2,220 research outputs found
A generic algorithm for reducing bias in parametric estimation
A general iterative algorithm is developed for the computation
of reduced-bias parameter estimates in regular statistical models through
adjustments to the score function. The algorithm unifies and provides appealing new interpretation for iterative methods that have been published
previously for some specific model classes. The new algorithm can usefully be viewed as a series of iterative bias corrections, thus facilitating the
adjusted score approach to bias reduction in any model for which the first-
order bias of the maximum likelihood estimator has already been derived.
The method is tested by application to a logit-linear multiple regression
model with beta-distributed responses; the results confirm the effectiveness
of the new algorithm, and also reveal some important errors in the existing
literature on beta regression
Jeffreys-prior penalty, finiteness and shrinkage in binomial-response generalized linear models
Penalization of the likelihood by Jeffreys' invariant prior, or by a positive
power thereof, is shown to produce finite-valued maximum penalized likelihood
estimates in a broad class of binomial generalized linear models. The class of
models includes logistic regression, where the Jeffreys-prior penalty is known
additionally to reduce the asymptotic bias of the maximum likelihood estimator;
and also models with other commonly used link functions such as probit and
log-log. Shrinkage towards equiprobability across observations, relative to the
maximum likelihood estimator, is established theoretically and is studied
through illustrative examples. Some implications of finiteness and shrinkage
for inference are discussed, particularly when inference is based on Wald-type
procedures. A widely applicable procedure is developed for computation of
maximum penalized likelihood estimates, by using repeated maximum likelihood
fits with iteratively adjusted binomial responses and totals. These theoretical
results and methods underpin the increasingly widespread use of reduced-bias
and similarly penalized binomial regression models in many applied fields
Brook Auto: High-Level Certification-Friendly Programming for GPU-powered Automotive Systems
Modern automotive systems require increased performance to implement Advanced Driving Assistance Systems (ADAS). GPU-powered platforms are promising candidates for such computational tasks, however current low-level programming models challenge the accelerator software certification process, while they limit the hardware selection to a fraction of the available platforms. In this paper we present Brook Auto, a high-level programming language for automotive GPU systems which removes these limitations. We describe the challenges and solutions we faced in its implementation, as well as a complete evaluation in terms of performance and productivity, which shows the effectiveness of our method.This work has been partially supported by the Spanish Ministry of Science and Innovation under grant TIN2015-65316-P and the HiPEAC Network of Excellence.Peer ReviewedPostprint (author's final draft
Liquidity commonality does not imply liquidity resilience commonality: A functional characterisation for ultra-high frequency cross-sectional LOB data
We present a large-scale study of commonality in liquidity and resilience
across assets in an ultra high-frequency (millisecond-timestamped) Limit Order
Book (LOB) dataset from a pan-European electronic equity trading facility. We
first show that extant work in quantifying liquidity commonality through the
degree of explanatory power of the dominant modes of variation of liquidity
(extracted through Principal Component Analysis) fails to account for heavy
tailed features in the data, thus producing potentially misleading results. We
employ Independent Component Analysis, which both decorrelates the liquidity
measures in the asset cross-section, but also reduces higher-order statistical
dependencies.
To measure commonality in liquidity resilience, we utilise a novel
characterisation as the time required for return to a threshold liquidity
level. This reflects a dimension of liquidity that is not captured by the
majority of liquidity measures and has important ramifications for
understanding supply and demand pressures for market makers in electronic
exchanges, as well as regulators and HFTs. When the metric is mapped out across
a range of thresholds, it produces the daily Liquidity Resilience Profile (LRP)
for a given asset. This daily summary of liquidity resilience behaviour from
the vast LOB dataset is then amenable to a functional data representation. This
enables the comparison of liquidity resilience in the asset cross-section via
functional linear sub-space decompositions and functional regression. The
functional regression results presented here suggest that market factors for
liquidity resilience (as extracted through functional principal components
analysis) can explain between 10 and 40% of the variation in liquidity
resilience at low liquidity thresholds, but are less explanatory at more
extreme levels, where individual asset factors take effect
Transient flows in active porous media
Stimuli-responsive materials that modify their shape in response to changes
in environmental conditions -- such as solute concentration, temperature, pH,
and stress -- are widespread in nature and technology. Applications include
micro- and nanoporous materials used in filtration and flow control. The
physiochemical mechanisms that induce internal volume modifications have been
widely studies. The coupling between induced volume changes and solute
transport through porous materials, however, is not well understood. Here, we
consider advective and diffusive transport through a small channel linking two
large reservoirs. A section of stimulus-responsive material regulates the
channel permeability, which is a function of the local solute concentration. We
derive an exact solution to the coupled transport problem and demonstrate the
existence of a flow regime in which the steady state is reached via a damped
oscillation around the equilibrium concentration value. Finally, the
feasibility of an experimental observation of the phenomena is discussed.
Please note that this version of the paper has not been formally peer reviewed,
revised or accepted by a journal
Evolution of Vocabulary on Scale-free and Random Networks
We examine the evolution of the vocabulary of a group of individuals
(linguistic agents) on a scale-free network, using Monte Carlo simulations and
assumptions from evolutionary game theory. It is known that when the agents are
arranged in a two-dimensional lattice structure and interact by diffusion and
encounter, then their final vocabulary size is the maximum possible. Knowing
all available words is essential in order to increase the probability to
``survive'' by effective reproduction. On scale-free networks we find a
different result. It is not necessary to learn the entire vocabulary available.
Survival chances are increased by using the vocabulary of the ``hubs'' (nodes
with high degree). The existence of the ``hubs'' in a scale-free network is the
source of an additional important fitness generating mechanism.Comment: 10 pages, 3 Figures, accepted in Physica
Explosive Percolation: Unusual Transitions of a Simple Model
In this paper we review the recent advances on explosive percolation, a very
sharp phase transition first observed by Achlioptas et al. (Science, 2009).
There a simple model was proposed, which changed slightly the classical
percolation process so that the emergence of the spanning cluster is delayed.
This slight modification turns out to have a great impact on the percolation
phase transition. The resulting transition is so sharp that it was termed
explosive, and it was at first considered to be discontinuous. This surprising
fact stimulated considerable interest in "Achlioptas processes". Later work,
however, showed that the transition is continuous (at least for Achlioptas
processes on Erdos networks), but with very unusual finite size scaling. We
present a review of the field, indicate open "problems" and propose directions
for future research.Comment: 27 pages, 4 figures, Review pape
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