159 research outputs found
A Modern Take on the Bias-Variance Tradeoff in Neural Networks
The bias-variance tradeoff tells us that as model complexity increases, bias
falls and variances increases, leading to a U-shaped test error curve. However,
recent empirical results with over-parameterized neural networks are marked by
a striking absence of the classic U-shaped test error curve: test error keeps
decreasing in wider networks. This suggests that there might not be a
bias-variance tradeoff in neural networks with respect to network width, unlike
was originally claimed by, e.g., Geman et al. (1992). Motivated by the shaky
evidence used to support this claim in neural networks, we measure bias and
variance in the modern setting. We find that both bias and variance can
decrease as the number of parameters grows. To better understand this, we
introduce a new decomposition of the variance to disentangle the effects of
optimization and data sampling. We also provide theoretical analysis in a
simplified setting that is consistent with our empirical findings
Accretion, feedback and galaxy bimodality: a comparison of the GalICS semi-analytic model and cosmological SPH simulations
We compare the galaxy population of an SPH simulation to those predicted by
the GalICS semi-analytic model and a stripped down version without supernova
and AGN feedback. The SPH simulation and the no-feedback GalICS model make
similar predictions for the baryonic mass functions of galaxies and for the
dependence of these mass functions on environment and redshift. The two methods
also make similar predictions for the galaxy content of dark matter haloes as a
function of halo mass and for the gas accretion history of galaxies. Both the
SPH and no-feedback GalICS models predict a bimodal galaxy population at z=0.
The "red'' sequence of gas poor, old galaxies is populated mainly by satellite
systems while, contrary to observations, the central galaxies of massive haloes
lie on the "blue'' star-forming sequence as a result of continuing hot gas
accretion at late times. Furthermore, both models overpredict the observed
baryonic mass function, especially at the high mass end. In the full GalICS
model, supernova-driven outflows reduce the masses of low and intermediate mass
galaxies by about a factor of two. AGN feedback suppresses gas cooling in large
haloes, producing a sharp cut-off in the baryonic mass function and moving the
central galaxies of these massive haloes to the red sequence. Our results imply
that the observational failings of the SPH simulation and the no-feedback
GalICS model are a consequence of missing input physics rather than
computational inaccuracies, that truncating gas accretion by satellite galaxies
automatically produces a bimodal galaxy distribution with a red sequence, but
that explaining the red colours of the most massive galaxies requires a
mechanism like AGN feedback that suppresses the accretion onto central galaxies
in large haloes.Comment: 17 pages, 11 figures, submitted to MNRA
Elemental signatures of Red Drum (Sciaenops ocellatus) otoliths from the Gulf of Mexico and western Atlantic
Discriminating age-O Red Snapper, Lutjanus campechanus, nursery areas in the northern Gulf of Mexico using otolith microchemistry
Influence of grain size, shape and compaction on georadar waves: example of an Aeolian dune
Many Ground Penetrating Radar (GPR) profiles acquired in dry aeolian
environment have shown good reflectivity inside present-day dunes. We show that
the origin of this reflectivity is related to changes in grain size
distribution, packing and/or grain shape in a sandy material. We integrate
these three parameters into analytical models for bulk permittivity in order to
predict the reflections and the velocity of GPR waves. We consider two GPR
cross-sections acquired over Aeolian dunes in the Chadian desert. The 2D
migration of GPR data suggests that dunes contain different kinds of bounding
surfaces. We discuss and model three kinds of reflections using reasonable
geological hypothesis about Aeolian sedimentation processes. The propagation
and the reflection of radar waves are calculated using the 1D wavelet modelling
method in spectral domain. The results of the forward modelling are in good
accordance with real observed data
Non-parametric clustering over user features and latent behavioral functions with dual-view mixture models
International audienceWe present a dual-view mixture model to cluster users based on their features and latent behavioral functions. Every component of the mixture model represents a probability density over a feature view for observed user attributes and a behavior view for latent behavioral functions that are indirectly observed through user actions or behaviors. Our task is to infer the groups of users as well as their latent behavioral functions. We also propose a non-parametric version based on a Dirichlet Process to automatically infer the number of clusters. We test the properties and performance of the model on a synthetic dataset that represents the participation of users in the threads of an online forum. Experiments show that dual-view models outperform single-view ones when one of the views lacks information
Sequential Quasi-Monte Carlo
We derive and study SQMC (Sequential Quasi-Monte Carlo), a class of
algorithms obtained by introducing QMC point sets in particle filtering. SQMC
is related to, and may be seen as an extension of, the array-RQMC algorithm of
L'Ecuyer et al. (2006). The complexity of SQMC is , where is
the number of simulations at each iteration, and its error rate is smaller than
the Monte Carlo rate . The only requirement to implement SQMC is
the ability to write the simulation of particle given as a
deterministic function of and a fixed number of uniform variates.
We show that SQMC is amenable to the same extensions as standard SMC, such as
forward smoothing, backward smoothing, unbiased likelihood evaluation, and so
on. In particular, SQMC may replace SMC within a PMCMC (particle Markov chain
Monte Carlo) algorithm. We establish several convergence results. We provide
numerical evidence that SQMC may significantly outperform SMC in practical
scenarios.Comment: 55 pages, 10 figures (final version
Planet Hunters: The First Two Planet Candidates Identified by the Public using the Kepler Public Archive Data
Planet Hunters is a new citizen science project, designed to engage the
public in an exoplanet search using NASA Kepler public release data. In the
first month after launch, users identified two new planet candidates which
survived our checks for false- positives. The follow-up effort included
analysis of Keck HIRES spectra of the host stars, analysis of pixel centroid
offsets in the Kepler data and adaptive optics imaging at Keck using NIRC2.
Spectral synthesis modeling coupled with stellar evolutionary models yields a
stellar density distribution, which is used to model the transit orbit. The
orbital periods of the planet candidates are 9.8844 \pm0.0087 days (KIC
10905746) and 49.7696 \pm0.00039 (KIC 6185331) days and the modeled planet
radii are 2.65 and 8.05 R\oplus. The involvement of citizen scientists as part
of Planet Hunters is therefore shown to be a valuable and reliable tool in
exoplanet detection.Comment: Submitted to MNRAS, added 1 line to table
ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP) : Detection of Extremely High-density Compact Structure of Prestellar Cores and Multiple Substructures Within
Prestellar cores are self-gravitating dense and cold structures within molecular clouds where future stars are born. They are expected, at the stage of transitioning to the protostellar phase, to harbor centrally concentrated dense (sub)structures that will seed the formation of a new star or the binary/multiple stellar systems. Characterizing this critical stage of evolution is key to our understanding of star formation. In this work, we report the detection of high-density (sub)structures on the thousand-astronomical-unit (au) scale in a sample of dense prestellar cores. Through our recent ALMA observations toward the Orion Planck Galactic Cold Clumps, we have found five extremely dense prestellar cores, which have centrally concentrated regions of similar to 2000 au in size, and several 10(7) cm(-3) in average density. Masses of these centrally dense regions are in the range of 0.30 to 6.89 M. For the first time, our higher resolution observations (0.8 '' similar to 320 au) further reveal that one of the cores shows clear signatures of fragmentation; such individual substructures/fragments have sizes of 800-1700 au, masses of 0.08 to 0.84 M, densities of 2 - 8 x 10(7) cm(-3), and separations of similar to 1200 au. The substructures are massive enough (greater than or similar to 0.1 M) to form young stellar objects and are likely examples of the earliest stage of stellar embryos that can lead to widely (similar to 1200 au) separated multiple systems.Peer reviewe
Working with Language: A Refocused Research Agenda for Cultural Leadership Studies
© 2016 British Academy of Management and John Wiley & Sons Ltd This paper critically reviews existing contributions from the field of cultural leadership studies with a view to highlighting the conceptual and methodological limitations of the dominant etic, cross-cultural approach in leadership studies and illuminating implications of the relative dominance and unreflective use of the English language as the academic and business lingua franca within this field. It subsequently outlines the negative implications of overlooking cultural and linguistic multiplicity for an understanding of culturally sensitive leadership practices. In drawing on lessons from this critical review and the emergent fields of emic, non-positivist cultural leadership studies, this analysis argues that the field of cultural leadership studies requires an alternative research agenda focused on language multiplicity, which enables the field to move towards emic, qualitative research that helps to empower individual cultural voices and explore cultural intra- and interrelationships, tensions and paradoxes embedded in leadership processes. The paper concludes by offering suggestions on methodological approaches for emic cultural leadership studies that are centred on the exploration of language as a cultural voice
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