3,245 research outputs found

    On optimality of kernels for approximate Bayesian computation using sequential Monte Carlo

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    Approximate Bayesian computation (ABC) has gained popularity over the past few years for the analysis of complex models arising in population genetics, epidemiology and system biology. Sequential Monte Carlo (SMC) approaches have become work-horses in ABC. Here we discuss how to construct the perturbation kernels that are required in ABC SMC approaches, in order to construct a sequence of distributions that start out from a suitably defined prior and converge towards the unknown posterior. We derive optimality criteria for different kernels, which are based on the Kullback-Leibler divergence between a distribution and the distribution of the perturbed particles. We will show that for many complicated posterior distributions, locally adapted kernels tend to show the best performance. We find that the added moderate cost of adapting kernel functions is easily regained in terms of the higher acceptance rate. We demonstrate the computational efficiency gains in a range of toy examples which illustrate some of the challenges faced in real-world applications of ABC, before turning to two demanding parameter inference problems in molecular biology, which highlight the huge increases in efficiency that can be gained from choice of optimal kernels. We conclude with a general discussion of the rational choice of perturbation kernels in ABC SMC settings

    Housing and Urbanization: A Socio-Spatial Analysis of Resettlement Projects in Hồ ChĂ­ Minh City

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    As Hồ ChĂ­ Minh City continues to undergo rapid urbanization, especially with the creation of a multitude of new urban zone developments on the periphery of the inner districts, the resettling of people has become common. Families who live within areas that are selected for urban upgrading or, as in other cases for the construction of new miniature cities, must face the realities of relocation. Many issues arise in the complicated process of resettling the displaced, due to complex land-use laws, bureaucratic dissonance, and lack of investment in actual resettlement housing. The authorities of Hồ ChĂ­ Minh City have faced palpable challenges in facilitating the many processes of resettlement, from persuading developers to invest in resettlement housing to establishing suitable compensation packages. Confusing legal labyrinths, delays in plan approval, and miscommunications between agencies, results in tangible affects on the highly vulnerable displaced families. Additionally, a serious disconnect arises between planners’ envisioned solution for resettlement housing and the real needs of the resettled, who are usually low-income workers. When the precise needs of displaced families and their prior sources of economic livelihood are disregarded, the general result is unsuitable design and the disordering of previously established socio-spatial networks. Additionally the displaced tend to be sent to occupy less advantageous space, as a result of gentrification, and are spatially repositioned in more excluded, disconnected marginal zones. Past and present resettlement procedures have faltered due especially to a lack of socio-spatial planning, which has resulted in undesirable threats to equitable metropolisation and rising potentials for urban fragmentation

    Shielding effectiveness of original and modified building materials

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    This contribution deals with the determination of the shielding effectiveness of building materials used for office, factory and government buildings. Besides the examination of standard materials, measurements were also performed on modified materials, e.g. ferro concrete with enhanced shielding effectiveness due to a changed mixture or structure of the reinforcement. The measurements of original and modified materials were carried out in a fully anechoic room (FAR) according to IEEE 299-1997 from 80 MHz up to 10 GHz
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