2,844 research outputs found
Approximate Bayesian inference for doubly robust estimation
Doubly robust estimators are typically constructed by combining outcome regression and propensity score models to satisfy moment restrictions that ensure consistent estimation of causal quantities provided at least one of the component models is correctly specified. Standard Bayesian methods are difficult to apply because restricted moment models do not imply fully specified likelihood functions. This paper proposes a Bayesian bootstrap approach to derive approximate posterior predictive distributions that are doubly robust for estimation of causal quantities. Simulations show that the approach performs well under various sources of misspecification of the outcome regression or propensity score models. The estimator is applied in a case study of the effect of area deprivation on the incidence of child pedestrian casualties in British cities
Fish oil omega-3 fatty acids partially prevent lipid-induced insulin resistance in human skeletal muscle without limiting acylcarnitine accumulation
This is the author accepted manuscript. The final version is available from Portland Press via the DOI in this record Acylcarnitine accumulation in skeletal muscle and plasma has been observed in numerous models of mitochondrial lipid overload and insulin resistance. Fish oil n3PUFA (omega-3 polyunsaturated fatty acids) are thought to protect against lipid-induced insulin resistance. The present study tested the hypothesis that the addition of n3PUFA to an intravenous lipid emulsion would limit muscle acylcarnitine accumulation and reduce the inhibitory effect of lipid overload on insulin action. On three occasions, six healthy young men underwent a 6-h euglycaemic-hyperinsulinaemic clamp accompanied by intravenous infusion of saline (Control), 10% Intralipid® [n6PUFA (omega-6 polyunsaturated fatty acids)] or 10% Intralipid®+10% Omegaven® (2:1; n3PUFA). The decline in insulin-stimulated whole-body glucose infusion rate, muscle PDCa (pyruvate dehydrogenase complex activation) and glycogen storage associated with n6PUFA compared with Control was prevented with n3PUFA. Muscle acetyl-CoA accumulation was greater following n6PUFA compared with Control and n3PUFA, suggesting that mitochondrial lipid overload was responsible for the lower insulin action observed. Despite these favourable metabolic effects of n3PUFA, accumulation of total muscle acylcarnitine was not attenuated when compared with n6PUFA. These findings demonstrate that n3PUFA exert beneficial effects on insulin-stimulated skeletal muscle glucose storage and oxidation independently of total acylcarnitine accumulation, which does not always reflect mitochondrial lipid overload.This research study was funded by The Royal Society [Grant RG100575]
Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 and AM
1343-452, with the aim of investigating the number and luminosity of any upper
asymptotic giant branch (AGB) stars present. The galaxies are members of the
Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc)
satellites of Cen A. The analysis is based on near-IR photometry for individual
red giant stars, derived from images obtained with ISAAC on the VLT. The
photometry, along with optical data derived from WFPC2 images retrieved from
the HST science archive, enable us to investigate the stellar populations of
the dEs in the vicinity of the red giant branch (RGB) tip. In both systems we
find stars above the RGB tip, which we interpret as intermediate-age upper-AGB
stars. The presence of such stars is indicative of extended star formation in
these dEs similar to that seen in many, but not all, dEs in the Local Group.
For AM 1339-445, the brightest of the upper-AGB stars have Mbol~-4.5 while
those in AM 1343-452 have Mbol~-4.8 mag. These luminosities suggest ages of
approximately 6.5+/-1 and 4+/-1 Gyr as estimates for the epoch of the last
episode of significant star formation in these systems. In both cases the
number of upper-AGB stars suggests that ~15% of the total stellar population is
in the form of intermediate-age stars, considerably less than is the case for
outlying dE satellites of the Milky Way such as Fornax and LeoI.Comment: 18 pages, 19 figures, A&A accepted; high resolution version available
from: http://www.eso.org/~mrejkuba/CenA_dEs_I.pd
The origin of power-law distributions in deterministic walks: the influence of landscape geometry
We investigate the properties of a deterministic walk, whose locomotion rule
is always to travel to the nearest site. Initially the sites are randomly
distributed in a closed rectangular ( landscape and, once
reached, they become unavailable for future visits. As expected, the walker
step lengths present characteristic scales in one () and two () dimensions. However, we find scale invariance for an intermediate
geometry, when the landscape is a thin strip-like region. This result is
induced geometrically by a dynamical trapping mechanism, leading to a power law
distribution for the step lengths. The relevance of our findings in broader
contexts -- of both deterministic and random walks -- is also briefly
discussed.Comment: 7 pages, 11 figures. To appear in Phys. Rev.
Collisionless dynamics in Globular Clusters
Since globular clusters (GCs) are old, low-N systems their dynamics is widely
believed to be fully dominated by collisional two-body processes, and their
surface brightness profiles are fit by King models. However, for many GCs,
especially those with HST-resolved central regions, and `extra-tidal' features,
King models provide poor fits. We suggest that this is partly because
collisionless dynamics is also important and contribute to shaping the cluster
properties. We show using time-scale and length-scale arguments that except for
the very centers of clusters, collisionless dynamics should be more important
than collisional. We then fit 38 GCs analyzed by Noyola and Gebhardt (2006)
with (collisional) King and (collisionless) DARKexp models over the full
available radial range, and find that the latter provide a better fit to 29
GCs; for six of these the fit is at least ~5x better in term of rms. DARKexp
models are theoretically derived maximum entropy equilibrium states of
self-gravitating collisionless systems and have already been shown to fit the
results of dark matter N-body simulations. (We do not attempt fits with ad hoc
fitting functions.)Comment: 27 pages, 18 figures; accepted to MNRA
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