1,043 research outputs found
Empirical Models of Auctions
Many important economic questions arising in auctions can be answered only with knowledge of the underlying primitive distributions governing bidder demand and information. An active literature has developed aiming to estimate these primitives by exploiting restrictions from economic theory as part of the econometric model used to interpret auction data. We review some highlights of this recent literature, focusing on identification and empirical applications. We describe three insights that underlie much of the recent methodological progress in this area and discuss some of the ways these insights have been extended to richer models allowing more convincing empirical applications. We discuss several recent empirical studies using these methods to address a range of important economic questions
lassopack: Model selection and prediction with regularized regression in Stata
This article introduces lassopack, a suite of programs for regularized
regression in Stata. lassopack implements lasso, square-root lasso, elastic
net, ridge regression, adaptive lasso and post-estimation OLS. The methods are
suitable for the high-dimensional setting where the number of predictors
may be large and possibly greater than the number of observations, . We
offer three different approaches for selecting the penalization (`tuning')
parameters: information criteria (implemented in lasso2), -fold
cross-validation and -step ahead rolling cross-validation for cross-section,
panel and time-series data (cvlasso), and theory-driven (`rigorous')
penalization for the lasso and square-root lasso for cross-section and panel
data (rlasso). We discuss the theoretical framework and practical
considerations for each approach. We also present Monte Carlo results to
compare the performance of the penalization approaches.Comment: 52 pages, 6 figures, 6 tables; submitted to Stata Journal; for more
information see https://statalasso.github.io
Elemental Abundances in the X-Ray Gas of Early-Type Galaxies with XMM and Chandra Observations
The source of hot gas in elliptical galaxies is thought to be due to stellar
mass loss, with contributions from supernova events and possibly from infall
from a surrounding environment. This picture predicts supersolar values for the
metallicity of the gas toward the inner part of the galaxy, which can be tested
by measuring the gas phase abundances. We use high-quality data for 10 nearby
early-type galaxy from XMM-Newton, featuring both the EPIC and the Reflection
Grating Spectrometer, where the strongest emission lines are detected with
little blending; some Chandra data are also used. We find excellent consistency
in the elemental abundances between the different XMM instruments and good
consistency with Chandra. Differences in abundances with aperture size and
model complexity are examined, but large differences rarely occur. For a
two-temperature thermal model plus a point source contribution, the median Fe
and O abundances are 0.86 and 0.44 of the Solar value, while Si and Mg
abundances are similar to that for Fe. This is similar to stellar abundances
for these galaxies but supernovae were expected to enhance the gas phase
abundances considerably, which is not observed.Comment: 35 pages, 10 figures, accepted for publication in Astrophysical
Journa
Diagnosis of lung cancer – improving survival rates
Lung cancer is a major global health burden with high incidence rates but poor long-term survival. Currently, the majority of cases are diagnosed at an advanced stage when surgical resection is not feasible. Screening for lung cancer has been a major focus of research for the last 40 years. Despite this, there is still a lack of evidence to promote its use outside clinical trials. More recently, interest has focused on promoting earlier recognition of symptomatic disease among both the general public and primary care physicians in order to encourage more timely investigation and referral to secondary care. The hope is that this approach may increase the proportion of disease identified in the early tages, allowing more surgical resections and improved five-year survival rates. This article provides an overview of the current evidence base in terms of early diagnosis of lung cancer and provides some examples of innovations to promote this
The Lack of Very Ultraluminous X-ray Sources in Early-type Galaxies
We have searched for ultraluminous X-ray sources (ULXs) in a sample of 28
elliptical and S0 galaxies observed with Chandra. We find that the number of
X-ray sources detected at a flux level that would correspond to a 0.3-10 keV
X-ray luminosity of ~2 x 10^39 ergs/s or greater (for which we have used the
designation very ultraluminous X-ray sources; VULXs) at the distance of each
galaxy is equal to the number of expected foreground/background objects. In
addition, the VULXs are uniformly distributed over the Chandra field of view
rather than distributed like the optical light of the galaxies, strengthening
the argument that the high flux sources are unassociated with the galaxies. We
have also taken the VULX candidate list of Colbert and Ptak and determined the
spatial distribution of VULXs in early-type galaxies and late-type galaxies
separately. While the spiral galaxy VULXs are clearly concentrated toward the
centers of the galaxies, the early-type galaxy VULXs are distributed randomly
over the ROSAT HRI field of view, again indicating that they are not associated
with the galaxies themselves. We conclude that with the exception of two rare
high luminosity objects within globular clusters of the elliptical galaxy
NGC1399, VULXs are generally not found within old stellar systems. However, we
do find a significant population of sources with luminosities of 1-2 x 10^39
ergs/s that reside within the sample galaxies that can be explained by
accretion onto 10-20 solar mass black holes. Given our results, we propose that
ULXs be defined as X-ray sources with L_X(0.3-10 keV) > 2 x 10^39 ergs/s.Comment: 5 pages in emulateapj style with 2 embedded Postscript figure,
accepted by ApJ Letter
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