1,030 research outputs found
The potential for bias in principal causal effect estimation when treatment received depends on a key covariate
Motivated by a potential-outcomes perspective, the idea of principal
stratification has been widely recognized for its relevance in settings
susceptible to posttreatment selection bias such as randomized clinical trials
where treatment received can differ from treatment assigned. In one such
setting, we address subtleties involved in inference for causal effects when
using a key covariate to predict membership in latent principal strata. We show
that when treatment received can differ from treatment assigned in both study
arms, incorporating a stratum-predictive covariate can make estimates of the
"complier average causal effect" (CACE) derive from observations in the two
treatment arms with different covariate distributions. Adopting a Bayesian
perspective and using Markov chain Monte Carlo for computation, we develop
posterior checks that characterize the extent to which incorporating the
pretreatment covariate endangers estimation of the CACE. We apply the method to
analyze a clinical trial comparing two treatments for jaw fractures in which
the study protocol allowed surgeons to overrule both possible randomized
treatment assignments based on their clinical judgment and the data contained a
key covariate (injury severity) predictive of treatment received.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS477 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Causal Impact of the Hospital Readmissions Reduction Program on Hospital Readmissions and Mortality
Estimating causal effects of the Hospital Readmissions Reduction Program
(HRRP), part of the Affordable Care Act, has been very controversial.
Associational studies have demonstrated decreases in hospital readmissions,
consistent with the intent of the program, although analyses with different
data sources and methods have differed in estimating effects on patient
mortality. To address these issues, we define the estimands of interest in the
context of potential outcomes, we formalize a Bayesian structural time-series
model for causal inference, and discuss the necessary assumptions for
estimation of effects using observed data. The method is used to estimate the
effect of the passage of HRRP on both the 30-day readmissions and 30-day
mortality. We show that for acute myocardial infarction and congestive heart
failure, HRRP caused reduction in readmissions while it had no statistically
significant effect on mortality. However, for pneumonia, HRRP had no
statistically significant effect on readmissions but caused an increase in
mortality.Comment: 10 pages, 1 figure, 2 table
Causal health impacts of power plant emission controls under modeled and uncertain physical process interference
Causal inference with spatial environmental data is often challenging due to
the presence of interference: outcomes for observational units depend on some
combination of local and non-local treatment. This is especially relevant when
estimating the effect of power plant emissions controls on population health,
as pollution exposure is dictated by (i) the location of point-source
emissions, as well as (ii) the transport of pollutants across space via dynamic
physical-chemical processes. In this work, we estimate the effectiveness of air
quality interventions at coal-fired power plants in reducing two adverse health
outcomes in Texas in 2016: pediatric asthma ED visits and Medicare all-cause
mortality. We develop methods for causal inference with interference when the
underlying network structure is not known with certainty and instead must be
estimated from ancillary data. We offer a Bayesian, spatial mechanistic model
for the interference mapping which we combine with a flexible non-parametric
outcome model to marginalize estimates of causal effects over uncertainty in
the structure of interference. Our analysis finds some evidence that emissions
controls at upwind power plants reduce asthma ED visits and all-cause
mortality, however accounting for uncertainty in the interference renders the
results largely inconclusive.Comment: 22 pages, 5 figures. Associated code and supplementary material can
be found at https://github.com/nbwikle/estimating-interferenc
Acceleration Of Protons To Above 6 MeV Using H2O >Snow> Nanowire Targets
A scheme is presented for using H2O >snow> nanowire targets for the generation of fast protons. This novel method may relax the requirements for very high laser intensities, thus reducing the size and cost of laser based ion acceleration system.Physic
Characterization of self-injected electron beams from LWFA experiments at SPARC_LAB
The plasma-based acceleration is an encouraging technique to overcome the
limits of the accelerating gradient in the conventional RF acceleration. A
plasma accelerator is able to provide accelerating fields up to hundreds of
, paving the way to accelerate particles to several MeV over a short
distance (below the millimetre range). Here the characteristics of preliminary
electron beams obtained with the self-injection mechanism produced with the
FLAME high-power laser at the SPARC_LAB test facility are shown. In detail,
with an energy laser on focus of and a pulse temporal length (FWHM) of
, we obtained an electron plasma density due to laser ionization of
about , electron energy up to and beam
charge in the range .Comment: 6 pages, 11 figures, conference EAAC201
5.5-7.5 MeV Proton generation by a moderate intensity ultra-short laser interaction with H2O nano-wire targets
We report on the first generation of 5.5-7.5 MeV protons by a moderate
intensity short-pulse laser (4.5 \times 1017 W/cm^2, 50 fsec) interacting with
H2O nano-wires (snow) deposited on a Sapphire substrate. In this setup, the
laser intensity is locally enhanced by the tip of the snow nano-wire, leading
to high spatial gradients. Accordingly, the plasma near the tip is subject to
enhanced ponderomotive potential, and confined charge separation is obtained.
Electrostatic fields of extremely high intensities are produced over the short
scale length, and protons are accelerated to MeV-level energies.Comment: submitted to PRL, under press embargo. 6 figure
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