15,618 research outputs found
Multivariate Hierarchical Frameworks for Modelling Delayed Reporting in Count Data
In many fields and applications count data can be subject to delayed
reporting. This is where the total count, such as the number of disease cases
contracted in a given week, may not be immediately available, instead arriving
in parts over time. For short term decision making, the statistical challenge
lies in predicting the total count based on any observed partial counts, along
with a robust quantification of uncertainty. In this article we discuss
previous approaches to modelling delayed reporting and present a multivariate
hierarchical framework where the count generating process and delay mechanism
are modelled simultaneously. Unlike other approaches, the framework can also be
easily adapted to allow for the presence of under-reporting in the final
observed count. To compare our approach with existing frameworks, one of which
we extend to potentially improve predictive performance, we present a case
study of reported dengue fever cases in Rio de Janeiro. Based on both
within-sample and out-of-sample posterior predictive model checking and
arguments of interpretability, adaptability, and computational efficiency, we
discuss the advantages and disadvantages of each modelling framework.Comment: Biometrics (2019
A theoretical and semiemprical correction to the long-range dispersion power law of stretched graphite
In recent years intercalated and pillared graphitic systems have come under
increasing scrutiny because of their potential for modern energy technologies.
While traditional \emph{ab initio} methods such as the LDA give accurate
geometries for graphite they are poorer at predicting physicial properties such
as cohesive energies and elastic constants perpendicular to the layers because
of the strong dependence on long-range dispersion forces. `Stretching' the
layers via pillars or intercalation further highlights these weaknesses. We use
the ideas developed by [J. F. Dobson et al, Phys. Rev. Lett. {\bf 96}, 073201
(2006)] as a starting point to show that the asymptotic dependence
of the cohesive energy on layer spacing in bigraphene is universal to all
graphitic systems with evenly spaced layers. At spacings appropriate to
intercalates, this differs from and begins to dominate the power
law for dispersion that has been widely used previously. The corrected power
law (and a calculated coefficient) is then unsuccesfully employed in the
semiempirical approach of [M. Hasegawa and K. Nishidate, Phys. Rev. B {\bf 70},
205431 (2004)] (HN). A modified, physicially motivated semiempirical method
including some effects allows the HN method to be used
successfully and gives an absolute increase of about to the predicted
cohesive energy, while still maintaining the correct asymptotics
Deriving Information Requirements from Responsibility Models
This paper describes research in understanding the requirements for complex information systems that are constructed from one or more generic COTS systems. We argue that, in these cases, behavioural requirements are largely defined by the underlying system and that the goal of the requirements engineering process is to understand the information requirements of system stakeholders. We discuss this notion of information requirements and propose that an understanding of how a socio-technical system is structured in terms of responsibilities is an effective way of discovering this type of requirement. We introduce the idea of responsibility modelling and show, using an example drawn from the domain of emergency planning, how a responsibility model can be used to derive information requirements for a system that coordinates the multiple agencies dealing with an emergency
Nonuniversality of the dispersion interaction: analytic benchmarks for van der Waals energy functionals
We highlight the non-universality of the asymptotic behavior of dispersion
forces, such that a sum of inverse sixth power contributions is often
inadequate. We analytically evaluate the cross-correlation energy Ec between
two pi-conjugated layers separated by a large distance D within the
electromagnetically non-retarded Random Phase Approximation, via a
tight-binding model. For two perfect semimetallic graphene sheets at T=0K we
find Ec = C D^{-3}, in contrast to the "insulating" D^{-4} dependence predicted
by currently accepted approximations. We also treat the case where one graphene
layer is replaced by a thin metal, a model relevant to the exfoliation of
graphite. Our general considerations also apply to nanotubes, nanowires and
layered metals.Comment: 4 pages, 0 fig
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
Tracking endogenous and grafted neural progenitor cells in normal and ischaemic brains using MRI contrast agents and genetic labelling
Cerebral ischaemia is a major cause of mortality and morbidity globally. Neural stem and
progenitor cells (NPC) have the potential to contribute to brain repair and regeneration after an
ischaemic event. Both endogenous and grafted NPC have been shown to migrate towards the
ischaemic lesion, and differentiate into neurons. This thesis investigates methods of labeling and
tracking the migration neural progenitor cells to a site of cerebral ischaemic injury, using magnetic
resonance imaging (MRI) contrast agents and transgenic lineage tracing techniques.
First, labeling of exogenous NPC populations was investigated, for use in cell tracking in grafting
studies. Cell labeling was optimized in vitro with fetal NPC using the iron oxide-based MRI
contrast agent. A labeling method was developed using the FePro contrast agent, which
maximized iron oxide uptake, was non-toxic to NPC, and did not interfere with NPC
proliferation and differentiation. Labelled cells were then grafted into the brain after cerebral
ischaemia, and imaged over four weeks using MRI. NPC migration was not observed in vivo, but
an endogenous contrast evolved over time within the lesioned tissue, which presented a source of
confounding signal for cell tracking. Endogenous ferric iron was observed in the lesion on
histological sections. Several limitations of using MRI-based iron oxide contrast agents were
highlighted in this study. To circumvent these limitations, we considered the development of
gadolinium-based MRI contrast agents for cellular labeling and tracking, in collaboration with
Imperial College chemistry department. Polymeric Gd-DOTA chelates were synthesized and
designed for maximal r1 relaxivity, and their relaxivity and effects on cell viability were assessed.
Through this approach, we identified a number of candidate polymeric Gd-DOTA chelates with
high relaxivity and low cytotoxicity for use in cellular imaging and tracking studies.
Next, cell tracking of endogenous NPC was investigated, using MRI contrast agent and transgenic
lineage tracing approaches. A method of in situ labeling of endogenous NPC with the MRI
contrast agent FePro was developed. NPC were labeled with FePro in situ, and their normal
migration to the olfactory bulb, where they contribute to neurogenesis, could be imaged in vivo
and ex vivo. In a second study, the migration of NPC constitutively expressing green fluorescent
protein (GPF) under the promoters of genes of two developmentally distinct cortical and striatal
NPC populations, was investigated following cerebral ischaemia. Both cortical and striatal
populations of NPC were observed to contribute to the migrating streams of NPC that were
observed in the striatum after five weeks post-ischaemia.
These studies demonstrate that MRI contrast agents offer the potential for in vivo, longitudinal
tracking of NPC migration, in both grafted and endogenous NPC populations. Coupled with
transgenic lineage tracing, and used in animal models of CNS injury such as cerebral ischaemia,
labeling and tracking the migration of NSC with MRI contrast agents can contribute to our
understanding of NPC biology in pathological environments
Synthesis of cubic diamond in the graphite-magnesium carbonate and graphite-K2Mg(CO3)(2) systems at high pressure of 9-10 GPa region
Cubic diamond was synthesized with two systems, (1) graphite with pure magnesium carbonate (magnesite) and (2) graphite with mixed potassium and magnesium carbonate at pressures and temperatures above 9.5 GPa, 1600 degrees C and 9 GPa, 1650 degrees C, respectively. At these conditions (1) the pure magnesite is solid, whereas (2) the mixed carbonate exists as a melt. In this pressure range, graphite seems to be partially transformed into hexagonal diamond. Measured carbon isotope delta(13)C values for all the materials suggest that the origin of the carbon source to form cubic diamond was the initial graphite powder, and not the carbonates
Mesoscale monitoring of the soil freeze/thaw boundary from orbital microwave radiometry
The fundamental objectives are to test the feasibility of delineating the lateral boundary between frozen and thawed condition in the surface layer of soil from orbital microwave radiometry and secondly to examine the sensitivity of general circulation models to an explicit parameterization of the boundary condition. Physical models were developed to relate emissivity to scene properties and a simulation package was developed to predict brightness temperature as a function of emissivity and physical temperature in order to address issues of heterogeneity, scaling, and scene dynamics. Radiative transfer models were develped for both bare soil surfaces and those obscured by an intervening layer of vegetation or snow. These models relate the emissivity to the physical properties of the soil and to those of the snow or vegetation cover. A SMMR simulation package was developed to evaluate the adequacy of the emission models and the limiting effects of scaling for realistic scenarios incorporating spatially heterogeneous scenes with dynamic moisture and temperature gradients at the pixel scale
Kohn-Sham Exchange Potential for a Metallic Surface
The behavior of the surface barrier that forms at the metal-vacuum interface
is important for several fields of surface science. Within the Density
Functional Theory framework, this surface barrier has two non-trivial
components: exchange and correlation. Exact results are provided for the
exchange component, for a jellium metal-vacuum interface, in a slab geometry.
The Kohn-Sham exact-exchange potential has been generated by using
the Optimized Effective Potential method, through an accurate numerical
solution, imposing the correct boundary condition. It has been proved
analytically, and confirmed numerically, that ; this conclusion is not affected by the inclusion of correlation
effects. Also, the exact-exchange potential develops a shoulder-like structure
close to the interface, on the vacuum side. The issue of the classical image
potential is discussed.Comment: Phys. Rev. Lett. (to appear
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