A Three-Part Bayesian Network for Modeling Dwelling Fires and Their Impact upon People and Property.

In the United Kingdom, dwelling fires are responsible for the majority of all fire-related fatalities. The development of these incidents involves the interaction of a multitude of variables that combine in many different ways. Consequently, assessment of dwelling fire risk can be complex, which often results in ambiguity during fire safety planning and decision making. In this article, a three-part Bayesian network model is proposed to study dwelling fires from ignition through to extinguishment in order to improve confidence in dwelling fire safety assessment. The model incorporates both hard and soft data, delivering posterior probabilities for selected outcomes. Case studies demonstrate how the model functions and provide evidence of its use for planning and accident investigation

Assessment of a Space Shuttle trajectory evaluation system (DOLILU II)

DOLILU II is a ground control system that generates space shuttle\u27s launch trajectories, first stage guidance commands and verifies whether the generated trajectories are safe for the flight. It is a safety critical system and a high degree of confidence in its safety and reliability must be gained through assessment. We addressed three issues related to its safety and reliability assessment. We developed a reliability assessment framework for DOLILU II system. We proposed techniques to speed up test case execution and designed methodologies for the generation of input conditions needed to test the system.;We used a Bayesian statistical framework for reliability assessment. Bayesian statistics uses knowledge about the system to be incorporated into the reliability model before testing. DOLILU II has been operational for nearly five years. We use this information when developing the reliability model. This information is introduced in the form of prior beliefs.;DOLILU II system requires an average time of 30 minutes for each test run. This translates into a large time period required for testing to demonstrate that DOLILU II exhibits the required failure rate. Vertical slicing, a semantic transformation technique, is used to prove the possibility of parallel execution and enhance each test case execution.;DOLILU II is an on-demand system. Many test trajectories are needed for its assessment. Regression methods were used to develop models for the generation of input data

Scoping study on the significance of mesh resolution vs. scenario uncertainty in the CFD modelling of residential smoke control systems

Computational fluid dynamics (CFD) modelling is a commonly applied tool adopted to support the specification and design of common corridor ventilation systems in UK residential buildings. Inputs for the CFD modelling of common corridor ventilation systems are typically premised on a ‘reasonable worst case’, i.e. no specific uncertainty quantification process is undertaken to evaluate the safety level. As such, where the performance of a specific design sits on a probability spectrum is not defined. Furthermore, mesh cell sizes adopted are typically c. 100 – 200 mm. For a large eddy simulation (LES) based CFD code, this is considered coarse for this application and creates a further uncertainty in respect of capturing key behaviours in the CFD model. Both co-existing practices summarised above create uncertainty, either due to parameter choice or the (computational fire and smoke) model. What is not clear is the relative importance of these uncertainties. This paper summarises a scoping study that subjects the noted common corridor CFD application to a probabilistic risk assessment (PRA), using the MaxEnt method. The uncertainty associated with the performance of a reference design is considered at different grid scales (achieving different ‘a posteriori’ mesh quality indicators), with the aim of quantifying the relative importance of uncertainties associated with inputs and scenarios, vs. the fidelity of the CFD model. For the specific case considered herein, it is found that parameter uncertainty has a more significant impact on the confidence of a given design solution relative to that arising from grid resolution, for grid sizes of 100 mm or less. Above this grid resolution, it was found that uncertainty associated with the model dictates. Given the specific ventilation arrangement modelled in this work care should be undertaken in generalising such conclusions

The MaxEnt method for probabilistic structural fire engineering : performance for multi-modal outputs

Probabilistic Risk Assessment (PRA) methodologies are gaining traction in fire engineering practice as a (necessary) means to demonstrate adequate safety for uncommon buildings. Further, an increasing number of applications of PRA based methodologies in structural fire engineering can be found in the contemporary literature. However, to date, the combination of probabilistic methods and advanced numerical fire engineering tools has been limited due to the absence of a methodology which is both efficient (i.e. requires a limited number of model evaluations) and unbiased (i.e. without prior assumptions regarding the output distribution type). An uncertainty quantification methodology (termed herein as MaxEnt) has recently been presented targeted at an unbiased assessment of the model output probability density function (PDF), using only a limited number of model evaluations. The MaxEnt method has been applied to structural fire engineering problems, with some applications benchmarked against Monte Carlo Simulations (MCS) which showed excellent agreement for single-modal distributions. However, the power of the method is in application for those cases where ‘validation’ is not computationally practical, e.g. uncertainty quantification for problems reliant upon complex modes (such as FEA or CFD). A recent study by Gernay, et al., applied the MaxEnt method to determine the PDF of maximum permissible applied load supportable by a steel-composite slab panel undergoing tensile membrane action (TMA) when subject to realistic (parametric) fire exposures. The study incorporated uncertainties in both the manifestation of the fire and the mechanical material parameters. The output PDF of maximum permissible load was found to be bi-modal, highlighting different failure modes depending upon the combinations of stochastic parameters. Whilst this outcome highlighted the importance of an un-biased approximation of the output PDF, in the absence of a MCS benchmark the study concluded that some additional studies are warranted to give users confidence and guidelines in such situations when applying the MaxEnt method. This paper summarises one further study, building upon Case C as presented in Gernay, et al

CONSUMER'S PERCEPTIONS OF ENVIRONMENTAL RISKS AND THE DEMAND FOR FOOD SAFETY

Public concern regarding food safety has emerged as a major policy issue. Chemicals and biotechnological processes are perceived as risks of food safety despite their contribution to an efficient, low cost agriculture and food industry. Increases in uses of biotechnological processes for foods are expected to be a major potential source of productivity improvements for Alberta and Canadian agriculture in future years. However, the demand for food safety involves increasing awareness and concern by consumers of chemical inputs and biotechnological processes in the agriculture and food industries. Nonetheless, there is a lack of basic economic and agricultural economic theory and methodology to analyze these issues and a need for policy, socioeconomics and marketing research on biotechnology and other environmental risk situations in the agricultural and food industry. This project was directed at developing and applying economic theory and methodology to help fill this gap. A major contribution of the project is the identification and verification of methodologies of stated choice to analyse tradeoffs arising from food safety perceptions of concerns by consumers. One component of the research project involved assessment of Alberta consumers' stated preferences and purchase behaviour for foods exhibiting a range of environmental risks, including perceptions of pesticide residues and hormonal treatments derived from biotechnological processes. The results of this survey indicated that Albertans were more concerned about pesticide use in food production than about the use of hormones. In contingent valuation questions developed for the study, more Albertans wish to restrict pesticide use (relative to a base case of not restricting either hormones or pesticides). They tended to persist in these choices in the face of potential increases in food costs, reflecting a higher level of concern with pesticides than hormones. Increasing education increased this concern. Increasing food cost decreased the probability of choosing to restrict pesticide or hormone use. Women appeared to perceive pesticide use in food production as a greater food safety risk than was perceived by men. The inferred average willingness to pay to restrict pesticide and growth hormone use in food production amounted to about 25% and 13% respectively of the average Albertan's food expenditures respectively. In the second survey of consumers' food/environmental risk perceptions undertaken relative to this project, the responses of a random sample of consumers to the use of recombinant bovine somatotrophin (rBST) in milk production were elicited using a stated preference methodology. A conditional logit model of consumer choice was developed and tested to analyse consumers' choices of milk with varying characteristics of fat content, price, freshness and rBST treatment. Awareness of rBST presence or otherwise is implied by labelling. The approach attempts to simulate market conditions with and without rBST labelled milk and to predict consumers' responses to variations in these conditions. Welfare calculations for a representative consumer indicate welfare losses with the introduction of rBST. These were slightly less for a male than a female household food purchaser and were less for food purchasers with higher levels of income and education. There was a small welfare gain when the representative food purchaser was offered a full range of "rBST" and "non-rBST" milks. The results suggest that making appropriately labelled "rBST-free" milk available to consumers could decrease consumer welfare losses associated with any introduction of rBST. The outcomes from application of these methodologies were related to the evidence of consumers' purchasing behaviour after licensing of the technology of rBST for use in the United States; this introduction did not require labelling. The assessment suggests a critical impact of product labelling policies and strategies on potential market impacts. An assessment is also made of Canada's food safety regulatory framework. The need for increased transparency and greater public participation in regulatory processes as means to increase public confidence in such food safety regulatory processes, specifically relating to biotechnology, is also identified.Food Consumption/Nutrition/Food Safety,

Confidence: Its role in dependability cases for risk assessment

Society is increasingly requiring quantitative assessment of risk and associated dependability cases. Informally, a dependability case comprises some reasoning, based on assumptions and evidence, that supports a dependability claim at a particular level of confidence. In this paper we argue that a quantitative assessment of claim confidence is necessary for proper assessment of risk. We discuss the way in which confidence depends upon uncertainty about the underpinnings of the dependability case (truth of assumptions, correctness of reasoning, strength of evidence), and propose that probability is the appropriate measure of uncertainty. We discuss some of the obstacles to quantitative assessment of confidence (issues of composability of subsystem claims; of the multi-dimensional, multi-attribute nature of dependability claims; of the difficult role played by dependence between different kinds of evidence, assumptions, etc). We show that, even in simple cases, the confidence in a claim arising from a dependability case can be surprisingly low

A methodology for the requirements analysis of critical real-time systems

PhD ThesisThis thesis describes a methodology for the requirements analysis of critical real-time systems. The methodology is based on formal methods, and provides a systematic way in which requirements can be analysed and specifications produced. The proposed methodology consists of a framework with distinct phases of analysis, a set oftechniques appropriate for the issues to be analysed at each phase of the framework, a hierarchical structure of the specifications obtained from the process of analysis, and techniques to perform quality assessment of the specifications. The phases of the framework, which are abstraction levels for the analysis of the requirements, follow directly from a general structure adopted for critical real-time systems. The intention is to define abstraction levels, or domains, in which the analysis of requirements can be performed in terms of specific properties of the system, thus reducing the inherent complexity of the analysis. Depending on the issues to be analysed in each domain, the choice of the appropriate formalism is determined by the set of features, related to that domain, that a formalism should possess. In this work, instead of proposing new formalisms we concentrate on identifying and enumerating those features that a formalism should have. The specifications produced at each phase of the framework are organised by means of a specification hierarchy, which facilitates our assessment of the quality of the requirements specifications, and their traceability. Such an assessment should be performed by qualitative and quantitative means in order to obtain high confidence (assurance) that the level of safety is acceptable. In order to exemplify the proposed methodology for the requirements analysis of critical real-time systems we discuss a case study based on a crossing of two rail tracks (in a model railway), which raises safety issues that are similar to those found at a traditional level crossing (i.e. rail-road)CAPES/Ministry of Education (Brazil

Quantitative Risk-Based Analysis for Military Counterterrorism Systems

The article of record as published may be found at http://dx.doi.org/10.1002/sysThis paper presents a realistic and practical approach to quantitatively assess the risk-reduction capabilities of military counterterrorism systems in terms of damage cost and casualty figures. The comparison of alternatives is thereby based on absolute quantities rather than an aggregated utility or value provided by multicriteria decision analysis methods. The key elements of the approach are (1) the use of decision-attack event trees for modeling and analyzing scenarios, (2) a portfolio model approach for analyzing multiple threats, and (3) the quantitative probabilistic risk assessment matrix for communicating the results. Decision-attack event trees are especially appropriate for modeling and analyzing terrorist attacks where the sequence of events and outcomes are time-sensitive. The actions of the attackers and the defenders are modeled as decisions and the outcomes are modeled as probabilistic events. The quantitative probabilistic risk assessment matrix provides information about the range of the possible outcomes while retaining the simplicity of the classic safety risk assessment matrix based on Mil-Std-882D. It therefore provides a simple and reliable tool for comparing alternatives on the basis of risk including confidence levels rather than single point estimates. This additional valuable information requires minimal additional effort. The proposed approach is illustrated using a simplified but realistic model of a destroyer operating in inland restricted waters. The complex problem of choosing a robust counterterrorism protection system against multiple terrorist threats is analyzed by introducing a surrogate multi-threat portfolio. The associated risk profile provides a practical approach for assessing the robustness of different counterterrorism systems against plausible terrorist threats. The paper documents the analysis for a hypothetical case of three potential threats.This work was performed as part of the Naval Postgraduate School institutionally funded research