual QRA. This chapter discusses the importance of defining the right problem for analysis and selecting the right analysis techniques it also provides an overview (not a how to) of the various classes of QRA techniques. Chapter 4 discusses ways to interpret and use QRA results. Conclusions about the future of QRA in the CPI are offered in Chapter 5. 

The decision to use QRA to satisfy a particular purpose may be the result of many compounding circumstances. There is no single way that the... 

There are a variety of ways to express absolute QRA results. Absolute frequency results are estimates of the statistical likelihood of an accident occurring. Table 3 contains examples of typical statements of absolute frequency estimates. These estimates for complex system failures are usually synthesized using basic equipment failure and operator error data. Depending upon the availability, specificity, and quality of failure data, the estimates may have considerable statistical uncertainty (e.g., factors of 10 or more because of uncertainties in the input data alone). When reporting single-point estimates or best estimates of the expected frequency of rare events (i.e., events not expected to occur within the operating life of a plant), analysts sometimes provide a measure of the sensitivity of the results arising from data uncertainties. 

The advantage of using relative risk results is that you can decide on the best way to improve safety at a facility without having to defend the absolute accuracy of the results. Relative results are also much less likely to be misinterpreted by people unfamiliar with QRA. The disadvantage of using relative results is that they, by definition, cannot give direct advice... 

Quantitative risk analysis is subject to several theoretical limitations. Table 13 lists five of the most global limitations of QRA. Some of these may be relatively unimportant for a specific study, and others may be minimized through care in execution and by limiting one s expectations about the applicability of the results. However, you must respect these limitations when chartering a QRA study and when using the results for decision-making purposes. 

Probably the least appreciated weakness of QRA is that the results are difficult to duplicate by independent experts. Even with the variety of sophisticated tools available for use, QRA is still largely dependent on good engineering judgment. The subtle assumptions of analysts performing QRA studies can often be the driving force behind the results. Many times these assumptions are at best arguable, and at worst arbitrary. 

Another way of interpreting absolute risk estimates is through the use of benchmarks or goals. Consider a company that operates 50 chemical process facilities. It is determined (through other, purely qualitative means) that Plant A has exhibited acceptable safety performance over the years. A QRA is performed on Plant A, and the absolute estimates are established as calibration points, or benchmarks, for the rest of the firm s facilities. Over the years, QRAs are performed on other facilities to aid in making decisions about safety maintenance and improvement. As these studies are completed, the results are carefully scrutinized against the benchmark facility. The frequency/consequence estimates are not the only results compared—the lists of major risk contributors, the statistical risk importance of safety systems, and other types of QRA results are also compared. As more and more facility results are accumulated, resources are allocated to any plant areas that are out of line with respect to the benchmark facility. 

In the past, qualitative approaches for hazard evaluation and risk analysis have been able to satisfy the majority of decision makers needs. In the future, there will be an increasing motivation to use QRA. For the special situations that appear to demand quantitative support for safety-related decisions, QRA can be effective in increasing the manager s understanding of the level of risk associated with a company activity. Whenever possible, decision makers should design QRA studies to produce relative results that support their information requirements. QRA studies used in this way are not subject to nearly as many of the numbers problems and limitations to which absolute risk studies are subject, and the results are less likely to be misused. 

If decision makers choose to use QRA, they must then define the analysis objectives so the results will satisfy the particular decisionmaking requirement. Because the cost of performing QRA is dependent... 

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