6,806 research outputs found
A Block Minorization--Maximization Algorithm for Heteroscedastic Regression
The computation of the maximum likelihood (ML) estimator for heteroscedastic
regression models is considered. The traditional Newton algorithms for the
problem require matrix multiplications and inversions, which are bottlenecks in
modern Big Data contexts. A new Big Data-appropriate minorization--maximization
(MM) algorithm is considered for the computation of the ML estimator. The MM
algorithm is proved to generate monotonically increasing sequences of
likelihood values and to be convergent to a stationary point of the
log-likelihood function. A distributed and parallel implementation of the MM
algorithm is presented and the MM algorithm is shown to have differing time
complexity to the Newton algorithm. Simulation studies demonstrate that the MM
algorithm improves upon the computation time of the Newton algorithm in some
practical scenarios where the number of observations is large
Marine Benthic Habitat Mapping of Muir Inlet, Glacier Bay National Park and Preserve, Alaska With an Evaluation of the Coastal and Marine Ecological Classification Standard III
Seafloor geology and potential benthic habitats were mapped in Muir Inlet, Glacier Bay National Park and Preserve, Alaska, using multibeam sonar, ground-truth information, and geological interpretations. Muir Inlet is a recently deglaciated fjord that is under the influence of glacial and paraglacial marine processes. High glacially derived sediment and meltwater fluxes, slope instabilities, and variable bathymetry result in a highly dynamic estuarine environment and benthic ecosystem. We characterize the fjord seafloor and potential benthic habitats using the Coastal and Marine Ecological Classification Standard (CMECS) recently developed by the National Oceanic and Atmospheric Administration (NOAA) and NatureServe. Substrates within Muir Inlet are dominated by mud, derived from the high glacial debris flux. Water-column characteristics are derived from a combination of conductivity temperature depth (CTD) measurements and circulation-model results. We also present modern glaciomarine sediment accumulation data from quantitative differential bathymetry. These data show Muir Inlet is divided into two contrasting environments: a dynamic upper fjord and a relatively static lower fjord. The accompanying maps represent the first publicly available high-resolution bathymetric surveys of Muir Inlet. The results of these analyses serve as a test of the CMECS and as a baseline for continued mapping and correlations among seafloor substrate, benthic habitats, and glaciomarine processes
The physical significance of the Babak-Grishchuk gravitational energy-momentum tensor
We examine the claim of Babak and Grishchuk [1] to have solved the problem of
localising the energy and momentum of the gravitational field. After
summarising Grishchuk's flat-space formulation of gravity, we demonstrate its
equivalence to General Relativity at the level of the action. Two important
transformations are described (diffeomorphisms applied to all fields, and
diffeomorphisms applied to the flat-space metric alone) and we argue that both
should be considered gauge transformations: they alter the mathematical
representation of a physical system, but not the system itself. By examining
the transformation properties of the Babak-Grishchuk gravitational
energy-momentum tensor under these gauge transformations (infinitesimal and
finite) we conclude that this object has no physical significance.Comment: 10 pages. Submitted to Phys. Rev. D; acknowledgements adjuste
Comprehension as social and intellectual practice: Rebuilding curriculum in low socioeconomic and cultural minority schools
This article reframes the concept of comprehension as a social and intellectual practice. It reviews current approaches to reading instruction for linguistically and culturally diverse and low socioeconomic students, noting an emphasis on comprehension as autonomous skills. The Four Resources model (Freebody & Luke, 1990) is used to make the case for the integration of comprehension instruction with an emphasis on student cultural and community knowledge, and substantive intellectual and sociocultural content in elementary school curricula. Illustrations are drawn from research underway on the teaching of literacy in primary schools in low SES communities
Improvements in Total Column Ozone in GEOSCCM and Comparisons with a New Ozone-Depleting Substances Scenario
The evolution of ozone is examined in the latest version of the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) using old and new ozone-depleting substances (ODS) scenarios. This version of GEOSCCM includes a representation of the quasi-biennial oscillation, a more realistic implementation of ozone chemistry at high solar zenith angles, an improved air/sea roughness parameterization, and an extra 5 parts per trillion of CH3Br to account for brominated very short-lived substances. Together these additions improve the representation of ozone compared to observations. This improved version of GEOSCCM was used to simulate the ozone evolution for the A1 2010 and the newStratosphere-troposphere Processes and their Role in Climate (SPARC) 2013 ODS scenario derived using the SPARC Lifetimes Report 2013. This new ODS scenario results in a maximum Cltot increase of 65 parts per trillion by volume (pptv), decreasing slightly to 60 pptv by 2100. Approximately 72% of the increase is due to the longer lifetime of CFC-11. The quasi-global (60degS-60degN) total column ozone difference is relatively small and less than 1Dobson unit on average and consistent with the 3-4% larger 2050-2080 average Cly in the new SPARC 2013 scenario. Over high latitudes, this small change in Cly compared to the relatively large natural variabilitymakes it not possible to discern a significant impact on ozone in the second half of the 21st century in a single set of simulations
Hierarchical Bayesian CMB Component Separation with the No-U-Turn Sampler
Key to any cosmic microwave background (CMB) analysis is the separation of
the CMB from foreground contaminants. In this paper we present a novel
implementation of Bayesian CMB component separation. We sample from the full
posterior distribution using the No-U-Turn Sampler (NUTS), a gradient based
sampling algorithm. Alongside this, we introduce new foreground modelling
approaches. We use the mean-shift algorithm to define regions on the sky,
clustering according to naively estimated foreground spectral parameters. Over
these regions we adopt a complete pooling model, where we assume constant
spectral parameters, and a hierarchical model, where we model individual
spectral parameters as being drawn from underlying hyper-distributions. We
validate the algorithm against simulations of the LiteBIRD and C-BASS
experiments, with an input tensor-to-scalar ratio of .
Considering multipoles , we are able to recover estimates
for . With LiteBIRD only observations, and using the complete pooling model,
we recover . For C-BASS and LiteBIRD observations
we find using the complete pooling model, and
using the hierarchical model. By adopting the
hierarchical model we are able to eliminate biases in our cosmological
parameter estimation, and obtain lower uncertainties due to the smaller
Galactic emission mask that can be adopted for power spectrum estimation.
Measured by the rate of effective sample generation, NUTS offers performance
improvements of over using Metropolis-Hastings to fit the complete
pooling model. The efficiency of NUTS allows us to fit the more sophisticated
hierarchical foreground model, that would likely be intractable with
non-gradient based sampling algorithms.Comment: 19 pages, 9 figure
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