2,702 research outputs found
A generalization of moderated statistics to data adaptive semiparametric estimation in high-dimensional biology
The widespread availability of high-dimensional biological data has made the
simultaneous screening of numerous biological characteristics a central
statistical problem in computational biology. While the dimensionality of such
datasets continues to increase, the problem of teasing out the effects of
biomarkers in studies measuring baseline confounders while avoiding model
misspecification remains only partially addressed. Efficient estimators
constructed from data adaptive estimates of the data-generating distribution
provide an avenue for avoiding model misspecification; however, in the context
of high-dimensional problems requiring simultaneous estimation of numerous
parameters, standard variance estimators have proven unstable, resulting in
unreliable Type-I error control under standard multiple testing corrections. We
present the formulation of a general approach for applying empirical Bayes
shrinkage approaches to asymptotically linear estimators of parameters defined
in the nonparametric model. The proposal applies existing shrinkage estimators
to the estimated variance of the influence function, allowing for increased
inferential stability in high-dimensional settings. A methodology for
nonparametric variable importance analysis for use with high-dimensional
biological datasets with modest sample sizes is introduced and the proposed
technique is demonstrated to be robust in small samples even when relying on
data adaptive estimators that eschew parametric forms. Use of the proposed
variance moderation strategy in constructing stabilized variable importance
measures of biomarkers is demonstrated by application to an observational study
of occupational exposure. The result is a data adaptive approach for robustly
uncovering stable associations in high-dimensional data with limited sample
sizes
Cluster coherent potential approximation for electronic structure of disordered alloys
We extend the single-site coherent potential approximation (CPA) to include
the effects of non-local disorder correlations (alloy short-range order) on the
electronic structure of random alloy systems. This is achieved by mapping the
original Anderson disorder problem to that of a selfconsistently embedded
cluster. This cluster problem is then solved using the equations of motion
technique. The CPA is recovered for cluster size , and the disorder
averaged density-of-states (DOS) is always positive definite. Various new
features, compared to those observed in CPA, and related to repeated scattering
on pairs of sites, reflecting the effect of SRO are clearly visible in the DOS.
It is explicitly shown that the cluster-CPA method always yields
positive-definite DOS. Anderson localization effects have been investigated
within this approach. In general, we find that Anderson localization sets in
before band splitting occurs, and that increasing partial order drives a
continuous transition from an Anderson insulator to an incoherent metal.Comment: 7 pages, 6 figures. submitted to PR
The change in weight perception of weight status among the overweight: comparison of NHANES III (1988–1994) and 1999–2004 NHANES
<p>Abstract</p> <p>Objectives</p> <p>This study seeks to determine whether perception of weight status among the overweight has changed with the increasing overweight/obesity prevalence.</p> <p>Methods</p> <p>The perception of weight status was compared between overweight participants (BMI between 25.0–29.9 kg/m<sup>2</sup>) from NHANES III (1988–1994) and overweight participants from NHANES 1999–2004. Perception of weight status was assessed by asking participants to classify their weight as about the right weight, underweight or overweight. Comparisons were made across age groups, genders, race/ethnicities and various income levels.</p> <p>Results</p> <p>Fewer overweight people during the NHANES 1999–2004 survey perceived themselves as overweight when compared to overweight people during the NHANES III survey. The change in distortion between the survey periods was greatest among persons with lower income, males and African-Americans.</p> <p>Conclusion</p> <p>The increase in overweight/obesity between the survey years (NHANES III and NHANES 1999–2004 has been accompanied with fewer overweight people perceiving themselves as overweight.</p
Large surface meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during the record-warm year 2010 explained by detailed energy balance observations
This study uses data from six on-ice weather stations, calibrated MODIS-derived albedo and proglacial river gauging measurements to drive and validate an energy balance model. We aim to quantify the record-setting positive temperature anomaly in 2010 and its effect on mass balance and runoff from the Kangerlussuaq sector of the Greenland ice sheet. In 2010, the average temperature was 4.9 °C (2.7 standard deviations) above the 1974–2010 average in Kangerlussuaq. High temperatures were also observed over the ice sheet, with the magnitude of the positive anomaly increasing with altitude, particularly in August. Simultaneously, surface albedo was anomalously low in 2010, predominantly in the upper ablation zone. The low albedo was caused by high ablation, which in turn profited from high temperatures and low winter snowfall. Surface energy balance calculations show that the largest melt excess (&sim;170%) occurred in the upper ablation zone (above 1000 m), where higher temperatures and lower albedo contributed equally to the melt anomaly. At lower elevations the melt excess can be attributed to high atmospheric temperatures alone. In total, we calculate that 6.6 &plusmn; 1.0 km<sup>3</sup> of surface meltwater ran off the ice sheet in the Kangerlussuaq catchment in 2010, exceeding the reference year 2009 (based on atmospheric temperature measurements) by &sim;150%. During future warm episodes we can expect a melt response of at least the same magnitude, unless a larger wintertime snow accumulation delays and moderates the melt-albedo feedback. Due to the hypsometry of the ice sheet, yielding an increasing surface area with elevation, meltwater runoff will be further amplified by increases in melt forcings such as atmospheric heat
Spin Stiffness in the Hubbard model
The spin stiffness of the repulsive Hubbard model that occurs
in the hydrodynamic theory of antiferromagnetic spin waves is shown to be the
same as the thermodynamically defined stiffness involved in twisting the order
parameter. New expressions for are derived, which enable easier
interpretation, and connections with superconducting weight and gauge
invariance are discussed.Comment: 21 Pages LaTeX2e, to be published in Journal of Physics
Time Dependent Current Oscillations Through a Quantum Dot
Time dependent phenomena associated to charge transport along a quantum dot
in the charge quantization regime is studied. Superimposed to the Coulomb
blockade behaviour the current has novel non-linear properties. Together with
static multistabilities in the negative resistance region of the I-V
characteristic curve, strong correlations at the dot give rise to
self-sustained current and charge oscillations. Their properties depend upon
the parameters of the quantum dot and the external applied voltages.Comment: 4 pages, 3 figures; to appear in PR
Electrochemical deprotonation of phosphate on stainless steel
Voltammetric experiments performed in phosphate buffer at constant pH 8.0 on platinum and stainless steel revealed clear reduction
currents, which were correlated to the concentrations of phosphate. On the basis of the reactions proposed previously, a model was elaborated,
assuming that both H2PO4 and HPO4
2 underwent cathodic deprotonation, and including the acid–base equilibriums. A kinetic model was
derived by analogy with the equations generally used for hydrogen evolution. Numerical fitting of the experimental data confirmed that the
phosphate species may act as an efficient catalyst of hydrogen evolution via electrochemical deprotonation. This reaction may introduce an
unexpected reversible pathway of hydrogen formation in the mechanisms of anaerobic corrosion. The possible new insights offered by the
electrochemical deprotonation of phosphate in microbially influenced corrosion was finally discussed
Efficient data processing and quantum phenomena: Single-particle systems
We study the relation between the acquisition and analysis of data and
quantum theory using a probabilistic and deterministic model for photon
polarizers. We introduce criteria for efficient processing of data and then use
these criteria to demonstrate that efficient processing of the data contained
in single events is equivalent to the observation that Malus' law holds. A
strictly deterministic process that also yields Malus' law is analyzed in
detail. We present a performance analysis of the probabilistic and
deterministic model of the photon polarizer. The latter is an adaptive
dynamical system that has primitive learning capabilities. This additional
feature has recently been shown to be sufficient to perform event-by-event
simulations of interference phenomena, without using concepts of wave
mechanics. We illustrate this by presenting results for a system of two chained
Mach-Zehnder interferometers, suggesting that systems that perform efficient
data processing and have learning capability are able to exhibit behavior that
is usually attributed to quantum systems only.Comment: http://www.compphys.net/dl
Insulating phases of the infinite-dimensional Hubbard model
A theory is developed for the T=0 Mott-Hubbard insulating phases of the
infinite-dimensional Hubbard model at half-filling, including both the
antiferromagnetic (AF) and paramagnetic (P) insulators. Local moments are
introduced explicitly from the outset, enabling ready identification of the
dominant low energy scales for insulating spin- flip excitations. Dynamical
coupling of single-particle processes to the spin-flip excitations leads to a
renormalized self-consistent description of the single-particle propagators
that is shown to be asymptotically exact in strong coupling, for both the AF
and P phases. For the AF case, the resultant theory is applicable over the
entire U-range, and is discussed in some detail. For the P phase, we consider
in particular the destruction of the Mott insulator, the resultant critical
behaviour of which is found to stem inherently from proper inclusion of the
spin-flip excitations.Comment: 13 pages Revtex, 12 postscript figure
β7−/− donor T cells cause less graft-versus-host disease while preserving graft-versus-tumor activity
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