Quantitative risk analysis limitations

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. 

Quantitative risk analysis (QRA) is a powerful analysis approach used to help manage risk and improve safety in many industries. When properly performed with appropriate respect for its theoretical and practical limitations, QRA provides a rational basis for evaluating process safety and comparing improvement alternatives. However, QRA is not a panacea that can solve all problems, make decisions for a manager, or substitute for existing safety assurance and loss prevention activities. Even when QRA is preferred, qualitative results, which always form the foundation for QRA, should be used to verify and support any conclusions drawn from QRA. 

The numerical scores to be used in a (semi-) quantitative risk analysis have to apply to the factors severity, probability and detectability. The scale may be linear or non-linear, for instance logarithmic. Addition or multiplication of these scores will lead to risk scores. These can be put in a priority sequence the harm with the highest scores should be dealt with the highest priority (efforts and formalities). And the score can also be compared with a fixed limit, for instance in case of a decision if any action has to be taken at all. 

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. 

Hazard analysis (HAZAN) is a quantitative way of assessing the likelihood of failure. Other names associated with this technique are risk analysis, quantitative risk assessment (QRA), and probability risk assessment (PRA). Keltz [44] expressed the view that HAZAN is a selective technique while HAZOP can be readily applied to new design and major modification. Some limitations of HAZOP are its inability to detect every weakness in design such as in plant layout, or miss hazards due to leaks on lines that pass through or close to a unit but cany material that is not used on that unit. In any case, hazards should... 

In the final phase of risk analysis—risk characterization—one integrates outputs of effects and exposure assessments. Risk is expressed in qualitative or quantitative estimates by comparison with reference values (e.g., hazard quotient). The severity of potential or actual damage should be characterized with the degree of uncertainty of risk estimates. Assumptions, data uncertainties and limitations of analyses are to be described clearly and reflected in the conclusions. The final product is a report that communicates to the affected and interested parties the analysis findings (Byrd and Cothern, 2000). 

Section 18.2 Risk Cliaracterization Section 18.3 Cause-Consequence Analysis Section 18.4 Qualitative Hazard Risk Analysis Section 18.5 Quantitative Hazard Risk Analysis Section 18.6 Uncertainlies/Limitations Section 18.7 Public Perception of Risk Section 18.8 Risk Communication... 

FTA was originally developed in 1962 at Bell Laboratories, under a U.S. Air Force Ballistics Systems Division contract to evaluate the Minuteman I Intercontinental Ballistic Missile (ICBM) Launch Control System [2]. This method is widely used in the area of safety engineering, to quantitatively determine the probability of safety in an undesired or accidental event referred to as a top event. With reference to Fig. V/3.0-1A, at the beginning of the chapter the importance of boundaries is discussed. In the figure, there is some gap between the external boundary and actual problem part (actual problem boundary). This is the interface part, and is quite important in any risk analysis. In the case of FTA, it is also important to define limits of resolution. A system is usually divided into subsystems, and in some cases a part may be subdivided into smaller parts. Now, the smallest part determines the limit of resolution and internal boundary. If, in one case, fault level is to be determined up to card level, and in another case it is necessary to determine the fault up to component level, naturally in the latter case, resolution will be higher. Limit of resolution has been shown in Fig. V/3.0-1A. 

Although the lower limit of quantitation is established during assay validation and prior to microdosing, assay sensitivity remains an uncertainty until the actual analysis of the microdose samples as well. There is always the danger that plasma exposures from the microdose are lower than predicted and as a result plasma concentrations from some or all of the time points cannot be detected by the LC-MS/MS method. Reduction of this risk is achieved by collaborative communication between the bioanalytical chemist and the project team. Conservative estimates on bioavailability and clearance can be used to establish the necessary limit of detection needed to determine plasma concentrations for all time points. Updates on the progress of the assay development allow the team to decide if the achievable limit of detection will enable the determination of plasma concentrations from enough time points to make a go-no go decision. Of course, sensitivity is not an issue with AMS, which practically ensures that plasma concentrations will be determined, possibly for several days, enabling the observation of complex PK and clearance from deep compartments. 

The extent of accommodation and characterization of uncertainty in exposure assessment must necessarily be balanced against similar considerations with respect to hazard, since the outcome of any risk assessment is a function of comparison of the two. If, for example, there is limited information to inform quantitatively on hazard and, as a result, a need to rely on defaults, there is limited benefit to be gained in developing the exposure analysis such that any increase in certainty is cancelled by uncertainties of greater magnitude associated with quantification of critical hazard, as a basis for a complete risk assessment. 

Limit of quantitation — (LOQ) The lowest amount of - analyte in a -> sample that can be quantitatively determined [i]. The LOQ is related to a signal value (j/q ) at which the -> analyte can surely be detected. It is defined by the following equation yq = pb + /cgoy, pb and standard deviation of the blank signal. The use of A q = 9 is recommended in order to have only a 0.135% of risk that a single signal measured in quantitative analysis is below the limit where it can be surely detected [ii]. 

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