3,517 research outputs found
Multiply Robust Causal Inference with Double Negative Control Adjustment for Categorical Unmeasured Confounding
Unmeasured confounding is a threat to causal inference in observational
studies. In recent years, use of negative controls to mitigate unmeasured
confounding has gained increasing recognition and popularity. Negative controls
have a longstanding tradition in laboratory sciences and epidemiology to rule
out non-causal explanations, although they have been used primarily for bias
detection. Recently, Miao et al. (2018) have described sufficient conditions
under which a pair of negative control exposure and outcome variables can be
used to nonparametrically identify the average treatment effect (ATE) from
observational data subject to uncontrolled confounding. In this paper, we
establish nonparametric identification of the ATE under weaker conditions in
the case of categorical unmeasured confounding and negative control variables.
We also provide a general semiparametric framework for obtaining inferences
about the ATE while leveraging information about a possibly large number of
measured covariates. In particular, we derive the semiparametric efficiency
bound in the nonparametric model, and we propose multiply robust and locally
efficient estimators when nonparametric estimation may not be feasible. We
assess the finite sample performance of our methods in extensive simulation
studies. Finally, we illustrate our methods with an application to the
postlicensure surveillance of vaccine safety among children
Adaptive locomotion of artificial microswimmers
Bacteria can exploit mechanics to display remarkable plasticity in response
to locally changing physical and chemical conditions. Compliant structures play
a striking role in their taxis behavior, specifically for navigation inside
complex and structured environments. Bioinspired mechanisms with rationally
designed architectures capable of large, nonlinear deformation present
opportunities for introducing autonomy into engineered small-scale devices.
This work analyzes the effect of hydrodynamic forces and rheology of local
surroundings on swimming at low Reynolds number, identifies the challenges and
benefits of utilizing elastohydrodynamic coupling in locomotion, and further
develops a suite of machinery for building untethered microrobots with
self-regulated mobility. We demonstrate that coupling the structural and
magnetic properties of artificial microswimmers with the dynamic properties of
the fluid leads to adaptive locomotion in the absence of on-board sensors
An Empirically Derived Three-Dimensional Laplace Resonance in the Gliese 876 Planetary System
We report constraints on the three-dimensional orbital architecture for all
four planets known to orbit the nearby M dwarf Gliese 876 based solely on
Doppler measurements and demanding long-term orbital stability. Our dataset
incorporates publicly available radial velocities taken with the ELODIE and
CORALIE spectrographs, HARPS, and Keck HIRES as well as previously unpublished
HIRES velocities. We first quantitatively assess the validity of the planets
thought to orbit GJ 876 by computing the Bayes factors for a variety of
different coplanar models using an importance sampling algorithm. We find that
a four-planet model is preferred over a three-planet model. Next, we apply a
Newtonian MCMC algorithm to perform a Bayesian analysis of the planet masses
and orbits using an n-body model in three-dimensional space. Based on the
radial velocities alone, we find that a 99% credible interval provides upper
limits on the mutual inclinations for the three resonant planets
( for the "c" and "b" pair and for
the "b" and "e" pair). Subsequent dynamical integrations of our posterior
sample find that the GJ 876 planets must be roughly coplanar
( and ), suggesting the amount of
planet-planet scattering in the system has been low. We investigate the
distribution of the respective resonant arguments of each planet pair and find
that at least one argument for each planet pair and the Laplace argument
librate. The libration amplitudes in our three-dimensional orbital model
supports the idea of the outer-three planets having undergone significant past
disk migration.Comment: 19 pages, 11 figures, 8 tables. Accepted to MNRAS. Posterior samples
available at https://github.com/benelson/GJ87
The 55 Cancri Planetary System: Fully Self-Consistent N-body Constraints and a Dynamical Analysis
We present an updated study of the planets known to orbit 55 Cancri A using
1,418 high-precision radial velocity observations from four observatories
(Lick, Keck, Hobby-Eberly Telescope, Harlan J. Smith Telescope) and transit
time/durations for the inner-most planet, 55 Cancri "e" (Winn et al. 2011). We
provide the first posterior sample for the masses and orbital parameters based
on self-consistent n-body orbital solutions for the 55 Cancri planets, all of
which are dynamically stable (for at least years). We apply a GPU
version of Radial velocity Using N-body Differential evolution Markov Chain
Monte Carlo (RUN DMC; B. Nelson et al. 2014) to perform a Bayesian analysis of
the radial velocity and transit observations. Each of the planets in this
remarkable system has unique characteristics. Our investigation of high-cadence
radial velocities and priors based on space-based photometry yields an updated
mass estimate for planet "e" ( M), which affects its
density ( g cm) and inferred bulk composition.
Dynamical stability dictates that the orbital plane of planet "e" must be
aligned to within of the orbital plane of the outer planets (which we
assume to be coplanar). The mutual interactions between the planets "b" and "c"
may develop an apsidal lock about . We find 36-45% of all our model
systems librate about the anti-aligned configuration with an amplitude of
. Other cases showed short-term perturbations in the
libration of , circulation, and nodding, but we find the
planets are not in a 3:1 mean-motion resonance. A revised orbital period and
eccentricity for planet "d" pushes it further toward the closest known Jupiter
analog in the exoplanet population.Comment: 12 pages, 5 figures, 4 tables, accepted to MNRAS. Figure 2 (left) is
updated from published version. Posterior samples available at
http://www.personal.psu.edu/ben125/Downloads.htm
Overwintering Seeds as Reservoirs for Seedling Pathogens of Wetland Plant Species
Seed germination and seedling establishment are central to the distribution and abundance of plant species in wetlands. While fungal and oomycete pathogens are known to affect seed viability and emergence, relatively little is known about which fungi and oomycetes are associated with seeds in the soil or how these species affect seeds and seedlings. We characterized the fungi and oomycetes associated with overwintering seeds in wetlands and determined their potential to influence seed germination and subsequent seedling mortality. Fungi and oomycetes did not affect seed germination, despite the isolation of high frequencies of known seed and seedling pathogens in the fungal genera Alternaria, Peyronellaea, Epicoccum, and Fusarium. However, many of the most frequently isolated fungal species from overwintering seeds were highly virulent to seedlings. While both native and nonnative plant species were tested, we did not observe consistent differences in either seed germination or seedling susceptibility based on the invasive status of plants tested, contrary to what we expected given several established hypotheses for invasive success. The high seedling virulence of fungi from overwintering seeds coupled with the differential abundance of some of the more pathogenic fungi among seeds of different plant species, led us to the conclusion that the fungal pathogens that colonize seeds in the seed bank over winter are likely to strongly impact subsequent seedling establishment in wetlands the following spring despite not reducing overwintering seed germination in the seed bank or differently effecting invasive plant species
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