73,069 research outputs found
Analysis of Partially Observed Networks via Exponential-family Random Network Models
Exponential-family random network (ERN) models specify a joint representation
of both the dyads of a network and nodal characteristics. This class of models
allow the nodal characteristics to be modelled as stochastic processes,
expanding the range and realism of exponential-family approaches to network
modelling. In this paper we develop a theory of inference for ERN models when
only part of the network is observed, as well as specific methodology for
missing data, including non-ignorable mechanisms for network-based sampling
designs and for latent class models. In particular, we consider data collected
via contact tracing, of considerable importance to infectious disease
epidemiology and public health
Efficient Probabilistic Group Testing Based on Traitor Tracing
Inspired by recent results from collusion-resistant traitor tracing, we
provide a framework for constructing efficient probabilistic group testing
schemes. In the traditional group testing model, our scheme asymptotically
requires T ~ 2 K ln N tests to find (with high probability) the correct set of
K defectives out of N items. The framework is also applied to several noisy
group testing and threshold group testing models, often leading to improvements
over previously known results, but we emphasize that this framework can be
applied to other variants of the classical model as well, both in adaptive and
in non-adaptive settings.Comment: 8 pages, 3 figures, 1 tabl
Shape, shear and flexion II - Quantifying the flexion formalism for extended sources with the ray-bundle method
Flexion-based weak gravitational lensing analysis is proving to be a useful
adjunct to traditional shear-based techniques. As flexion arises from gradients
across an image, analytic and numerical techniques are required to investigate
flexion predictions for extended image/source pairs. Using the Schwarzschild
lens model, we demonstrate that the ray-bundle method for gravitational lensing
can be used to accurately recover second flexion, and is consistent with
recovery of zero first flexion. Using lens plane to source plane bundle
propagation, we find that second flexion can be recovered with an error no
worse than 1% for bundle radii smaller than {\Delta}{\theta} = 0.01 {\theta}_E
and lens plane impact pararameters greater than {\theta}_E + {\Delta}{\theta},
where {\theta}_E is the angular Einstein radius. Using source plane to lens
plane bundle propagation, we demonstrate the existence of a preferred flexion
zone. For images at radii closer to the lens than the inner boundary of this
zone, indicative of the true strong lensing regime, the flexion formalism
should be used with caution (errors greater than 5% for extended image/source
pairs). We also define a shear zone boundary, beyond which image shapes are
essentially indistinguishable from ellipses (1% error in ellipticity). While
suggestive that a traditional weak lensing analysis is satisfactory beyond this
boundary, a potentially detectable non-zero flexion signal remains.Comment: 14 pages, 13 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The non-Gaussianity of the cosmic shear likelihood - or: How odd is the Chandra Deep Field South?
(abridged) We study the validity of the approximation of a Gaussian cosmic
shear likelihood. We estimate the true likelihood for a fiducial cosmological
model from a large set of ray-tracing simulations and investigate the impact of
non-Gaussianity on cosmological parameter estimation. We investigate how odd
the recently reported very low value of really is as derived from
the \textit{Chandra} Deep Field South (CDFS) using cosmic shear by taking the
non-Gaussianity of the likelihood into account as well as the possibility of
biases coming from the way the CDFS was selected.
We find that the cosmic shear likelihood is significantly non-Gaussian. This
leads to both a shift of the maximum of the posterior distribution and a
significantly smaller credible region compared to the Gaussian case. We
re-analyse the CDFS cosmic shear data using the non-Gaussian likelihood.
Assuming that the CDFS is a random pointing, we find
for fixed . In a
WMAP5-like cosmology, a value equal to or lower than this would be expected in
of the times. Taking biases into account arising from the way the
CDFS was selected, which we model as being dependent on the number of haloes in
the CDFS, we obtain . Combining the CDFS data
with the parameter constraints from WMAP5 yields and for a flat
universe.Comment: 18 pages, 16 figures, accepted for publication in A&A; New Bayesian
treatment of field selection bia
Oceanographic Weather Maps: Using Oceanographic Models to Improve Seabed Mapping Planning and Acquisition
In a world of high precision sensors, one of the few remaining challenges in multibeam echosounding is that of refraction based uncertainty. A poor understanding of oceanographic variability can lead to inadequate sampling of the water mass and the uncertainties that result from this can dominate the uncertainty budget of even state-of-the-art echosounding systems. Though dramatic improvements have been made in sensor accuracies over the past few decades, survey accuracy and efficiency is still potentially limited by a poor understanding of the “underwater weather”. Advances in the sophistication of numerical oceanographic forecast modeling, combined with ever increasing computing power, allow for the timely operation and dissemination of oceanographic nowcast and forecast model systems on regional and global scales. These sources of information, when examined using sound speed uncertainty analysis techniques, have the potential to change the way hydrographers work by increasing our understanding of what to expect from the ocean and when to expect it. Sound speed analyses derived from ocean modeling system’s three-dimensional predictions could provide guidance for hydrographers during survey planning, acquisition and post-processing of hydrographic data. In this work, we examine techniques for processing and visualizing of predictions from global and regional operational oceanographic forecast models and climatological analyses from an ocean atlas to better understand how these data could best be put to use to in the field of hydrograph
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