Absolute risk

Response to SSRI in OCD Relative Risk Absolute Risk Number Needed... 

Finally, the statistical methods used to obtain estimates of relative risk, absolute rates of cancer, confidence intervals and significance tests, and to adjust for confounding should have been clearly stated by the authors. The methods used should preferably have been the generally accepted techniques that have been refined since the mid-1970s. These methods have been reviewed for case-control studies (Breslow Day, 1980) and for cohort studies (Breslow Day, 1987). 

Schechtman, E. (2002). Odds ratio, relative risk, absolute risk reduction, and the numberneeded to treat—Which of these should we use Value in Health, 5(5), 431-436. 

Results Relative ranking of risk Absolute margin Used for design and licensing decisions... 

Safety is the freedom from hazards and thus the absence of any associated risks. Unfoi tunately, absolute safety cannot be realized. 

Risk and Uncertainty Discounted-cash-flow rates of return (DCFRR) and net present values (NPV) for future projects can never be predicted absolutely because the cash-flow data for such projects are subject to uncertainty. Therefore, when stating predicted values of (DCFRR) and (NPV) for projects, it is also desirable to give a measure of confidence in the predictions. 

Risk indices are usually single-number estimates, which may be used to compare one risk with another or used in an absolute sense compared to a specific target. For risks to employees the fatal accident rate (FAR) is a commonly apphed measure. The FAR is a singlenumber index, which is the expected number of fatalities from a specific event based on 10 exposure hours. For workers in a chemical plant, the FAR could be calculated as follows ... 

Absolute and relative risks Major risk contributors Comparisons with other risks... 

Absolute risk results are specific numerical estimates of the frequencies and/or consequences of process facility accidents synthesized from... 

Relative risk results show only the difference between the levels of safety of one or more cases of interest and a reference, or baseline, case. Relative risk estimates can be used (as can absolute estimates) to determine the most efficient way to improve safety at a facility. But, the use of relative risk estimates alone does little to help ensure that the most efficient way is safe enough unless one of the cases meets qualitative safety criteria (e.g., compliance with relevant codes, standards, and/or regulations consistency with current industry practice). 

If both frequency and consequence values are calculated and reported on an absolute basis, then they may be reported graphically in combination with one another (Chapter 3), or simply as the product of frequency and consequence. Table 5 contains some examples of typical risk estimates (frequency and consequence products). Based on absolute risk estimates, you can decide whether the risk of a specific activity exceeds your threshold of risk tolerance (risk goal). If so, analysts can estimate the reduction in risk, given that certain improvements are made, assumptions changed, or operating circumstances eliminated. 

Then, the absolute reduction in frequency, consequence, or risk can be calculated and compared to the cost of implementing the improvement, allowing you to determine whether the change represents the best use of resources to improve safety. 

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... 

TABLE 7. Converting Absolute Risk Estimates to Relative Risk Estimates... 

System Absolute Risk Estimate Relative Risk Estimate... 

Whenever possible, relative comparisons of risk should be made (Step 8). Comparing relative risk estimates for alternative strategies avoids many of the problems associated with interpreting and defending absolute estimates. Table 9 contains examples of typical conclusions you can reach using relative risk estimates. In some cases, however, absolute estimates may be required to satisfy your needs. Table 10 contains a list of examples of typical conclusions possible using absolute risk estimates. 

TABLE 10. Examples of Possible Conclusions Using Absolute Risk Estimates... 

Adequate support from the facility staff is absolutely essential. The facility staff must help the analysis team gather pertinent documents (e.g., PSilDs, procedures, software descriptions, material inventories, meteorological data, population data) and must describe current operating and maintenance practices. The facility staff must then critique the logic model(s) and calculation(s) to ensure that the assumptions are correct and that the results seem reasonable. The facility staff should also be involved in developing any recommendations to reduce risk so they will fully understand the rationale behind all proposed improvements and can help ensure that the proposed improvements are feasible. Table 12 summarizes the types of facility resources and personnel needed for a typical QRA. 

If there is a lack of specific, appropriate data for a process facility, there can be considerable uncertainty in a frequency estimate like the one above. When study objectives require absolute risk estimates, it is customary for engineers to want to express their lack of confidence in an estimate by reporting a range estimate (e.g., 90% confidence limits of 8 X 10 per year to 1 X 10 per year) rather than a single-point estimate (e.g., 2 X 10per year). For this reason alone it may be necessary for you to require that an uncertainty analysis be performed. 

Both individual and societal risks may be presented on an absolute basis compared to a specific risk target or criterion. Or, they may be presented on a relative basis to avoid arguments regarding the adequacy of the absolute numbers while preserving the salient differences between alternatives. The end results of the risk presentation may be a single number (or a range of numbers if an uncertainty analysis was performed) or one or more graphs. 

The work required to evaluate risk results will be a function of the objectives of the study. For relative risk studies, this evaluation is usually not very time-consuming. For absolute risk studies, in which many uncertainty and sensitivity cases may have been produced, the risk evaluation step may account for to 35% of the total effort of a large-scale QRA. Chapter 4 discusses the problems associated with interpreting risk results. 

The accuracy of absolute risk results depends on (1) whether all the significant contributors to risk have been analyzed, (2) the realism of the mathematical models used to predict failure characteristics and accident phenomena, and (3) the statistical uncertainty associated with the various input data. The achievable accuracy of absolute risk results is very dependent on the type of hazard being analyzed. In studies where the dominant risk contributors can be calibrated with ample historical data (e.g., the risk of an engine failure causing an airplane crash), the uncertainty can be reduced to a few percent. However, many authors of published studies and other expert practitioners have recognized that uncertainties can be greater than 1 to 2 orders of magnitude in studies whose major contributors are rare, catastrophic events. 

Absolute risk estimates can be difficult to use when there is no apparent human experience against which to calibrate them. By definition, there never exists enough experience about catastrophic rare events (fortunately) with which to calibrate the thinking about their significance. If there were enough data, you would not have elected to do the QRA in the first place. So, now thatyou have a bottom line estimate... 

There are several widely used approaches for developing perspective about the significance of absolute risk estimates (Figure 16). The first approach is to compare the risk estimates to historical experience within your company, looking for similar events. Most companies have safety and loss recordkeeping programs that date back many years. But if directly related data are sparse, you may widen your comparison to extrapolate from near-miss incidents that could have caused the event of interest. You will not, however, frequently find solace from the company data—or even comparable industry data. 

Another approach is to use government and private mortality and injury statistics. Calculated absolute risk estimates (the probability per year of a worker being injured or killed) can be compared to those de facto worker risk standards. For example, in the United Kingdom, industry and government alike are using the fatal accident rate (FAR, see Glos-... 

FICURE 16. Means of establishing perspective with absolute risk estimates. 

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. 

Having a numeric criterion for tolerable risk would be everyone s choice when making decisions using absolute risk estimates. Unfortunately, no universally accepted or mandated criterion exists. Nevertheless, when attempting to establish risk guidelines satisfying the requirements described, an organization has a number of resources avail-able. 3 Some particularly valuable sources of information include ... 

However, because of the diversity of short-term and long-term hazards to the public and workers in the CPI, no single criterion will ever meet everyone s needs. Even if such a criterion could be developed, there would still be controversy over its use. Absolute risk estimates are not... 

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. 

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... 

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