Estimating the causal effect of a time-varying treatment on time-to-event using structural nested failure time models

In this paper we review an approach to estimating the causal effect of a time-varying treatment on time to some event of interest. This approach is designed for the situation where the treatment may have been repeatedly adapted to patient characteristics, which themselves may also be time-dependent. In this situation the effect of the treatment cannot simply be estimated by conditioning on the patient characteristics, as these may themselves be indicators of the treatment effect. This so-called time-dependent confounding is typical in observational studies. We discuss a new class of failure time models, structural nested failure time models, which can be used to estimate the causal effect of a time-varying treatment, and present methods for estimating and testing the parameters of these models

Betti number signatures of homogeneous Poisson point processes

The Betti numbers are fundamental topological quantities that describe the k-dimensional connectivity of an object: B_0 is the number of connected components and B_k effectively counts the number of k-dimensional holes. Although they are appealing natural descriptors of shape, the higher-order Betti numbers are more difficult to compute than other measures and so have not previously been studied per se in the context of stochastic geometry or statistical physics. As a mathematically tractable model, we consider the expected Betti numbers per unit volume of Poisson-centred spheres with radius alpha. We present results from simulations and derive analytic expressions for the low intensity, small radius limits of Betti numbers in one, two, and three dimensions. The algorithms and analysis depend on alpha-shapes, a construction from computational geometry that deserves to be more widely known in the physics community.Comment: Submitted to PRE. 11 pages, 10 figure

Pulsed pumping of a Bose-Einstein condensate

In this work, we examine a system for coherent transfer of atoms into a Bose-Einstein condensate. We utilize two spatially separate Bose-Einstein condensates in different hyperfine ground states held in the same dc magnetic trap. By means of a pulsed transfer of atoms, we are able to show a clear resonance in the timing of the transfer, both in temperature and number, from which we draw conclusions about the underlying physical process. The results are discussed in the context of the recently demonstrated pumped atom laser.Comment: 5 pages, 5 figures, published in Physical Review

Tactile Interactions with a Humanoid Robot : Novel Play Scenario Implementations with Children with Autism

Acknowledgments: This work has been partially supported by the European Commission under contract number FP7-231500-ROBOSKIN. Open Access: This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.The work presented in this paper was part of our investigation in the ROBOSKIN project. The project has developed new robot capabilities based on the tactile feedback provided by novel robotic skin, with the aim to provide cognitive mechanisms to improve human-robot interaction capabilities. This article presents two novel tactile play scenarios developed for robot-assisted play for children with autism. The play scenarios were developed against specific educational and therapeutic objectives that were discussed with teachers and therapists. These objectives were classified with reference to the ICF-CY, the International Classification of Functioning – version for Children and Youth. The article presents a detailed description of the play scenarios, and case study examples of their implementation in HRI studies with children with autism and the humanoid robot KASPAR.Peer reviewedFinal Published versio

Rb-85 tunable-interaction Bose-Einstein condensate machine

We describe our experimental setup for creating stable Bose-Einstein condensates of Rb-85 with tunable interparticle interactions. We use sympathetic cooling with Rb-87 in two stages, initially in a tight Ioffe-Pritchard magnetic trap and subsequently in a weak, large-volume crossed optical dipole trap, using the 155 G Feshbach resonance to manipulate the elastic and inelastic scattering properties of the Rb-85 atoms. Typical Rb-85 condensates contain 4 x 10^4 atoms with a scattering length of a=+200a_0. Our minimalist apparatus is well-suited to experiments on dual-species and spinor Rb condensates, and has several simplifications over the Rb-85 BEC machine at JILA (Papp, 2007; Papp and Wieman, 2006), which we discuss at the end of this article.Comment: 10 pages, 8 figure

Cold atom gravimetry with a Bose-Einstein Condensate

We present a cold atom gravimeter operating with a sample of Bose-condensed Rubidium-87 atoms. Using a Mach-Zehnder configuration with the two arms separated by a two-photon Bragg transition, we observe interference fringes with a visibility of 83% at T=3 ms. We exploit large momentum transfer (LMT) beam splitting to increase the enclosed space-time area of the interferometer using higher-order Bragg transitions and Bloch oscillations. We also compare fringes from condensed and thermal sources, and observe a reduced visibility of 58% for the thermal source. We suspect the loss in visibility is caused partly by wavefront aberrations, to which the thermal source is more susceptible due to its larger transverse momentum spread. Finally, we discuss briefly the potential advantages of using a coherent atomic source for LMT, and present a simple mean-field model to demonstrate that with currently available experimental parameters, interaction-induced dephasing will not limit the sensitivity of inertial measurements using freely-falling, coherent atomic sources.Comment: 6 pages, 4 figures. Final version, published PR

Southern African development community regional situation analysis

The Southern African Development Community (SADC) groups fourteen sovereign states in the southern and eastern Africa region for the main purpose of fostering co-operation for mutual benefit from development of the resources of the whole region. The region accounts for almost 70% gross domestic product of sub-Saharan Africa and is home to almost a third of its people. In the context of water resources, conditions in the SADC region are highly variable with respect to the relative reliance of each of the Member States on surface or groundwater sources. However, studies already indicate that water resources will be scarce in 9 of the 14 Member States within the next 10 to 30 years, most especially in the southern and eastern portion of the SADC region. Clearly, water resource conservation and comprehensive national and regional planning is going to be crucial. SADC recognised the critical importance of water to regional integration and economic development and established its own Water Sector in 1996. A SADC Protocol on Shared Watercourse Systems was adopted to set the rules for joint management of resources. A Regional Strategic Action Plan for Integrated Water Resource Development and Management has been compiled; this is being implemented to address key water management issues, concerning both surface water bodies and aquifers (groundwater). The region is also characterised by rapid population growth. Extremes of climate bring frequent drought and substantial flood events that impact on rural populations as well as national productivity. The region is already highly dependent on groundwater for rural water supply, and it is clear that groundwater is a key element in the alleviation of the effects of drought on rural communities. However, policy responses to drought have, in the past, been based on short-term crisis reactions, which have generally proved to be inefficient or ineffective. To address this undesirable situation, proactive, sustainable and integrated management of groundwater resources needs to be instigated, but with due sympathy to the requirements of ecosystems

Statistical Mechanics of Steiner trees

The Minimum Weight Steiner Tree (MST) is an important combinatorial optimization problem over networks that has applications in a wide range of fields. Here we discuss a general technique to translate the imposed global connectivity constrain into many local ones that can be analyzed with cavity equation techniques. This approach leads to a new optimization algorithm for MST and allows to analyze the statistical mechanics properties of MST on random graphs of various types