52,824 research outputs found
Techniques for the Fast Simulation of Models of Highly dependable Systems
With the ever-increasing complexity and requirements of highly dependable systems, their evaluation during design and operation is becoming more crucial. Realistic models of such systems are often not amenable to analysis using conventional analytic or numerical methods. Therefore, analysts and designers turn to simulation to evaluate these models. However, accurate estimation of dependability measures of these models requires that the simulation frequently observes system failures, which are rare events in highly dependable systems. This renders ordinary Simulation impractical for evaluating such systems. To overcome this problem, simulation techniques based on importance sampling have been developed, and are very effective in certain settings. When importance sampling works well, simulation run lengths can be reduced by several orders of magnitude when estimating transient as well as steady-state dependability measures. This paper reviews some of the importance-sampling techniques that have been developed in recent years to estimate dependability measures efficiently in Markov and nonMarkov models of highly dependable system
Guidelines for the scoping and environmental assessment of water resources projects. The environment and water resources projects - Volume 2
In its role as protector of the water environment, the Environment Agency requires significant water resources abstraction applications and schemes such as drought orders,
drought permits, time limited licences, and river transfers to be environmentally assessed leading to the production of an environmental report or statement. This may not take the
form of a formal Environmental Assessment, but is required to provide environmental information to support applications. (See Volume 1 - Guidance for Scoping and
Environmental Assessment for Water Resources Projects in North East Region). This second volume concentrates on the environmental monitoring component of environmental assessments
Efficient simulation of large deviation events for sums of random vectors using saddle-point representations
We consider the problem of efficient simulation estimation of the
density function at the tails, and the probability of large
deviations for a sum of independent, identically distributed (i.i.d.),
light-tailed and nonlattice random vectors. The latter problem
besides being of independent interest, also forms a building block
for more complex rare event problems that arise, for instance, in
queuing and financial credit risk modeling. It has been extensively
studied in the literature where state-independent, exponential-twisting-based
importance sampling has been shown to be asymptotically
efficient and a more nuanced state-dependent exponential twisting
has been shown to have a stronger bounded relative error property.
We exploit the saddle-point-based representations that exist for
these rare quantities, which rely on inverting the characteristic
functions of the underlying random vectors. These representations
reduce the rare event estimation problem to evaluating certain
integrals, which may via importance sampling be represented as
expectations. Furthermore, it is easy to identify and approximate the
zero-variance importance sampling distribution to estimate these
integrals. We identify such importance sampling measures and show
that they possess the asymptotically vanishing relative error
property that is stronger than the bounded relative error
property. To illustrate the broader applicability of the proposed
methodology, we extend it to develop an asymptotically vanishing
relative error estimator for the practically important expected
overshoot of sums of i.i.d. random variables
Small-body deflection techniques using spacecraft: techniques in simulating the fate of ejecta
We define a set of procedures to numerically study the fate of ejecta
produced by the impact of an artificial projectile with the aim of deflecting
an asteroid. Here we develop a simplified, idealized model of impact conditions
that can be adapted to fit the details of specific deflection-test scenarios,
such as what is being proposed for the AIDA project. Ongoing studies based upon
the methodology described here can be used to inform observational strategies
and safety conditions for an observing spacecraft. To account for ejecta
evolution, the numerical strategies we are employing are varied and include a
large N-Body component, a smoothed-particle hydrodynamics (SPH) component, and
an application of impactor scaling laws. Simulations that use SPH-derived
initial conditions show high-speed ejecta escaping at low angles of
inclination, and very slowly moving ejecta lofting off the surface at higher
inclination angles, some of which re-impacts the small-body surface. We are
currently investigating the realism of this and other models' behaviors. Next
steps will include the addition of solar perturbations to the model and
applying the protocol developed here directly to specific potential mission
concepts such as the proposed AIDA scenario.Comment: 19 pages, 11 figures, accepted for publication in Advances in Space
Research, Special Issue: Asteroids & Space Debri
A comparative assessment of methodologies used to evaluate competition policy
Research by academics and competition agencies on evaluating competition policy has grown rapidly during the last two decades. This paper surveys the literature in order to (i) assess the fitness for purpose of the main quantitative methodologies employed, and (ii) identify the main undeveloped areas and unanswered questions for future research. It suggests that policy evaluation is necessarily an imprecise science and that all existing methodologies have strengths and limitations. The areas where the need is most pressing for further work include: understanding why Article 102 cases are only infrequently evaluated; the need to bring conscious discussion of the counterfactual firmly into the foreground; a wider definition of policy to include success in deterrence and detection. At the heart of the discussion is the impact of selection bias on most aspects of evaluation. These topics are the focus of ongoing work in the CCP
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