1,296 research outputs found
Prediction Weighted Maximum Frequency Selection
Shrinkage estimators that possess the ability to produce sparse solutions
have become increasingly important to the analysis of today's complex datasets.
Examples include the LASSO, the Elastic-Net and their adaptive counterparts.
Estimation of penalty parameters still presents difficulties however. While
variable selection consistent procedures have been developed, their finite
sample performance can often be less than satisfactory. We develop a new
strategy for variable selection using the adaptive LASSO and adaptive
Elastic-Net estimators with diverging. The basic idea first involves
using the trace paths of their LARS solutions to bootstrap estimates of maximum
frequency (MF) models conditioned on dimension. Conditioning on dimension
effectively mitigates overfitting, however to deal with underfitting, these MFs
are then prediction-weighted, and it is shown that not only can consistent
model selection be achieved, but that attractive convergence rates can as well,
leading to excellent finite sample performance. Detailed numerical studies are
carried out on both simulated and real datasets. Extensions to the class of
generalized linear models are also detailed.Comment: This manuscript contains 41 pages and 14 figure
Spike and slab variable selection: Frequentist and Bayesian strategies
Variable selection in the linear regression model takes many apparent faces
from both frequentist and Bayesian standpoints. In this paper we introduce a
variable selection method referred to as a rescaled spike and slab model. We
study the importance of prior hierarchical specifications and draw connections
to frequentist generalized ridge regression estimation. Specifically, we study
the usefulness of continuous bimodal priors to model hypervariance parameters,
and the effect scaling has on the posterior mean through its relationship to
penalization. Several model selection strategies, some frequentist and some
Bayesian in nature, are developed and studied theoretically. We demonstrate the
importance of selective shrinkage for effective variable selection in terms of
risk misclassification, and show this is achieved using the posterior from a
rescaled spike and slab model. We also show how to verify a procedure's ability
to reduce model uncertainty in finite samples using a specialized forward
selection strategy. Using this tool, we illustrate the effectiveness of
rescaled spike and slab models in reducing model uncertainty.Comment: Published at http://dx.doi.org/10.1214/009053604000001147 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Shape Instabilities in the Dynamics of a Two-component Fluid Membrane
We study the shape dynamics of a two-component fluid membrane, using a
dynamical triangulation monte carlo simulation and a Langevin description.
Phase separation induces morphology changes depending on the lateral mobility
of the lipids. When the mobility is large, the familiar labyrinthine spinodal
pattern is linearly unstable to undulation fluctuations and breaks up into
buds, which move towards each other and merge. For low mobilities, the membrane
responds elastically at short times, preferring to buckle locally, resulting in
a crinkled surface.Comment: 4 pages, revtex, 3 eps figure
A Novel Monte Carlo Approach to the Dynamics of Fluids --- Single Particle Diffusion, Correlation Functions and Phase Ordering of Binary Fluids
We propose a new Monte Carlo scheme to study the late-time dynamics of a
2-dim hard sphere fluid, modeled by a tethered network of hard spheres.
Fluidity is simulated by breaking and reattaching the flexible tethers. We
study the diffusion of a tagged particle, and show that the velocity
autocorrelation function has a long-time tail. We investigate the
dynamics of phase separation of a binary fluid at late times, and show that the
domain size grows as for high viscosity fluids with a
crossover to for low viscosity fluids. Our scheme can accomodate
particles interacting with a pair potential ,and modified to study
dynamics of fluids in three dimensions.Comment: Latex, 4 pages, 4 figure
The Challenges of Conducting Clinical Trials for Patients with Cardiogenic Shock
Cardiogenic shock due to ST-segment elevation myocardial infarction is associated with high morbidity and mortality. Patients in shock are acutely ill, and clinicians may lack equipoise, thus presenting a challenge to developing high-quality evidence to guide practice. This review will summarize these challenges and offer possible solutions
Fence methods for mixed model selection
Many model search strategies involve trading off model fit with model
complexity in a penalized goodness of fit measure. Asymptotic properties for
these types of procedures in settings like linear regression and ARMA time
series have been studied, but these do not naturally extend to nonstandard
situations such as mixed effects models, where simple definition of the sample
size is not meaningful. This paper introduces a new class of strategies, known
as fence methods, for mixed model selection, which includes linear and
generalized linear mixed models. The idea involves a procedure to isolate a
subgroup of what are known as correct models (of which the optimal model is a
member). This is accomplished by constructing a statistical fence, or barrier,
to carefully eliminate incorrect models. Once the fence is constructed, the
optimal model is selected from among those within the fence according to a
criterion which can be made flexible. In addition, we propose two variations of
the fence. The first is a stepwise procedure to handle situations of many
predictors; the second is an adaptive approach for choosing a tuning constant.
We give sufficient conditions for consistency of fence and its variations, a
desirable property for a good model selection procedure. The methods are
illustrated through simulation studies and real data analysis.Comment: Published in at http://dx.doi.org/10.1214/07-AOS517 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
The Effectiveness of Silane and Siloxane Treatments on the Superhydrophobicity and Icephobicity of Concrete Surfaces
Icy roads lead to treacherous driving conditions in regions of the U.S., leading to over 450 fatalities per year. De-icing chemicals, such as road salt, leave much to be desired. In this report, commercially available silane, siloxane, and related materials were evaluated as solutions, simple emulsions, and complex emulsions with incorporated particulates, for their effectiveness as superhydrophobic treatments. Through the development and use of a basic impact test, the ease of ice removal (icephobicity) was examined as an application of the targeted superhydrophobicity.
A general correlation was found between icephobicity and hydrophobicity, with the amount of ice removed on impact increasing with increasing contact angle. However, the correlation was poor in the high performance region (high contact angle and high ice removal.)
Polymethylhydrogensiloxane was a top performer and was more effective when used as a shell type emulsion with silica fume particulates. An aqueous sodium methyl siliconate solution showed good performance for ice loss and contact angle, as did a commercial proprietary emulsion using a diethoxyoctylsilyl trimethylsilyl ester of silicic acid. These materials have sterically available functional groups that can react or associate with the concrete surface and are potentially film-forming. Materials with less reactive functional groups and a lower propensity to film-form did not perform as well
Unsupervised Bump Hunting Using Principal Components
Principal Components Analysis is a widely used technique for dimension
reduction and characterization of variability in multivariate populations. Our
interest lies in studying when and why the rotation to principal components can
be used effectively within a response-predictor set relationship in the context
of mode hunting. Specifically focusing on the Patient Rule Induction Method
(PRIM), we first develop a fast version of this algorithm (fastPRIM) under
normality which facilitates the theoretical studies to follow. Using basic
geometrical arguments, we then demonstrate how the PC rotation of the predictor
space alone can in fact generate improved mode estimators. Simulation results
are used to illustrate our findings.Comment: 24 pages, 9 figure
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