10,759 research outputs found
Estimating the causal effect of a time-varying treatment on time-to-event using structural nested failure time models
In this paper we review an approach to estimating the causal effect of a
time-varying treatment on time to some event of interest. This approach is
designed for the situation where the treatment may have been repeatedly adapted
to patient characteristics, which themselves may also be time-dependent. In
this situation the effect of the treatment cannot simply be estimated by
conditioning on the patient characteristics, as these may themselves be
indicators of the treatment effect. This so-called time-dependent confounding
is typical in observational studies. We discuss a new class of failure time
models, structural nested failure time models, which can be used to estimate
the causal effect of a time-varying treatment, and present methods for
estimating and testing the parameters of these models
Reducing Prawn-trawl Bycatch in Australia: An Overview and an Example from Queensland
Prawn trawling occurs in most states of Australia in tropical, subtropical, and temperate waters. Bycatch occurs
to some degree in all Australian trawl fisheries, and there is pressure to reduce the levels of trawl fishery bycatch. This paper gives a brief overview of the bycatch issues and
technological solutions that have been evaluated or adopted in Australian prawn-trawl fi sheries. Turtle excluder devices (TEDâs) and bycatch reduction devices (BRDâs) are
the principal solutions to bycatch in Australian prawn-trawl fisheries. This paper focuses on a major prawn-trawl fishery of northeastern Australia, and the results of
commercial use of TEDâs and BRDâs in the Queensland east coast trawl fishery are presented. New industry designs are
described, and the status of TED and BRD adoption and regulation is summarized. The implementation of technological solutions to reduce fishery bycatch is assumed generally to assist prawn-trawl fisheries within
Australia in achieving legislative requirements for minimal environmental impact and ecological sustainable development
The SADC Groundwater Data and Information Archive, Knowledge Sharing and Co-operation Project. Final report
The Southern African Development Community (SADC) Groundwater Data and Information
Archive, Knowledge Sharing and Co-operation Project, funded by the German Development
Cooperation (GIZ) and Department for International Development, UK (DFID), was initiated in
September 2009 to identify, catalogue and subsequently promote access to the large collection of
reports held in the UK by the British Geological Survey (BGS). The work has focused on a
wealth of unpublished so-called âgreyâ data and information which describes groundwater
occurrence and development in Southern Africa and was gathered by the BGS over its many
decades of involvement in the region.
The project has four main aims:
To catalogue and describe the "grey data" documents on SADC groundwater held by the
BGS within a digital metadatabase.
To identify a sub-set of scanned documents to be made freely available to groundwater
practitioners and managers in the SADC region by electronic distribution.
To link the metadatabase and digital sub-set of documents via a web portal hosted by the
BGS, to enable download of documents by SADC groundwater workers.
To strengthen links between BGS hydrogeologists with counterparts in SADC, and
provide an example of groundwater data sharing which could be emulated by other
European Geological Surveys with substantial data holdings on SADC groundwater.
The project has successfully met these aims. The assessment of BGS archived material produced
an electronic meta-database describing 1735 items held in hard copy. Of these, 1041 have been
scanned digitally to searchable Portable Document Format (PDF) format. A subset of 655 PDFs
including partial documents related to groundwater development from the colonial and post
independence period as well as BGS internal project reports and reports approved for web
dissemination by host countries are now available to download (free of charge) at
http://www.SADCgroundwaterarchive.com . Initial results indicate a good deal of interest both
from within SADC and elsewhere, accessed by directly addressing the website and via a search
engine such as Google. The information presented has already been used by in-region projects
such as the SADC Hydrogeological Mapping project and the Malawi Water Assessment Project.
This is essentially a pilot project providing an example of how Web delivery of the archive is an
important step forward for the well-being of the SADC region. It permits access to documents
few even new existed and will, it is hoped, provide a valuable dataset that should inhibit the
temptation to waste scarce resources by âre-inventing the wheelâ
Groundwater dependence and drought within the southern African development community
A groundwater situation analysis of the SADC region has been undertaken as part of the World Bank GEF Programme as a basis for ensuring equitable use of groundwater resources, particularly during periods of drought, both for human needs and for sustaining ecosystems. Much of the groundwater in the region occurs in weathered crystalline rocks suitable for dispersed supply to rural communities, although there are several aquifers capable of sustaining urban demand that contribute to the supply of several major cities and towns. A number of SADC Member States, such as Botswana, Namibia and South Africa, are very dependent on groundwater, whereas the Democratic Republic of Congo is least dependent. Groundwater dependence and groundwater demand, together providing an indication of drought vulnerability, have been assessed from the availability and coverage of groundwater data, but it is very apparent that reliable and comprehensive groundwater data are major deficiencies throughout the SADC region. Few attempts have thus been made to calculate renewable groundwater resource volumes or develop optimum use of groundwater, despite the fact that susceptibility of many Member States to drought requires them to consider mitigation strategies to lessen the hardships imposed largely on their rural population. Such strategy requires long-term intervention and not short-term emergency responses, a process that is directly related to availability of comprehensive groundwater datasets. Considerable effort in groundwater assessment and monitoring and the accumulation, evaluation and dissemination of essential datasets will thus be required to maintain population livelihoods in future years when water supply is projected to be in deficit in over half of the SADC Member States
Holography in Superspace
The AdS/CFT correspondence identifies the coordinates of the conformal
boundary of anti-de Sitter space with the coordinates of the conformal field
theory. We generalize this identification to theories formulated in superspace.
As an application of our results, we study a class of Wilson loops in N=4 SYM
theory. A gauge theory computation shows that the expectation values of these
loops are invariant under a local kappa-symmetry, except at intersections. We
identify this with the kappa-invariance of the associated string worldsheets in
the corresponding bulk superspace.Comment: 36 pages, LaTeX, references adde
Stability of continuously pumped atom lasers
A multimode model of a continuously pumped atom laser is shown to be unstable
below a critical value of the scattering length. Above the critical scattering
length, the atom laser reaches a steady state, the stability of which increases
with pumping. Below this limit the laser does not reach a steady state. This
instability results from the competition between gain and loss for the excited
states of the lasing mode. It will determine a fundamental limit for the
linewidth of an atom laser beam.Comment: 4 page
Betti number signatures of homogeneous Poisson point processes
The Betti numbers are fundamental topological quantities that describe the
k-dimensional connectivity of an object: B_0 is the number of connected
components and B_k effectively counts the number of k-dimensional holes.
Although they are appealing natural descriptors of shape, the higher-order
Betti numbers are more difficult to compute than other measures and so have not
previously been studied per se in the context of stochastic geometry or
statistical physics.
As a mathematically tractable model, we consider the expected Betti numbers
per unit volume of Poisson-centred spheres with radius alpha. We present
results from simulations and derive analytic expressions for the low intensity,
small radius limits of Betti numbers in one, two, and three dimensions. The
algorithms and analysis depend on alpha-shapes, a construction from
computational geometry that deserves to be more widely known in the physics
community.Comment: Submitted to PRE. 11 pages, 10 figure
Determination of Body Density of College-Age Women using Circumference Measurements
The purpose of this investigation was to determine an effective and simple method of estimating body density of college-age women (n=51), ages 18 to 26 years, using age and selected circumference measurements (thorax, waist, abdomen, hip, gluteal fold and thigh). Two subproblems were also investigated. The first subproblem was to determine if the sum of the selected circumferences was a better predictor of body density than when each circumference was used individually as a predictor. The second subproblem was to determine if body density could best be predicted by a linear or curvilinear relationship. Statistical analysis of the data included descriptive statistics for the subjects and multiple regression analyses to determine the best predictors of body density from circumferences. lntraclass correlations were computed to determine test-retest reliability of the data collected. A regression equation using four circumferences (thorax, abdomen, gluteal fold and thigh) was derived to predict body density (R 2=.65). Cross-validation was employed to determine the validity of the regression equation. The four independent circumferences (thorax, abdomen, gluteal fold and thigh) were significant (p\u3c.05) in their prediction of body density and the inclusion of five or all six did not add significantly to the prediction of body density (p\u3e.05). The sums of the circumferences, the square of these sums, the square of the independent circumferences and age were not significant predictors of body density (p\u3e.05). The data of the validation and cross-validation samples were highly correlated with the actual body density values for each respective sample (r=.70 and r=. 73, respectively)
Pulsed pumping of a Bose-Einstein condensate
In this work, we examine a system for coherent transfer of atoms into a
Bose-Einstein condensate. We utilize two spatially separate Bose-Einstein
condensates in different hyperfine ground states held in the same dc magnetic
trap. By means of a pulsed transfer of atoms, we are able to show a clear
resonance in the timing of the transfer, both in temperature and number, from
which we draw conclusions about the underlying physical process. The results
are discussed in the context of the recently demonstrated pumped atom laser.Comment: 5 pages, 5 figures, published in Physical Review
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