205 research outputs found
The Calculus of M-estimation in R with geex
M-estimation, or estimating equation, methods are widely applicable for point
estimation and asymptotic inference. In this paper, we present an R package
that can find roots and compute the empirical sandwich variance estimator for
any set of user-specified, unbiased estimating equations. Examples from the
M-estimation primer by Stefanski and Boos (2002) demonstrate use of the
software. The package also includes a framework for finite sample variance
corrections and a website with an extensive collection of tutorials
A Recipe for inferference: Start with Causal Inference. Add Interference. Mix Well with R.
In causal inference, interference occurs when the treatment of one subject affects the outcome of other subjects. Interference can distort research conclusions about causal effects when not accounted for properly. In the absence of interference, inverse probability weighted (IPW) estimators are commonly used to estimate causal effects from observational data. Recently, IPW estimators have been extended to handle interference. Tchetgen Tchetgen and VanderWeele (2012) proposed IPW methods to estimate direct and indirect (or spillover) effects that allow for interference between individuals within groups. In this paper, we present inferference, an R package that computes these IPW causal effect estimates when interference may be present within groups. We illustrate use of the package with examples from political science and infectious disease
Characterizing Signal Loss in the 21 cm Reionization Power Spectrum: A Revised Study of PAPER-64
The Epoch of Reionization (EoR) is an uncharted era in our Universe's history
during which the birth of the first stars and galaxies led to the ionization of
neutral hydrogen in the intergalactic medium. There are many experiments
investigating the EoR by tracing the 21cm line of neutral hydrogen. Because
this signal is very faint and difficult to isolate, it is crucial to develop
analysis techniques that maximize sensitivity and suppress contaminants in
data. It is also imperative to understand the trade-offs between different
analysis methods and their effects on power spectrum estimates. Specifically,
with a statistical power spectrum detection in HERA's foreseeable future, it
has become increasingly important to understand how certain analysis choices
can lead to the loss of the EoR signal. In this paper, we focus on signal loss
associated with power spectrum estimation. We describe the origin of this loss
using both toy models and data taken by the 64-element configuration of the
Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER).
In particular, we highlight how detailed investigations of signal loss have led
to a revised, higher 21cm power spectrum upper limit from PAPER-64.
Additionally, we summarize errors associated with power spectrum error
estimation that were previously unaccounted for. We focus on a subset of
PAPER-64 data in this paper; revised power spectrum limits from the PAPER
experiment are presented in a forthcoming paper by Kolopanis et al. (in prep.)
and supersede results from previously published PAPER analyses.Comment: 25 pages, 18 figures, Accepted by Ap
PAPER-64 Constraints On Reionization II: The Temperature Of The z=8.4 Intergalactic Medium
We present constraints on both the kinetic temperature of the intergalactic
medium (IGM) at z=8.4, and on models for heating the IGM at high-redshift with
X-ray emission from the first collapsed objects. These constraints are derived
using a semi-analytic method to explore the new measurements of the 21 cm power
spectrum from the Donald C. Backer Precision Array for Probing the Epoch of
Reionization (PAPER), which were presented in a companion paper, Ali et al.
(2015). Twenty-one cm power spectra with amplitudes of hundreds of mK^2 can be
generically produced if the kinetic temperature of the IGM is significantly
below the temperature of the Cosmic Microwave Background (CMB); as such, the
new results from PAPER place lower limits on the IGM temperature at z=8.4.
Allowing for the unknown ionization state of the IGM, our measurements find the
IGM temperature to be above ~5 K for neutral fractions between 10% and 85%,
above ~7 K for neutral fractions between 15% and 80%, or above ~10 K for
neutral fractions between 30% and 70%. We also calculate the heating of the IGM
that would be provided by the observed high redshift galaxy population, and
find that for most models, these galaxies are sufficient to bring the IGM
temperature above our lower limits. However, there are significant ranges of
parameter space that could produce a signal ruled out by the PAPER
measurements; models with a steep drop-off in the star formation rate density
at high redshifts or with relatively low values for the X-ray to star formation
rate efficiency of high redshift galaxies are generally disfavored. The PAPER
measurements are consistent with (but do not constrain) a hydrogen spin
temperature above the CMB temperature, a situation which we find to be
generally predicted if galaxies fainter than the current detection limits of
optical/NIR surveys are included in calculations of X-ray heating.Comment: companion paper to Ali et al. (2015), ApJ 809, 61; matches version
accepted to ApJ; 11 pages, 7 figure
The Hydrogen Epoch of Reionization Array Dish I: Beam Pattern Measurements and Science Implications
The Hydrogen Epoch of Reionization Array (HERA) is a radio interferometer
aiming to detect the power spectrum of 21 cm fluctuations from neutral hydrogen
from the Epoch of Reionization (EOR). Drawing on lessons from the Murchison
Widefield Array (MWA) and the Precision Array for Probing the Epoch of
Reionization (PAPER), HERA is a hexagonal array of large (14 m diameter) dishes
with suspended dipole feeds. Not only does the dish determine overall
sensitivity, it affects the observed frequency structure of foregrounds in the
interferometer. This is the first of a series of four papers characterizing the
frequency and angular response of the dish with simulations and measurements.
We focus in this paper on the angular response (i.e., power pattern), which
sets the relative weighting between sky regions of high and low delay, and
thus, apparent source frequency structure. We measure the angular response at
137 MHz using the ORBCOMM beam mapping system of Neben et al. We measure a
collecting area of 93 m^2 in the optimal dish/feed configuration, implying
HERA-320 should detect the EOR power spectrum at z~9 with a signal-to-noise
ratio of 12.7 using a foreground avoidance approach with a single season of
observations, and 74.3 using a foreground subtraction approach. Lastly we study
the impact of these beam measurements on the distribution of foregrounds in
Fourier space.Comment: 13 pages, 9 figures. Replaced to match accepted ApJ versio
Hydrogen Epoch of Reionization Array (HERA)
The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to
measure 21 cm emission from the primordial intergalactic medium (IGM)
throughout cosmic reionization (), and to explore earlier epochs of our
Cosmic Dawn (). During these epochs, early stars and black holes
heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is
designed to characterize the evolution of the 21 cm power spectrum to constrain
the timing and morphology of reionization, the properties of the first
galaxies, the evolution of large-scale structure, and the early sources of
heating. The full HERA instrument will be a 350-element interferometer in South
Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz.
Currently, 19 dishes have been deployed on site and the next 18 are under
construction. HERA has been designated as an SKA Precursor instrument.
In this paper, we summarize HERA's scientific context and provide forecasts
for its key science results. After reviewing the current state of the art in
foreground mitigation, we use the delay-spectrum technique to motivate
high-level performance requirements for the HERA instrument. Next, we present
the HERA instrument design, along with the subsystem specifications that ensure
that HERA meets its performance requirements. Finally, we summarize the
schedule and status of the project. We conclude by suggesting that, given the
realities of foreground contamination, current-generation 21 cm instruments are
approaching their sensitivity limits. HERA is designed to bring both the
sensitivity and the precision to deliver its primary science on the basis of
proven foreground filtering techniques, while developing new subtraction
techniques to unlock new capabilities. The result will be a major step toward
realizing the widely recognized scientific potential of 21 cm cosmology.Comment: 26 pages, 24 figures, 2 table
Dissociation constants and thermodynamic properties of amino acids used in CO2 absorption from (293 to 353) K
The second dissociation constants of the amino acids βalanine, taurine, sarcosine, 6-aminohexanoic acid, DL-methionine, glycine, L-phenylalanine, and L-proline and the third dissociation constants of L-glutamic acid and L-aspartic acid have been determined from electromotive force measurements at temperatures from (293 to 353) K. Experimental results are reported and compared to literature values. Values of the standard state thermodynamic properties are derived from the experimental results and compared to the values of commercially available amines used as absorbents for CO 2 capture.
Clinically Actionable Hypercholesterolemia and Hypertriglyceridemia in Children with Nonalcoholic Fatty Liver Disease
OBJECTIVE:
To determine the percentage of children with nonalcoholic fatty liver disease (NAFLD) in whom intervention for low-density lipoprotein cholesterol or triglycerides was indicated based on National Heart, Lung, and Blood Institute guidelines.
STUDY DESIGN:
This multicenter, longitudinal cohort study included children with NAFLD enrolled in the National Institute of Diabetes and Digestive and Kidney Diseases Nonalcoholic Steatohepatitis Clinical Research Network. Fasting lipid profiles were obtained at diagnosis. Standardized dietary recommendations were provided. After 1 year, lipid profiles were repeated and interpreted according to National Heart, Lung, and Blood Institute Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction. Main outcomes were meeting criteria for clinically actionable dyslipidemia at baseline, and either achieving lipid goal at follow-up or meeting criteria for ongoing intervention.
RESULTS:
There were 585 participants, with a mean age of 12.8 years. The prevalence of children warranting intervention for low-density lipoprotein cholesterol at baseline was 14%. After 1 year of recommended dietary changes, 51% achieved goal low-density lipoprotein cholesterol, 27% qualified for enhanced dietary and lifestyle modifications, and 22% met criteria for pharmacologic intervention. Elevated triglycerides were more prevalent, with 51% meeting criteria for intervention. At 1 year, 25% achieved goal triglycerides with diet and lifestyle changes, 38% met criteria for advanced dietary modifications, and 37% qualified for antihyperlipidemic medications.
CONCLUSIONS:
More than one-half of children with NAFLD met intervention thresholds for dyslipidemia. Based on the burden of clinically relevant dyslipidemia, lipid screening in children with NAFLD is warranted. Clinicians caring for children with NAFLD should be familiar with lipid management
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