11,994 research outputs found
Causal inference for continuous-time processes when covariates are observed only at discrete times
Most of the work on the structural nested model and g-estimation for causal
inference in longitudinal data assumes a discrete-time underlying data
generating process. However, in some observational studies, it is more
reasonable to assume that the data are generated from a continuous-time process
and are only observable at discrete time points. When these circumstances
arise, the sequential randomization assumption in the observed discrete-time
data, which is essential in justifying discrete-time g-estimation, may not be
reasonable. Under a deterministic model, we discuss other useful assumptions
that guarantee the consistency of discrete-time g-estimation. In more general
cases, when those assumptions are violated, we propose a controlling-the-future
method that performs at least as well as g-estimation in most scenarios and
which provides consistent estimation in some cases where g-estimation is
severely inconsistent. We apply the methods discussed in this paper to
simulated data, as well as to a data set collected following a massive flood in
Bangladesh, estimating the effect of diarrhea on children's height. Results
from different methods are compared in both simulation and the real
application.Comment: Published in at http://dx.doi.org/10.1214/10-AOS830 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Detecting periodicity in experimental data using linear modeling techniques
Fourier spectral estimates and, to a lesser extent, the autocorrelation
function are the primary tools to detect periodicities in experimental data in
the physical and biological sciences. We propose a new method which is more
reliable than traditional techniques, and is able to make clear identification
of periodic behavior when traditional techniques do not. This technique is
based on an information theoretic reduction of linear (autoregressive) models
so that only the essential features of an autoregressive model are retained.
These models we call reduced autoregressive models (RARM). The essential
features of reduced autoregressive models include any periodicity present in
the data. We provide theoretical and numerical evidence from both experimental
and artificial data, to demonstrate that this technique will reliably detect
periodicities if and only if they are present in the data. There are strong
information theoretic arguments to support the statement that RARM detects
periodicities if they are present. Surrogate data techniques are used to ensure
the converse. Furthermore, our calculations demonstrate that RARM is more
robust, more accurate, and more sensitive, than traditional spectral
techniques.Comment: 10 pages (revtex) and 6 figures. To appear in Phys Rev E. Modified
styl
Surrogate-assisted network analysis of nonlinear time series
The performance of recurrence networks and symbolic networks to detect weak
nonlinearities in time series is compared to the nonlinear prediction error.
For the synthetic data of the Lorenz system, the network measures show a
comparable performance. In the case of relatively short and noisy real-world
data from active galactic nuclei, the nonlinear prediction error yields more
robust results than the network measures. The tests are based on surrogate data
sets. The correlations in the Fourier phases of data sets from some surrogate
generating algorithms are also examined. The phase correlations are shown to
have an impact on the performance of the tests for nonlinearity.Comment: 9 pages, 5 figures, Chaos
(http://scitation.aip.org/content/aip/journal/chaos), corrected typo
Modulation of Thermoelectric Power of Individual Carbon Nanotubes
Thermoelectric power (TEP) of individual single walled carbon nanotubes
(SWNTs) has been measured at mesoscopic scales using a microfabricated heater
and thermometers. Gate electric field dependent TEP-modulation has been
observed. The measured TEP of SWNTs is well correlated to the electrical
conductance across the SWNT according to the Mott formula. At low temperatures,
strong modulations of TEP were observed in the single electron conduction
limit. In addition, semiconducting SWNTs exhibit large values of TEP due to the
Schottky barriers at SWNT-metal junctions.Comment: to be published in Phys. Rev. Let
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Results of an aqueous source term model for a radiological risk assessment of the Drigg LLW Site, U.K.
A radionuclide source term model has been developed which simulates the biogeochemical evolution of the Drigg low level waste (LLW) disposal site. The DRINK (DRIgg Near field Kinetic) model provides data regarding radionuclide concentrations in groundwater over a period of 100,000 years, which are used as input to assessment calculations for a groundwater pathway. The DRINK model also provides input to human intrusion and gaseous assessment calculations through simulation of the solid radionuclide inventory. These calculations are being used to support the Drigg post closure safety case. The DRINK model considers the coupled interaction of the effects of fluid flow, microbiology, corrosion, chemical reaction, sorption and radioactive decay. It represents the first direct use of a mechanistic reaction-transport model in risk assessment calculations
Optimal Restricted Estimation for More Efficient Longitudinal Causal Inference
Efficient semiparametric estimation of longitudinal causal effects is often analytically or computationally intractable. We propose a novel restricted estimation approach for increasing efficiency, which can be used with other techniques, is straightforward to implement, and requires no additional modeling assumptions
Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite
Electric field effect devices based on mesoscopic graphite are fabricated for
galvanomagnetic measurements. Strong modulation of magneto-resistance and Hall
resistance as a function of gate voltage is observed as sample thickness
approaches the screening length. Electric field dependent Landau level
formation is detected from Shubnikov de Haas oscillations in
magneto-resistance. The effective mass of electron and hole carriers has been
measured from the temperature dependant behavior of these oscillations.Comment: 4 pages, 4 figures included, submitted to Phys. Rev. Let
A haplome alignment and reference sequence of the highly polymorphic Ciona savignyi genome
The high degree of polymorphism in the genome of the sea squirt Ciona savignyi complicated the assembly of sequence contigs, but a new alignment method results in a much improved sequence
Elastic energy of polyhedral bilayer vesicles
In recent experiments [M. Dubois, B. Dem\'e, T. Gulik-Krzywicki, J.-C.
Dedieu, C. Vautrin, S. D\'esert, E. Perez, and T. Zemb, Nature (London) Vol.
411, 672 (2001)] the spontaneous formation of hollow bilayer vesicles with
polyhedral symmetry has been observed. On the basis of the experimental
phenomenology it was suggested [M. Dubois, V. Lizunov, A. Meister, T.
Gulik-Krzywicki, J. M. Verbavatz, E. Perez, J. Zimmerberg, and T. Zemb, Proc.
Natl. Acad. Sci. U.S.A. Vol. 101, 15082 (2004)] that the mechanism for the
formation of bilayer polyhedra is minimization of elastic bending energy.
Motivated by these experiments, we study the elastic bending energy of
polyhedral bilayer vesicles. In agreement with experiments, and provided that
excess amphiphiles exhibiting spontaneous curvature are present in sufficient
quantity, we find that polyhedral bilayer vesicles can indeed be energetically
favorable compared to spherical bilayer vesicles. Consistent with experimental
observations we also find that the bending energy associated with the vertices
of bilayer polyhedra can be locally reduced through the formation of pores.
However, the stabilization of polyhedral bilayer vesicles over spherical
bilayer vesicles relies crucially on molecular segregation of excess
amphiphiles along the ridges rather than the vertices of bilayer polyhedra.
Furthermore, our analysis implies that, contrary to what has been suggested on
the basis of experiments, the icosahedron does not minimize elastic bending
energy among arbitrary polyhedral shapes and sizes. Instead, we find that, for
large polyhedron sizes, the snub dodecahedron and the snub cube both have lower
total bending energies than the icosahedron
Using Case Description Information to Reduce Sensitivity to Bias for the Attributable Fraction Among the Exposed
The attributable fraction among the exposed (\textbf{AF}), also known as
the attributable risk or excess fraction among the exposed, is the proportion
of disease cases among the exposed that could be avoided by eliminating the
exposure. Understanding the \textbf{AF} for different exposures helps guide
public health interventions. The conventional approach to inference for the
\textbf{AF} assumes no unmeasured confounding and could be sensitive to
hidden bias from unobserved covariates. In this paper, we propose a new
approach to reduce sensitivity to hidden bias for conducting statistical
inference on the \textbf{AF} by leveraging case description information.
Case description information is information that describes the case, e.g., the
subtype of cancer. The exposure may have more of an effect on some types of
cases than other types. We explore how leveraging case description information
can reduce sensitivity to bias from unmeasured confounding through an
asymptotic tool, design sensitivity, and simulation studies. We allow for the
possibility that leveraging case definition information may introduce
additional selection bias through an additional sensitivity parameter. The
proposed methodology is illustrated by re-examining alcohol consumption and the
risk of postmenopausal invasive breast cancer using case description
information on the subtype of cancer (hormone-sensitive or insensitive) using
data from the Women's Health Initiative (WHI) Observational Study (OS).Comment: 30 pages, 8 tables, 1 figur
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