9,487 research outputs found
A linear noise approximation for stochastic epidemic models fit to partially observed incidence counts
Stochastic epidemic models (SEMs) fit to incidence data are critical to
elucidating outbreak dynamics, shaping response strategies, and preparing for
future epidemics. SEMs typically represent counts of individuals in discrete
infection states using Markov jump processes (MJPs), but are computationally
challenging as imperfect surveillance, lack of subject-level information, and
temporal coarseness of the data obscure the true epidemic. Analytic integration
over the latent epidemic process is impossible, and integration via Markov
chain Monte Carlo (MCMC) is cumbersome due to the dimensionality and
discreteness of the latent state space. Simulation-based computational
approaches can address the intractability of the MJP likelihood, but are
numerically fragile and prohibitively expensive for complex models. A linear
noise approximation (LNA) that approximates the MJP transition density with a
Gaussian density has been explored for analyzing prevalence data in
large-population settings, but requires modification for analyzing incidence
counts without assuming that the data are normally distributed. We demonstrate
how to reparameterize SEMs to appropriately analyze incidence data, and fold
the LNA into a data augmentation MCMC framework that outperforms deterministic
methods, statistically, and simulation-based methods, computationally. Our
framework is computationally robust when the model dynamics are complex and
applies to a broad class of SEMs. We evaluate our method in simulations that
reflect Ebola, influenza, and SARS-CoV-2 dynamics, and apply our method to
national surveillance counts from the 2013--2015 West Africa Ebola outbreak
A Simple BATSE Measure of GRB Duty Cycle
We introduce a definition of gamma-ray burst (GRB) duty cycle that describes
the GRB's efficiency as an emitter; it is the GRB's average flux relative to
the peak flux. This GRB duty cycle is easily described in terms of measured
BATSE parameters; it is essentially fluence divided by the quantity peak flux
times duration. Since fluence and duration are two of the three defining
characteristics of the GRB classes identified by statistical clustering
techniques (the other is spectral hardness), duty cycle is a potentially
valuable probe for studying properties of these classes.Comment: 4 pages, 1 figure, presented at the 5th Huntsville Gamma-Ray Burst
Symposiu
An analytical and experimental study of injection-locked two-port oscillators
A Ku-band IMPATT oscillator with two distinct output power ports was injection-locked alternately at both ports. The transmission locking bandwidth was nearly the same for either port. The lower free running power port had a reflection locking bandwidth that was narrower than its transmission locking one. Just the opposite was found at the other port. A detailed analytical model for two-port injection-locked oscillators is presented, and its results agree quite well with the experiments. A critique of the literature on this topic is included to clear up misconceptions and errors. It is concluded that two-port injection-locked oscillators may prove useful in certain communication systems
The Age, Metallicity and Alpha-Element Abundance of Galactic Globular Clusters from Single Stellar Population Models
Establishing the reliability with which stellar population parameters can be
measured is vital to extragalactic astronomy. Galactic GCs provide an excellent
medium in which to test the consistency of Single Stellar Population (SSP)
models as they should be our best analogue to a homogeneous (single) stellar
population. Here we present age, metallicity and -element abundance
measurements for 48 Galactic globular clusters (GCs) as determined from
integrated spectra using Lick indices and SSP models from Thomas, Maraston &
Korn, Lee & Worthey and Vazdekis et al. By comparing our new measurements to
independent determinations we are able to assess the ability of these SSPs to
derive consistent results -- a key requirement before application to
heterogeneous stellar populations like galaxies.
We find that metallicity determinations are extremely robust, showing good
agreement for all models examined here, including a range of enhancement
methods. Ages and -element abundances are accurate for a subset of our
models, with the caveat that the range of these parameters in Galactic GCs is
limited. We are able to show that the application of published Lick index
response functions to models with fixed abundance ratios allows us to measure
reasonable -element abundances from a variety of models. We also
examine the age-metallicity and [/Fe]-metallicity relations predicted
by SSP models, and characterise the possible effects of varied model horizontal
branch morphology on our overall results.Comment: 22 pages, 19 figures, accepted for publication in MNRA
Efficient data augmentation for fitting stochastic epidemic models to prevalence data
Stochastic epidemic models describe the dynamics of an epidemic as a disease
spreads through a population. Typically, only a fraction of cases are observed
at a set of discrete times. The absence of complete information about the time
evolution of an epidemic gives rise to a complicated latent variable problem in
which the state space size of the epidemic grows large as the population size
increases. This makes analytically integrating over the missing data infeasible
for populations of even moderate size. We present a data augmentation Markov
chain Monte Carlo (MCMC) framework for Bayesian estimation of stochastic
epidemic model parameters, in which measurements are augmented with
subject-level disease histories. In our MCMC algorithm, we propose each new
subject-level path, conditional on the data, using a time-inhomogeneous
continuous-time Markov process with rates determined by the infection histories
of other individuals. The method is general, and may be applied, with minimal
modifications, to a broad class of stochastic epidemic models. We present our
algorithm in the context of multiple stochastic epidemic models in which the
data are binomially sampled prevalence counts, and apply our method to data
from an outbreak of influenza in a British boarding school
Origin of Concentric Banding in the Spines of the Tropical Echinoid Heterocentrotus
The concentric bands or rings which are
evident in transverse sections of echinoid spines
have attracted the interest of biologists for more
than a hundred years. First, the basic cause of
ring formation remains obscure although at
least two controversial hypotheses have been
proposed, and elucidation of the ring-forming
mechanism is important to a better understanding
of the general processes involved in growth.
Second, there is a practical aspect to the "growthring
problem" because, if the number of rings
in a spine is closely and directly related to the
age of the animal, killing the urchin for age
determination is obviated, thus facilitating ecological,
population, and other practical studies
in marine research
Expansion Potential for Irrigation within the Mississippi Delta Region
17.6 million acres, or 73 percent, of the Mississippi Delta Region is currently cropland and possesses the physical characteristics of slope, texture and soil type which are recommended for irrigation. Economic feasibility of expanding irrigation by flood, furrow and center pivot methods were examined under 24 scenarios representing two sets of crop prices, yield levels, production costs, opportunity costs and six crop rotations. Irrigation was economically feasible for 56 to 100 percent of the cropland across all scenarios. Approximately 88 percent of the cropland can be economically irrigated with flood or furrow in its present form, 8 percent yield highest net returns if furrow irrigated following land forming and 4 percent can be economically irrigated only with center pivot systems
What absent switch costs and mixing costs during bilingual language comprehension can tell us about language control.
Epub 2019 Mar 28.In the current study, we set out to investigate language control, which is the process that minimizes cross-language interference, during bilingual language comprehension. According to current theories of bilingual language comprehension, language-switch costs, which are a marker for reactive language control, should be observed. However, a closer look at the literature shows that this is not always the case. Furthermore, little to no evidence for language-mixing costs, which are a marker for proactive language control, has been observed in the bilingual language comprehension literature. This is in line with current theories of bilingual language comprehension, as they do not explicitly account for proactive language control. In the current study, we further investigated these two markers of language control and found no evidence for comprehension-based language-switch costs in six experiments, even though other types of switch costs were observed with the exact same setup (i.e., task-switch costs, stimulus modality-switch costs, and production-based language-switch costs). Furthermore, only one out of three experiments showed comprehension-based language-mixing costs, providing the first tentative evidence for proactive language control during bilingual language comprehension. The implications of the absence and occurrence of these costs are discussed in terms of processing speed and parallel language activation. (PsycINFO Database Record (c) 2019 APA, all rights reserved)This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 706128. This research was also supported by grants ANR-11-LABX-0036 (BLRI), ANR-16-CONV-0002 (ILCB), and ANR-11-IDEX-0001-02 from the French National Research Council (ANR)
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