3,754 research outputs found
Bayesian computation for statistical models with intractable normalizing constants
This paper deals with some computational aspects in the Bayesian analysis of
statistical models with intractable normalizing constants. In the presence of
intractable normalizing constants in the likelihood function, traditional MCMC
methods cannot be applied. We propose an approach to sample from such posterior
distributions. The method can be thought as a Bayesian version of the MCMC-MLE
approach of Geyer and Thompson (1992). To the best of our knowledge, this is
the first general and asymptotically consistent Monte Carlo method for such
problems. We illustrate the method with examples from image segmentation and
social network modeling. We study as well the asymptotic behavior of the
algorithm and obtain a strong law of large numbers for empirical averages.Comment: 20 pages, 4 figures, submitted for publicatio
ABC likelihood-freee methods for model choice in Gibbs random fields
Gibbs random fields (GRF) are polymorphous statistical models that can be
used to analyse different types of dependence, in particular for spatially
correlated data. However, when those models are faced with the challenge of
selecting a dependence structure from many, the use of standard model choice
methods is hampered by the unavailability of the normalising constant in the
Gibbs likelihood. In particular, from a Bayesian perspective, the computation
of the posterior probabilities of the models under competition requires special
likelihood-free simulation techniques like the Approximate Bayesian Computation
(ABC) algorithm that is intensively used in population genetics. We show in
this paper how to implement an ABC algorithm geared towards model choice in the
general setting of Gibbs random fields, demonstrating in particular that there
exists a sufficient statistic across models. The accuracy of the approximation
to the posterior probabilities can be further improved by importance sampling
on the distribution of the models. The practical aspects of the method are
detailed through two applications, the test of an iid Bernoulli model versus a
first-order Markov chain, and the choice of a folding structure for two
proteins.Comment: 19 pages, 5 figures, to appear in Bayesian Analysi
Extensive light profile fitting of galaxy-scale strong lenses
We investigate the merits of a massive forward modeling of ground-based
optical imaging as a diagnostic for the strong lensing nature of Early-Type
Galaxies, in the light of which blurred and faint Einstein rings can hide. We
simulate several thousand mock strong lenses under ground- and space-based
conditions as arising from the deflection of an exponential disk by a
foreground de Vaucouleurs light profile whose lensing potential is described by
a Singular Isothermal Ellipsoid. We then fit for the lensed light distribution
with sl_fit after having subtracted the foreground light emission off (ideal
case) and also after having fitted the deflector's light with galfit. By
setting thresholds in the output parameter space, we can decide the
lens/not-a-lens status of each system. We finally apply our strategy to a
sample of 517 lens candidates present in the CFHTLS data to test the
consistency of our selection approach. The efficiency of the fast modeling
method at recovering the main lens parameters like Einstein radius, total
magnification or total lensed flux, is quite comparable under CFHT and HST
conditions when the deflector is perfectly subtracted off (only possible in
simulations), fostering a sharp distinction between the good and the bad
candidates. Conversely, for a more realistic subtraction, a substantial
fraction of the lensed light is absorbed into the deflector's model, which
biases the subsequent fitting of the rings and then disturbs the selection
process. We quantify completeness and purity of the lens finding method in both
cases. This suggests that the main limitation currently resides in the
subtraction of the foreground light. Provided further enhancement of the
latter, the direct forward modeling of large numbers of galaxy-galaxy strong
lenses thus appears tractable and could constitute a competitive lens finder in
the next generation of wide-field imaging surveys.Comment: A&A accepted version, minor changes (13 pages, 10 figures
Demographic consequences of agricultural practices on a long-lived avian predator
Includes bibliographical references.2022 Fall.To view the abstract, please see the full text of the document
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