375 research outputs found
Multilevel ensemble Kalman filtering
This work embeds a multilevel Monte Carlo sampling strategy into the Monte
Carlo step of the ensemble Kalman filter (EnKF) in the setting of finite
dimensional signal evolution and noisy discrete-time observations. The signal
dynamics is assumed to be governed by a stochastic differential equation (SDE),
and a hierarchy of time grids is introduced for multilevel numerical
integration of that SDE. The resulting multilevel EnKF is proved to
asymptotically outperform EnKF in terms of computational cost versus
approximation accuracy. The theoretical results are illustrated numerically
Toward Better Understanding on How Group A <em>Streptococcus</em> Manipulates Human Fibrinolytic System
Group A Streptococcus pyogenes (GAS) is a human pathogen that commonly causes superficial infections such as pharyngitis, but can also lead to systemic and fatal diseases. GAS infection remains to be a major threat in regions with insufficient medical infrastructures, leading to half a million deaths annually worldwide. The pathogenesis of GAS is mediated by a number of virulence factors, which function to facilitate bacterial colonization, immune evasion, and deep tissue invasion. In this review, we will discuss the mechanism of molecular interaction between the host protein and virulence factors that target the fibrinolytic system, including streptokinase (SK), plasminogen-binding group A streptococcal M-like protein (PAM), and streptococcal inhibitor of complement (SIC). We will discuss our current understanding, through structural studies, on how these proteins manipulate the fibrinolytic system during infection
Gastric cancer and Helicobacter pylori: a combined analysis of 12 case control studies nested within prospective cohorts
BACKGROUND: The magnitude of the association
between Helicobacter pylori and
incidence of gastric cancer is unclear. H
pylori infection and the circulating antibody
response can be lost with development
of cancer; thus retrospective studies
are subject to bias resulting from classifi-
cation of cases as H pylori negative when
they were infected in the past.
AIMS: To combine data from all case control
studies nested within prospective
cohorts to assess more reliably the relative
risk of gastric cancer associated with H
pylori infection.To investigate variation in
relative risk by age, sex, cancer type and
subsite, and interval between blood sampling
and cancer diagnosis.
METHODS: Studies were eligible if blood
samples for H pylori serology were collected
before diagnosis of gastric cancer in
cases. Identified published studies and two
unpublished studies were included. Individual
subject data were obtained for
each. Matched odds ratios (ORs) and 95%
confidence intervals (95% CI) were calculated
for the association between H pylori
and gastric cancer.
RESULTS: Twelve studies with 1228 gastric
cancer cases were considered. The association
with H pylori was restricted to noncardia
cancers (OR 3.0; 95% CI 2.3–3.8)
and was stronger when blood samples for
H pylori serology were collected 10+ years
before cancer diagnosis (5.9; 3.4–10.3). H
pylori infection was not associated with an
altered overall risk of cardia cancer (1.0;
0.7–1.4).
CONCLUSIONS: These results suggest that
5.9 is the best estimate of the relative risk
of non-cardia cancer associated with H
pylori infection and that H pylori does not
increase the risk of cardia cancer. They
also support the idea that when H pylori
status is assessed close to cancer diagnosis,
the magnitude of the non-cardia
association may be underestimated
A jump-growth model for predator-prey dynamics: derivation and application to marine ecosystems
This paper investigates the dynamics of biomass in a marine ecosystem. A
stochastic process is defined in which organisms undergo jumps in body size as
they catch and eat smaller organisms. Using a systematic expansion of the
master equation, we derive a deterministic equation for the macroscopic
dynamics, which we call the deterministic jump-growth equation, and a linear
Fokker-Planck equation for the stochastic fluctuations. The McKendrick--von
Foerster equation, used in previous studies, is shown to be a first-order
approximation, appropriate in equilibrium systems where predators are much
larger than their prey. The model has a power-law steady state consistent with
the approximate constancy of mass density in logarithmic intervals of body mass
often observed in marine ecosystems. The behaviours of the stochastic process,
the deterministic jump-growth equation and the McKendrick--von Foerster
equation are compared using numerical methods. The numerical analysis shows two
classes of attractors: steady states and travelling waves.Comment: 27 pages, 4 figures. Final version as published. Only minor change
MATHICSE Technical Report : Multilevel ensemble Kalman filtering for spatio-temporal processes
This work concerns state-space models, in which the state-space is an infinite-dimensional spatial field, and the evolution is in continuous time, hence requiring approximation in space and time. The multilevel Monte Carlo (MLMC) sampling strategy is leveraged in the Monte Carlo step of the ensemble Kalman filter (EnKF), thereby yielding a multilevel ensemble Kalman filter (MLEnKF) for spatio-temporal models, which has provably superior as- ymptotic error/cost ratio. A practically relevant stochastic partial differential equation (SPDE) example is presented, and numerical experiments with this example support our theoretical findings
Observations and assessment of forest carbon dynamics following disturbance in North America
Disturbance processes of various types substantially modify ecosystem carbon dynamics both temporally and spatially, and constitute a fundamental part of larger landscape-level dynamics. Forests typically lose carbon for several years to several decades following severe disturbance, but our understanding of the duration and dynamics of post-disturbance forest carbon fluxes remains limited. Here we capitalize on a recent North American Carbon Program disturbance synthesis to discuss techniques and future work needed to better understand carbon dynamics after forest disturbance. Specifically, this paper addresses three topics: (1) the history, spatial distribution, and characteristics of different types of disturbance (in particular fire, insects, and harvest) in North America; (2) the integrated measurements and experimental designs required to quantify forest carbon dynamics in the years and decades after disturbance, as presented in a series of case studies; and (3) a synthesis of the greatest uncertainties spanning these studies, as well as the utility of multiple types of observations (independent but mutually constraining data) in understanding their dynamics. The case studies—in the southeast U.S., central boreal Canada, U.S. Rocky Mountains, and Pacific Northwest—explore how different measurements can be used to constrain and understand carbon dynamics in regrowing forests, with the most important measurements summarized for each disturbance type. We identify disturbance severity and history as key but highly uncertain factors driving post-disturbance carbon source-sink dynamics across all disturbance types. We suggest that imaginative, integrative analyses using multiple lines of evidence, increased measurement capabilities, shared models and online data sets, and innovative numerical algorithms hold promise for improved understanding and prediction of carbon dynamics in disturbance-prone forests
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
ETICA Workshop on Computer Ethics: Exploring Normative Issues
The ETICA project aims to identify emerging information and communication technologies. These technologies are then analysed and evaluated from an ethical perspective. The aim of this analysis is to suggest possible governance arrangements that will allow paying proactive attention to such ethical issues. During the ETICA workshop at the summer school, participants were asked to choose one of the 11 technologies that ETICA had identified. For each of these technologies there was a detailed description developed by work package 1 of the project. Workshop participants were asked to reflect on the ethical issues they saw as relevant and likely to arise from the technology. This paper discusses the ethical views of the workshop participants and contrasts them with the findings of the ethical analysis within the ETICA project
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