8,876 research outputs found
Sensitivity Analysis for Unmeasured Confounding in Meta-Analyses
Random-effects meta-analyses of observational studies can produce biased
estimates if the synthesized studies are subject to unmeasured confounding. We
propose sensitivity analyses quantifying the extent to which unmeasured
confounding of specified magnitude could reduce to below a certain threshold
the proportion of true effect sizes that are scientifically meaningful. We also
develop converse methods to estimate the strength of confounding capable of
reducing the proportion of scientifically meaningful true effects to below a
chosen threshold. These methods apply when a "bias factor" is assumed to be
normally distributed across studies or is assessed across a range of fixed
values. Our estimators are derived using recently proposed sharp bounds on
confounding bias within a single study that do not make assumptions regarding
the unmeasured confounders themselves or the functional form of their
relationships to the exposure and outcome of interest. We provide an R package,
ConfoundedMeta, and a freely available online graphical user interface that
compute point estimates and inference and produce plots for conducting such
sensitivity analyses. These methods facilitate principled use of random-effects
meta-analyses of observational studies to assess the strength of causal
evidence for a hypothesis
Chandra discovery of the intracluster medium around UM425 at redshift 1.47
We report on a discovery of a candidate cluster of galaxies at redshift
z=1.47 based on Chandra observations in the field of quasars UM425 A & B. We
detect with high significance diffuse emission due the intracluster hot gas
around the quasar pair. This is the second highest redshift cluster candidate
after 3C294 at z=1.786. The diffuse emission is elliptical in shape with about
17" extent. If indeed at z=1.47, this corresponds to a physical size of 140
h_{70}^{-1} Kpc and 2--10 keV luminosity of about 3 times 10^{43} erg/s. The
cluster is unlikely to be the long sought gravitational lens invoked to explain
unusual brightness of UM425 A and the close quasar pair. Coexistence of the
quasars with the cluster suggests a link of activity to cluster environment.
The unusual brightness of UM425 A may then be due to a higher accretion rate.
We also comment briefly on the X-ray spectra of UM 425 A & B which also happen
to be broad absorption line quasars. We argue that present evidence suggests
that the quasars are just a pair and not lensed images of the same quasar.Comment: Submitted to ApJ Letter
Bone marrow mononuclear cells and acute myocardial infarction
PMCID: PMC3340546This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited
Chandra LETGS spectroscopy of the Quasar MR2251-178 and its warm absorber
We present an analysis of our Chandra Low Energy Transmission Grating
Spectrometer (LETGS) observation of the quasar MR2251-178. The warm absorber of
MR2251-178 is well described by a hydrogen column density, N_H~2x10^21 cm^-2,
and an ionization parameter log(xi)~0.6. We find in the spectrum weak evidence
for narrow absorption lines from Carbon and Nitrogen which indicate that the
ionized material is in outflow. We note changes (in time) of the absorption
structure in the band (0.6-1) keV (around the UTAs plus the OVII and OVIII
K-edges) at different periods of the observation. We measure a (0.1-2) keV flux
of 2.58x10^-11 ergs cm^-2 s^-1. This flux implies that the nuclear source of
MR2251-178 is in a relatively low state. No significant variability is seen in
the light curve. We do not find evidence for an extra cold material in the line
of sight, and set an upper limit of N_H~1.2x10^20 cm^-2. The X-ray spectrum
does not appear to show evidence for dusty material, though an upper limit in
the neutral carbon and oxygen column densities can only be set to N_CI~2x10^19
cm^-2 and N_OI~9x10^19 cm^-2, respectively.Comment: 42 pages, 12 figures, Accepted in Apj. Typo in abstract (ver2): "We
do not find evidence for an extra...
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