15,891 research outputs found
Upper Limits from Counting Experiments with Multiple Pipelines
In counting experiments, one can set an upper limit on the rate of a Poisson
process based on a count of the number of events observed due to the process.
In some experiments, one makes several counts of the number of events, using
different instruments, different event detection algorithms, or observations
over multiple time intervals. We demonstrate how to generalize the classical
frequentist upper limit calculation to the case where multiple counts of events
are made over one or more time intervals using several (not necessarily
independent) procedures. We show how different choices of the rank ordering of
possible outcomes in the space of counts correspond to applying different
levels of significance to the various measurements. We propose an ordering that
is matched to the sensitivity of the different measurement procedures and show
that in typical cases it gives stronger upper limits than other choices. As an
example, we show how this method can be applied to searches for
gravitational-wave bursts, where multiple burst-detection algorithms analyse
the same data set, and demonstrate how a single combined upper limit can be set
on the gravitational-wave burst rate.Comment: 26 pages (CQG style), 8 figures. Added study of robustness of limits
Systematic review and meta-analysis. small intestinal bacterial overgrowth in chronic pancreatitis
BACKGROUND:
Evidence on small intestinal bacterial overgrowth (SIBO) in patients with chronic pancreatitis (CP) is conflicting.
AIM:
The purpose of this study was to perform a systematic review and meta-analysis on the prevalence of SIBO in CP and to examine the relationship of SIBO with symptoms and nutritional status.
METHODS:
Case-control and cross-sectional studies investigating SIBO in CP patients were analysed. The prevalence of positive tests was pooled across studies, and the rate of positivity between CP cases and controls was calculated.
RESULTS:
In nine studies containing 336 CP patients, the pooled prevalence of SIBO was 36% (95% confidence interval (CI) 17-60%) with considerable heterogeneity (I2â=â91%). A sensitivity analysis excluding studies employing lactulose breath test gave a pooled prevalence of 21.7% (95% CI 12.7-34.5%) with lower heterogeneity (I2â=â56%). The odds ratio for a positive test in CP vs controls was 4.1 (95% CI 1.6-10.4) (I2â=â59.7%). The relationship between symptoms and SIBO in CP patients varied across studies, and the treatment of SIBO was associated with clinical improvement.
CONCLUSIONS:
One-third of CP patients have SIBO, with a significantly increased risk over controls, although results are heterogeneous, and studies carry several limitations. The impact of SIBO and its treatment in CP patients deserve further investigation
Simple threshold rules solve explore/exploit tradeâoffs in a resource accumulation search task
How, and how well, do people switch between exploration and exploitation to search for and accumulate resources? We study the decision processes underlying such exploration/exploitation tradeâoffs using a novel card selection task that captures the common situation of searching among multiple resources (e.g., jobs) that can be exploited without depleting. With experience, participants learn to switch appropriately between exploration and exploitation and approach optimal performance. We model participants' behavior on this task with random, threshold, and sampling strategies, and find that a linear decreasing threshold rule best fits participants' results. Further evidence that participants use decreasing thresholdâbased strategies comes from reaction time differences between exploration and exploitation; however, participants themselves report nonâdecreasing thresholds. Decreasing threshold strategies that âfrontâloadâ exploration and switch quickly to exploitation are particularly effective in resource accumulation tasks, in contrast to optimal stopping problems like the Secretary Problem requiring longer exploration
Variability of Atlantic Ocean heat transport and its effects on the atmosphere
The variability of the Atlantic meridional Ocean Heat Transport (OHT) has been diagnosed from a simulation of
a coupled ocean-atmosphere general circulation model, and the mechanisms responsible for this variability have
been elucidated. It has been demonstrated that the interannual variability in Atlantic OHT is dominated by
windstress-driven Ekman fluctuations. In contrast, the decadal and multidecadal variability is associated with the
fluctuations of the Thermohaline Circulation (THC), driven by the fluctuations in deep convection over the
Greenland-Iceland-Norwegian (GIN) Sea. The fluctuations of OHT induce Ocean Heat Content (OHC), and Sea
Surface Temperature (SST) anomalies over the tropical and subtropical North Atlantic. The SST anomalies, in
turn, have an impact on the atmosphere. The lead-lag relationships between the fluctuations of THC-related OHT
and those of OHC and SST raise the possibility that a knowledge of OHT fluctuations could be used to predict
variations in Atlantic Sea surface temperatures, and perhaps aspects of climate, several years in advance. A
comparison of results from a second, independent, coupled model simulation is also presented, and similar
conclusions reached
Impurity segregation in graphene nanoribbons
The electronic properties of low-dimensional materials can be engineered by
doping, but in the case of graphene nanoribbons (GNR) the proximity of two
symmetry-breaking edges introduces an additional dependence on the location of
an impurity across the width of the ribbon. This introduces energetically
favorable locations for impurities, leading to a degree of spatial segregation
in the impurity concentration. We develop a simple model to calculate the
change in energy of a GNR system with an arbitrary impurity as that impurity is
moved across the ribbon and validate its findings by comparison with ab initio
calculations. Although our results agree with previous works predicting the
dominance of edge disorder in GNR, we argue that the distribution of adsorbed
impurities across a ribbon may be controllable by external factors, namely an
applied electric field. We propose that this control over impurity segregation
may allow manipulation and fine-tuning of the magnetic and transport properties
of GNRs.Comment: 5 pages, 4 figures, submitte
Predicting Future Instance Segmentation by Forecasting Convolutional Features
Anticipating future events is an important prerequisite towards intelligent
behavior. Video forecasting has been studied as a proxy task towards this goal.
Recent work has shown that to predict semantic segmentation of future frames,
forecasting at the semantic level is more effective than forecasting RGB frames
and then segmenting these. In this paper we consider the more challenging
problem of future instance segmentation, which additionally segments out
individual objects. To deal with a varying number of output labels per image,
we develop a predictive model in the space of fixed-sized convolutional
features of the Mask R-CNN instance segmentation model. We apply the "detection
head'" of Mask R-CNN on the predicted features to produce the instance
segmentation of future frames. Experiments show that this approach
significantly improves over strong baselines based on optical flow and
repurposed instance segmentation architectures
Test Results of the RS-44 Integrated Component Evaluator Liquid Oxygen/Hydrogen Rocket Engine
An advanced LOX/LH2 expander cycle rocket engine, producing 15,000 lbf thrust for Orbital Transfer Vehicle missions, was tested to determine ignition, transition, and main stage characteristics. Detail design and fabrication of the pump fed RS44 integrated component evaluator (ICE) was accomplished using company discretionary resources and was tested under this contracted effort. Successful demonstrations were completed to about the 50 percent fuel turbopump power level (87,000 RPM), but during this last test, a high pressure fuel turbopump (HPFTP) bearing failed curtailing the test program. No other hardware were affected by the HPFTP premature shutdown. The ICE operations matched well with the predicted start transient simulations. The tests demonstrated the feasibility of a high performance advanced expander cycle engine. All engine components operated nominally, except for the HPFTP, during the engine hot-fire tests. A failure investigation was completed using company discretionary resources
Josephson Coupling through a Quantum Dot
We derive, via fourth order perturbation theory, an expression for the
Josephson current through a gated interacting quantum dot. We analyze our
expression for two different models of the superconductor-dot-superconductor
(SDS) system. When the matrix elements connecting dot and leads are featureless
constants, we compute the Josephson coupling J_c as a function of the gate
voltage and Coulomb interaction. In the diffusive dot limit, we compute the
probability distribution P(J_c) of Josephson couplings. In both cases, pi
junction behavior (J_c < 0) is possible, and is not simply dependent on the
parity of the dot occupancy.Comment: 9 pages; 3 encapsulated PostScript figure
Evolution of the Pairwise Peculiar Velocity Distribution Function in Lagrangian Perturbation Theory
The statistical distribution of the radial pairwise peculiar velocity of
galaxies is known to have an exponential form as implied by observations and
explicitly shown in N-body simulations. Here we calculate its statistical
distribution function using the Zel'dovich approximation assuming that the
primordial density fluctuations are Gaussian distributed. We show that the
exponential distribution is realized as a transient phenomena on megaparsec
scales in the standard cold-dark-matter model.Comment: 19 pages, 8 Postscript figures, AAS LaTe
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