19,464 research outputs found
Nested Markov Properties for Acyclic Directed Mixed Graphs
Directed acyclic graph (DAG) models may be characterized in at least four
different ways: via a factorization, the d-separation criterion, the
moralization criterion, and the local Markov property. As pointed out by Robins
(1986, 1999), Verma and Pearl (1990), and Tian and Pearl (2002b), marginals of
DAG models also imply equality constraints that are not conditional
independences. The well-known `Verma constraint' is an example. Constraints of
this type were used for testing edges (Shpitser et al., 2009), and an efficient
marginalization scheme via variable elimination (Shpitser et al., 2011).
We show that equality constraints like the `Verma constraint' can be viewed
as conditional independences in kernel objects obtained from joint
distributions via a fixing operation that generalizes conditioning and
marginalization. We use these constraints to define, via Markov properties and
a factorization, a graphical model associated with acyclic directed mixed
graphs (ADMGs). We show that marginal distributions of DAG models lie in this
model, prove that a characterization of these constraints given in (Tian and
Pearl, 2002b) gives an alternative definition of the model, and finally show
that the fixing operation we used to define the model can be used to give a
particularly simple characterization of identifiable causal effects in hidden
variable graphical causal models.Comment: 67 pages (not including appendix and references), 8 figure
Sparse Nested Markov models with Log-linear Parameters
Hidden variables are ubiquitous in practical data analysis, and therefore
modeling marginal densities and doing inference with the resulting models is an
important problem in statistics, machine learning, and causal inference.
Recently, a new type of graphical model, called the nested Markov model, was
developed which captures equality constraints found in marginals of directed
acyclic graph (DAG) models. Some of these constraints, such as the so called
`Verma constraint', strictly generalize conditional independence. To make
modeling and inference with nested Markov models practical, it is necessary to
limit the number of parameters in the model, while still correctly capturing
the constraints in the marginal of a DAG model. Placing such limits is similar
in spirit to sparsity methods for undirected graphical models, and regression
models. In this paper, we give a log-linear parameterization which allows
sparse modeling with nested Markov models. We illustrate the advantages of this
parameterization with a simulation study.Comment: Appears in Proceedings of the Twenty-Ninth Conference on Uncertainty
in Artificial Intelligence (UAI2013
Assessing the potential impact of strengthening food safety regulations on developing countries: The US Food Safety and Modernization Act
This paper’s goal is to assess the extent to which producers in Developing Countries have coped stricter US food safety regulations. We approach the question by calculating refusals/imports ratios and their trends for a sample of Developing Countries. We conclude there is a learning process in low value added products.food safety, traceability, value chain analysis, trade, Latin America and the Caribbean, Food Consumption/Nutrition/Food Safety, International Development, International Relations/Trade,
Tolerability of topical NSAIDs in the elderly - Do they really convey a safety advantage?
Non-steroidal anti-inflammatory drugs (NSAIDs) are very widely prescribed but they have a poor safety profile, with a range of potential adverse effects. NSAIDs that are used in topical formulation have been developed in the last fifteen years and their use is increasing. The purpose is to achieve a high local concentration of the active ingredient at the affected site, with as low a plasma concentration as possible to minimise possible systemic side-effects. This paper reviews the evidence available regarding the safety of topical NSAIDs, with particular reference to gastrointestinal and renal toxicity which have recently been highlighted in the medical press. The effects on the elderly are also discussed. The authors conclude that more formal epidemiological evaluation of the safety of topical NSAIDs is required in the post-marketing situation
Exoplanet Transmission Spectroscopy using KMOS
KMOS (K-Band Multi Object Spectrograph) is a novel integral field
spectrograph installed in the VLT's ANTU unit. The instrument offers an ability
to observe 24 2.8"2.8" sub-fields positionable within a 7.2' patrol
field, each sub-field producing a spectrum with a 1414-pixel spatial
resolution. The main science drivers for KMOS are the study of galaxies, star
formation, and molecular clouds, but its ability to simultaneously measure
spectra of multiple stars makes KMOS an interesting instrument for exoplanet
atmosphere characterization via transmission spectroscopy. We set to test
whether transmission spectroscopy is practical with KMOS, and what are the
conditions required to achieve the photometric precision needed, based on
observations of a partial transit of WASP-19b, and full transits of GJ 1214b
and HD 209458b. Our analysis uses the simultaneously observed comparison stars
to reduce the effects from instrumental and atmospheric sources, and Gaussian
processes to model the residual systematics. We show that KMOS can, in theory,
deliver the photometric precision required for transmission spectroscopy.
However, this is shown to require a) pre-imaging to ensure accurate centering
and b) a very stable night with optimal observing conditions (seeing
0.8"). Combining these two factors with the need to observe several
transits, each with a sufficient out-of-transit baseline (and with the fact
that similar or better precision can be reached with telescopes and instruments
with smaller pressure,) we conclude that transmission spectroscopy is not the
optimal science case to take advantage of the abilities offered by KMOS and
VLT.Comment: 11 pages, accepted to MNRA
A uniform analysis of HD209458b Spitzer/IRAC lightcurves with Gaussian process models
We present an analysis of Spitzer/IRAC primary transit and secondary eclipse
lightcurves measured for HD209458b, using Gaussian process models to
marginalise over the intrapixel sensitivity variations in the 3.6 micron and
4.5 micron channels and the ramp effect in the 5.8 micron and 8.0 micron
channels. The main advantage of this approach is that we can account for a
broad range of degeneracies between the planet signal and systematics without
actually having to specify a deterministic functional form for the latter. Our
results do not confirm a previous claim of water absorption in transmission.
Instead, our results are more consistent with a featureless transmission
spectrum, possibly due to a cloud deck obscuring molecular absorption bands.
For the emission data, our values are not consistent with the thermal inversion
in the dayside atmosphere that was originally inferred from these data.
Instead, we agree with another re-analysis of these same data, which concluded
a non-inverted atmosphere provides a better fit. We find that a solar-abundance
clear-atmosphere model without a thermal inversion underpredicts the measured
emission in the 4.5 micron channel, which may suggest the atmosphere is
depleted in carbon monoxide. An acceptable fit to the emission data can be
achieved by assuming that the planet radiates as an isothermal blackbody with a
temperature of K.Comment: 18 pages, 5 figures, 6 tables. Accepted by MNRA
Discovery of Seven Companions To Intermediate-Mass Stars With Extreme Mass Ratios in the Scorpius-Centaurus Association
We report the detection of seven low-mass companions to intermediate-mass stars (SpT B/A/F; M similar to 1.5-4.5M(circle dot)) in the Scorpius-Centaurus (Sco-Cen) Association using nonredundant aperture masking interferometry. Our newly detected objects have contrasts Delta L' approximate to 4-6, corresponding to masses as low as similar to 20 M-Jup and mass ratios of q approximate to 0.01-0.08, depending on the assumed age of the target stars. With projected separations rho approximate to 10-30 AU, our aperture masking detections sample an orbital region previously unprobed by conventional adaptive optics imaging of intermediate-mass Sco-Cen stars covering much larger orbital radii (similar to 30-3000 AU). At such orbital separations, these objects resemble higher-mass versions of the directly imaged planetary mass companions to the 10-30 Myr, intermediate-mass stars HR 8799, beta Pictoris, and HD 95086. These newly discovered companions span the brown dwarf desert, and their masses and orbital radii provide a new constraint on models of the Formation of low-mass stellar and substellar companions to intermediate-mass stars.NASA through the Sagan Fellowship ProgramNSF Astronomy and Astrophysics Postdoctoral Fellowship AST-1203023Clay FellowshipNASA through Hubble Fellowship 51257.01AURA, Inc., for NASA NAS 5-26555W. M. Keck FoundationAstronom
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