2,730 research outputs found
The Spontaneous Emergence of Social Influence in Online Systems
Social influence drives both offline and online human behaviour. It pervades
cultural markets, and manifests itself in the adoption of scientific and
technical innovations as well as the spread of social practices. Prior
empirical work on the diffusion of innovations in spatial regions or social
networks has largely focused on the spread of one particular technology among a
subset of all potential adopters. It has also been difficult to determine
whether the observed collective behaviour is driven by natural influence
processes, or whether it follows external signals such as media or marketing
campaigns. Here, we choose an online context that allows us to study social
influence processes by tracking the popularity of a complete set of
applications installed by the user population of a social networking site, thus
capturing the behaviour of all individuals who can influence each other in this
context. By extending standard fluctuation scaling methods, we analyse the
collective behaviour induced by 100 million application installations, and show
that two distinct regimes of behaviour emerge in the system. Once applications
cross a particular threshold of popularity, social influence processes induce
highly correlated adoption behaviour among the users, which propels some of the
applications to extraordinary levels of popularity. Below this threshold, the
collective effect of social influence appears to vanish almost entirely in a
manner that has not been observed in the offline world. Our results demonstrate
that even when external signals are absent, social influence can spontaneously
assume an on-off nature in a digital environment. It remains to be seen whether
a similar outcome could be observed in the offline world if equivalent
experimental conditions could be replicated
Statistical correlation analysis for comparing vibration data from test and analysis
A theory was developed to compare vibration modes obtained by NASTRAN analysis with those obtained experimentally. Because many more analytical modes can be obtained than experimental modes, the analytical set was treated as expansion functions for putting both sources in comparative form. The dimensional symmetry was developed for three general cases: nonsymmetric whole model compared with a nonsymmetric whole structural test, symmetric analytical portion compared with a symmetric experimental portion, and analytical symmetric portion with a whole experimental test. The theory was coded and a statistical correlation program was installed as a utility. The theory is established with small classical structures
Bounds on Quantum Correlations in Bell Inequality Experiments
Bell inequality violation is one of the most widely known manifestations of
entanglement in quantum mechanics; indicating that experiments on physically
separated quantum mechanical systems cannot be given a local realistic
description. However, despite the importance of Bell inequalities, it is not
known in general how to determine whether a given entangled state will violate
a Bell inequality. This is because one can choose to make many different
measurements on a quantum system to test any given Bell inequality and the
optimization over measurements is a high-dimensional variational problem. In
order to better understand this problem we present algorithms that provide, for
a given quantum state, both a lower bound and an upper bound on the maximal
expectation value of a Bell operator. Both bounds apply techniques from convex
optimization and the methodology for creating upper bounds allows them to be
systematically improved. In many cases these bounds determine measurements that
would demonstrate violation of the Bell inequality or provide a bound that
rules out the possibility of a violation. Examples are given to illustrate how
these algorithms can be used to conclude definitively if some quantum states
violate a given Bell inequality.Comment: 13 pages, 1 table, 2 figures. Updated version as published in PR
The Dynamics of Viral Marketing
We present an analysis of a person-to-person recommendation network,
consisting of 4 million people who made 16 million recommendations on half a
million products. We observe the propagation of recommendations and the cascade
sizes, which we explain by a simple stochastic model. We analyze how user
behavior varies within user communities defined by a recommendation network.
Product purchases follow a 'long tail' where a significant share of purchases
belongs to rarely sold items. We establish how the recommendation network grows
over time and how effective it is from the viewpoint of the sender and receiver
of the recommendations. While on average recommendations are not very effective
at inducing purchases and do not spread very far, we present a model that
successfully identifies communities, product and pricing categories for which
viral marketing seems to be very effective
A Finite Element Computation of the Gravitational Radiation emitted by a Point-like object orbiting a Non-rotating Black Hole
The description of extreme-mass-ratio binary systems in the inspiral phase is
a challenging problem in gravitational wave physics with significant relevance
for the space interferometer LISA. The main difficulty lies in the evaluation
of the effects of the small body's gravitational field on itself. To that end,
an accurate computation of the perturbations produced by the small body with
respect the background geometry of the large object, a massive black hole, is
required. In this paper we present a new computational approach based on Finite
Element Methods to solve the master equations describing perturbations of
non-rotating black holes due to an orbiting point-like object. The numerical
computations are carried out in the time domain by using evolution algorithms
for wave-type equations. We show the accuracy of the method by comparing our
calculations with previous results in the literature. Finally, we discuss the
relevance of this method for achieving accurate descriptions of
extreme-mass-ratio binaries.Comment: RevTeX 4. 18 pages, 8 figure
On the Matter of Time
Drawing on several disciplinary areas, this article considers diverse cultural concepts of time, space, and materiality. It explores historical shifts in ideas about time, observing that these have gone full circle, from visions in which time and space were conflated, through increasingly divergent linear understandings of the relationship between them, to their reunion in contemporary notions of space-time. Making use of long-term ethnographic research and explorations of the topic of Time at Durham University’s Institute of Advanced Study (2012–13), the article considers Aboriginal Australian ideas about relationality and the movement of matter through space and time. It asks why these earliest explanations of the cosmos, though couched in a wholly different idiom, seem to have more in common with the theories proposed by contemporary physicists than with the ideas that dominated the period between the Holocene and the Anthropocene. The analysis suggests that such unexpected resonance between these oldest and newest ideas about time and space may spring from the fact that they share an intense observational focus on material events. Comparing these vastly different but intriguingly compatible worldviews meets interdisciplinary aims in providing a fresh perspective on both of them
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On the application of optimal wavelet filter banks for ECG signal classification
This paper discusses ECG signal classification after parametrizing the ECG waveforms in the wavelet domain. Signal decomposition using perfect reconstruction quadrature mirror filter banks can provide a very parsimonious representation of ECG signals. In the current work, the filter parameters are adjusted by a numerical optimization algorithm in order to minimize a cost function associated to the filter cut-off sharpness. The goal consists of achieving a better compromise between frequency selectivity and time resolution at each decomposition level than standard orthogonal filter banks such as those of the Daubechies and Coiflet families. Our aim is to optimally decompose the signals in the wavelet domain so that they can be subsequently used as inputs for training to a neural network classifier
An economic evaluation of contingency management for completion of hepatitis B vaccination in those on treatment for opiate dependence
Aims: To determine whether the provision of contingency management using financial incentives to improve hepatitis B vaccine completion in people who inject drugs entering community treatment represents a cost-effective use of healthcare resources.
Design: A probabilistic cost-effectiveness analysis was conducted, using a decision-tree to estimate the short-term clinical and healthcare cost impact of the vaccination strategies, followed by a Markov process to evaluate the long-term clinical consequences and costs associated with hepatitis B infection.
Settings and participants: Data on attendance to vaccination from a UK cluster randomised trial.
Intervention: Two contingency management options were examined in the trial: fixed vs. escalating schedule financial incentives.
Measurement: Lifetime healthcare costs and quality-adjusted life years discounted at 3.5% annually; incremental cost-effectiveness ratios.
Findings: The resulting estimate for the incremental lifetime healthcare cost of the contingency management strategy versus usual care was £22 (95% CI: -£12 to £40) per person offered the incentive. For 1,000 people offered the incentive, the incremental reduction in numbers of hepatitis B infections avoided over their lifetime was estimated at 19 (95% CI: 8 to 30). The probabilistic incremental cost per quality adjusted life year gained of the contingency management programme was estimated to be £6,738 (95% CI: £6,297 to £7,172), with an 89% probability of being considered cost-effective at a threshold of £20,000 per quality-adjusted life years gained (98% at £30,000).
Conclusions: Using financial incentives to increase hepatitis B vaccination completion in people who inject drugs could be a cost-effective use of healthcare resources in the UK as long as the incidence remains above 1.2%
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