36,949 research outputs found
Survivor-complier effects in the presence of selection on treatment, with application to a study of prompt ICU admission
Pre-treatment selection or censoring (`selection on treatment') can occur
when two treatment levels are compared ignoring the third option of neither
treatment, in `censoring by death' settings where treatment is only defined for
those who survive long enough to receive it, or in general in studies where the
treatment is only defined for a subset of the population. Unfortunately, the
standard instrumental variable (IV) estimand is not defined in the presence of
such selection, so we consider estimating a new survivor-complier causal
effect. Although this effect is generally not identified under standard IV
assumptions, it is possible to construct sharp bounds. We derive these bounds
and give a corresponding data-driven sensitivity analysis, along with
nonparametric yet efficient estimation methods. Importantly, our approach
allows for high-dimensional confounding adjustment, and valid inference even
after employing machine learning. Incorporating covariates can tighten bounds
dramatically, especially when they are strong predictors of the selection
process. We apply the methods in a UK cohort study of critical care patients to
examine the mortality effects of prompt admission to the intensive care unit,
using ICU bed availability as an instrument
Barbaetis: A New Genus of Eastern Nearctic Mayflies (Ephemeroptera: Baetidae)
The new genus Barbaetis Waltz and McCafferty, and new species Barbaetis benfieldi Kennedy are described from larvae collected from the New River, Virginia. Barbaetis is easily told from Baetis by the presence of procoxal osmobranchia. Cladistics of B. benfieldi, related Pseudocloeon species, and the lutheri and pavidus complexes of Baetis are presented and indicate the need for further taxonomic revision. The habitat of B. benfieldi is described in terms of several ecological parameters. The new species demonstrates a univoltine life history with postembryonic development restricted to a short springtime period
Data Power in Material Contexts: Introduction
This short piece introduces the special issue of Television & New Media (TVNM) on data power in material contexts, which brings together papers which analyze the operations of data power across a range of real-world domains. It highlights the increasing connectedness of digital data tracking, aggregation, and analytics across domains that include and move beyond media, as data are increasingly combined and shared across diverse digital spaces. Thus, it connects media and communications scholarship concerned with datafication to debates in other related and overlapping fields, as part of the larger project of building data studies as an interdisciplinary and critical field. It briefly introduces the papers in the special issue, all of which constitute detailed empirical investigations that ground the study of data power in specific, material contexts
Transforming fixed-length self-avoiding walks into radial SLE_8/3
We conjecture a relationship between the scaling limit of the fixed-length
ensemble of self-avoiding walks in the upper half plane and radial SLE with
kappa=8/3 in this half plane from 0 to i. The relationship is that if we take a
curve from the fixed-length scaling limit of the SAW, weight it by a suitable
power of the distance to the endpoint of the curve and then apply the conformal
map of the half plane that takes the endpoint to i, then we get the same
probability measure on curves as radial SLE. In addition to a non-rigorous
derivation of this conjecture, we support it with Monte Carlo simulations of
the SAW. Using the conjectured relationship between the SAW and radial SLE, our
simulations give estimates for both the interior and boundary scaling
exponents. The values we obtain are within a few hundredths of a percent of the
conjectured values
Phase separation due to quantum mechanical correlations
Can phase separation be induced by strong electron correlations? We present a
theorem that affirmatively answers this question in the Falicov-Kimball model
away from half-filling, for any dimension. In the ground state the itinerant
electrons are spatially separated from the classical particles.Comment: 4 pages, 1 figure. Note: text and figure unchanged, title was
misspelle
Non-parabolicity of the conduction band of wurtzite GaN
Using cyclotron resonance, we measure the effective mass, *, of electrons
in AlGaN/GaN heterostructures with densities, cm. From our extensive data, we extrapolate a band edge
mass of . By comparing our * data with the results of a
multi-band \textbf{k.p} calculation we infer that the effect of remote bands is
essential in explaining the observed conduction band non-parabolicity (NP). Our
calculation of polaron mass corrections -- including finite width and screening
- suggests those to be negligible. It implies that the behavior of
can be understood solely in terms of NP. Finally, using our NP and polaron
corrections, we are able to reduce the large scatter in the published band edge
mass values
Correction of Optical Aberrations in Elliptic Neutron Guides
Modern, nonlinear ballistic neutron guides are an attractive concept in
neutron beam delivery and instrumentation, because they offer increased
performance over straight or linearly tapered guides. However, like other
ballistic geometries they have the potential to create significantly
non-trivial instrumental resolution functions. We address the source of the
most prominent optical aberration, namely coma, and we show that for extended
sources the off-axis rays have a different focal length from on-axis rays,
leading to multiple reflections in the guide system. We illustrate how the
interplay between coma, sources of finite size, and mirrors with non-perfect
reflectivity can therefore conspire to produce uneven distributions in the
neutron beam divergence, the source of complicated resolution functions. To
solve these problems, we propose a hybrid elliptic-parabolic guide geometry.
Using this new kind of neutron guide shape, it is possible to condition the
neutron beam and remove almost all of the aberrations, whilst providing the
same performance in beam current as a standard elliptic neutron guide. We
highlight the positive implications for a number of neutron scattering
instrument types that this new shape can bring.Comment: Presented at NOP2010 Conference in Alpe d'Huez, France, in March 201
Considering long-memory when testing for changepoints in surface temperature:a classification approach based on the time-varying spectrum
Changepoint models are increasingly used to represent changes in the rate of warming in surface temperature records. On the opposite hand, a large body of literature has suggested long‐memory processes to characterize long‐term behavior in surface temperatures. While these two model representations provide different insights into the underlying mechanisms, they share similar spectrum properties that create “ambiguity” and challenge distinguishing between the two classes of models. This study aims to compare the two representations to explain temporal changes and variability in surface temperatures. To address this question, we extend a recently developed time‐varying spectral procedure and assess its accuracy through a synthetic series mimicking observed global monthly surface temperatures. We vary the length of the synthetic series to determine the number of observations needed to be able to accurately distinguish between changepoints and long‐memory models. We apply the approach to two gridded surface temperature data sets. Our findings unveil regions in the oceans where long‐memory is prevalent. These results imply that the presence of long‐memory in monthly sea surface temperatures may impact the significance of trends, and special attention should be given to the choice of model representing memory (short versus long) when assessing long‐term changes
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