9,905 research outputs found
Multiscale inference for a multivariate density with applications to X-ray astronomy
In this paper we propose methods for inference of the geometric features of a
multivariate density. Our approach uses multiscale tests for the monotonicity
of the density at arbitrary points in arbitrary directions. In particular, a
significance test for a mode at a specific point is constructed. Moreover, we
develop multiscale methods for identifying regions of monotonicity and a
general procedure for detecting the modes of a multivariate density. It is is
shown that the latter method localizes the modes with an effectively optimal
rate. The theoretical results are illustrated by means of a simulation study
and a data example. The new method is applied to and motivated by the
determination and verification of the position of high-energy sources from
X-ray observations by the Swift satellite which is important for a
multiwavelength analysis of objects such as Active Galactic Nuclei.Comment: Keywords and Phrases: multiple tests, modes, multivariate density,
X-ray astronomy AMS Subject Classification: 62G07, 62G10, 62G2
Modelling Joint Development of Light Rail Transit Stations and Land Use - The Case of Tel-Aviv
Light Rail Transit (LRT) has been gaining popularity as a means of decreasing private automobile dependency and thus reducing car pollutants, relieving congestion and enhancing community liveability. LRT is also perceived as an important generator of economic growth, mainly in old urban centers. Through the improvement of accessibility to CBDs (Central Business Districts) planners and decision makers expect to revitalize central cities' vis-a-vis the increasing competition from the growing suburban shopping malls. More specifically, the objective of this paper is to explore the complex relationship between transportation and land use by analyzing the optimal composition of land use around the proposed light rail stations. Density and diversity are the two most important characteristics of urban land use development. We examine changes in land use adjacent to the LRT stations in metropolitan Tel-Aviv, and their impact on the demand for total travel in particular. These changes include hypothetical scenarios of alternative land use compositions, densities and intensities of residential, employment, and commercial land uses. In order to measure the impact of these changes on travel, a demand model is calibrated. The traditional four-step transportation model is retrofitted with alternative land use density and diversity variables. Among these are: residential density, job-population balance etc. As such, the reÂŹstructured model is more sensitive to the different hypothetical land use scenarios and is expected to predict ridership demand changes more accurately. The results have shown that some of the land use variables are extremely important for trip generation trends forecasts, especially trip attraction trends. Furthermore, the simulations of the various land use policies are able to display the spatial reaction of trip rates to land use function, density, degree of mix, and household characteristics. The results of this study could serve to better assess urban transportation ridership demands, especially since they serve as input for mode choice analyses. Moreover, by exploring this subject even further, planners and decision makers will be able to attain a clearer and more comprehensive picture of optimal land use patterns surrounding station areas, and in doing so, improving the quality of life of urban dwellers, commuters and visitors.
Bayesian Repulsive Gaussian Mixture Model
We develop a general class of Bayesian repulsive Gaussian mixture models that
encourage well-separated clusters, aiming at reducing potentially redundant
components produced by independent priors for locations (such as the Dirichlet
process). The asymptotic results for the posterior distribution of the proposed
models are derived, including posterior consistency and posterior contraction
rate in the context of nonparametric density estimation. More importantly, we
show that compared to the independent prior on the component centers, the
repulsive prior introduces additional shrinkage effect on the tail probability
of the posterior number of components, which serves as a measurement of the
model complexity. In addition, an efficient and easy-to-implement
blocked-collapsed Gibbs sampler is developed based on the exchangeable
partition distribution and the corresponding urn model. We evaluate the
performance and demonstrate the advantages of the proposed model through
extensive simulation studies and real data analysis. The R code is available at
https://drive.google.com/open?id=0B_zFse0eqxBHZnF5cEhsUFk0cVE
Development of an Analytic Nodal Diffusion Solver in Multigroups for 3D Reactor Cores with Rectangular or Hexagonal Assemblies.
More accurate modelling of physical phenomena involved in present and future nuclear reactors requires a multi-scale and multi-physics approach. This challenge can be accomplished by the coupling of best-estimate core-physics, thermal-hydraulics and multi-physics solvers. In order to make viable that coupling, the current trends in reactor simulations are along the development of a new generation of tools based on user-friendly, modular, easily linkable, faster and more accurate codes to be integrated in common platforms. These premises are in the origin of the NURESIM Integrated Project within the 6th European Framework Program, which is envisaged to provide the initial step towards a Common European Standard Software Platform for nuclear reactors simulations. In the frame of this project and to reach the above-mentioned goals, a 3-D multigroup nodal solver for neutron diffusion calculations called ANDES (Analytic Nodal Diffusion Equation Solver) has been developed and tested in-depth in this Thesis. ANDES solves the steady-state and time-dependent neutron diffusion equation in threedimensions and any number of energy groups, utilizing the Analytic Coarse-Mesh Finite-Difference (ACMFD) scheme to yield the nodal coupling equations. It can be applied to both Cartesian and triangular-Z geometries, so that simulations of LWR as well as VVER, HTR and fast reactors can be performed. The solver has been implemented in a fully encapsulated way, enabling it as a module to be readily integrated in other codes and platforms. In fact, it can be used either as a stand-alone nodal code or as a solver to accelerate the convergence of whole core pin-by-pin code systems. Verification of performance has shown that ANDES is a code with high order definition for whole core realistic nodal simulations. In this paper, the methodology developed and involved in ANDES is presented
Flow Logic
Flow networks have attracted a lot of research in computer science. Indeed,
many questions in numerous application areas can be reduced to questions about
flow networks. Many of these applications would benefit from a framework in
which one can formally reason about properties of flow networks that go beyond
their maximal flow. We introduce Flow Logics: modal logics that treat flow
functions as explicit first-order objects and enable the specification of rich
properties of flow networks. The syntax of our logic BFL* (Branching Flow
Logic) is similar to the syntax of the temporal logic CTL*, except that atomic
assertions may be flow propositions, like or , for
, which refer to the value of the flow in a vertex, and
that first-order quantification can be applied both to paths and to flow
functions. We present an exhaustive study of the theoretical and practical
aspects of BFL*, as well as extensions and fragments of it. Our extensions
include flow quantifications that range over non-integral flow functions or
over maximal flow functions, path quantification that ranges over paths along
which non-zero flow travels, past operators, and first-order quantification of
flow values. We focus on the model-checking problem and show that it is
PSPACE-complete, as it is for CTL*. Handling of flow quantifiers, however,
increases the complexity in terms of the network to , even
for the LFL and BFL fragments, which are the flow-counterparts of LTL and CTL.
We are still able to point to a useful fragment of BFL* for which the
model-checking problem can be solved in polynomial time. Finally, we introduce
and study the query-checking problem for BFL*, where under-specified BFL*
formulas are used for network exploration
Clustering South African households based on their asset status using latent variable models
The Agincourt Health and Demographic Surveillance System has since 2001
conducted a biannual household asset survey in order to quantify household
socio-economic status (SES) in a rural population living in northeast South
Africa. The survey contains binary, ordinal and nominal items. In the absence
of income or expenditure data, the SES landscape in the study population is
explored and described by clustering the households into homogeneous groups
based on their asset status. A model-based approach to clustering the Agincourt
households, based on latent variable models, is proposed. In the case of
modeling binary or ordinal items, item response theory models are employed. For
nominal survey items, a factor analysis model, similar in nature to a
multinomial probit model, is used. Both model types have an underlying latent
variable structure - this similarity is exploited and the models are combined
to produce a hybrid model capable of handling mixed data types. Further, a
mixture of the hybrid models is considered to provide clustering capabilities
within the context of mixed binary, ordinal and nominal response data. The
proposed model is termed a mixture of factor analyzers for mixed data (MFA-MD).
The MFA-MD model is applied to the survey data to cluster the Agincourt
households into homogeneous groups. The model is estimated within the Bayesian
paradigm, using a Markov chain Monte Carlo algorithm. Intuitive groupings
result, providing insight to the different socio-economic strata within the
Agincourt region.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS726 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Smallpox did reduce height : a reply to our critics.
Between them our critics span the entire range of this Journalâs readership. On the one hand Razzell accuses us of âthe abandonment of traditional scholarly proceduresâ. He argues that our plight âwill provide a salutary lesson for the new economic history. No amount of sophisticated statistical analysis will provide a substitute for careful study of original sources.â In contrast, Heintel and Baten use far more sophisticated statistical techniques - including a continuous kernel density estimator and truncation point estimators - in an attempt to justify their claim that our âconclusions are without empirical or statistical foundation.â Because these two comments are so totally different we will look at each in turn.
The circular SiZer, inferred persistence of shape parameters and application to early stem cell differentiation
We generalize the SiZer of Chaudhuri and Marron (J. Amer. Statist. Assoc. 94
(1999) 807-823, Ann. Statist. 28 (2000) 408-428) for the detection of shape
parameters of densities on the real line to the case of circular data. It turns
out that only the wrapped Gaussian kernel gives a symmetric, strongly Lipschitz
semi-group satisfying "circular" causality, that is, not introducing possibly
artificial modes with increasing levels of smoothing. Some notable differences
between Euclidean and circular scale space theory are highlighted. Based on
this, we provide an asymptotic theory to make inference about the persistence
of shape features. The resulting circular mode persistence diagram is applied
to the analysis of early mechanically-induced differentiation in adult human
stem cells from their actin-myosin filament structure. As a consequence, the
circular SiZer based on the wrapped Gaussian kernel (WiZer) allows the
verification at a controlled error level of the observation reported by Zemel
et al. (Nat. Phys. 6 (2010) 468-473): Within early stem cell differentiation,
polarizations of stem cells exhibit preferred directions in three different
micro-environments.Comment: Published at http://dx.doi.org/10.3150/15-BEJ722 in the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
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