Reducing Reparameterization Gradient Variance

Optimization with noisy gradients has become ubiquitous in statistics and machine learning. Reparameterization gradients, or gradient estimates computed via the "reparameterization trick," represent a class of noisy gradients often used in Monte Carlo variational inference (MCVI). However, when these gradient estimators are too noisy, the optimization procedure can be slow or fail to converge. One way to reduce noise is to use more samples for the gradient estimate, but this can be computationally expensive. Instead, we view the noisy gradient as a random variable, and form an inexpensive approximation of the generating procedure for the gradient sample. This approximation has high correlation with the noisy gradient by construction, making it a useful control variate for variance reduction. We demonstrate our approach on non-conjugate multi-level hierarchical models and a Bayesian neural net where we observed gradient variance reductions of multiple orders of magnitude (20-2,000x)

Investigation of police decision making using combined EEG and virtual reality methods

Police officers in the UK are granted additional powers to allow them to protect life and property crime. Of these powers, the sanction to use stopping, potentially lethal, force given to Authorised Firearms Officers (AFO) is arguably the most salient. Each decision made by an AFO to discharge their firearm or not has great impact and so it is important we research the cognitive processes that lead to such a decision.One challenge of researching these cognitive processes is eliciting ecologically valid behaviour while maintaining internal validity. We approached this challenge by developing combined electroencephalography (EEG) and virtual reality research methods. Using these methods, we produced scenarios that reflected features of AFO training. First, we tested simple versions of the scenarios on a novice population. Following this, we increased the complexity of the scenarios and collected data from both AFOs and novices.We found that participants were fastest when responding to threatening scenarios. Further, AFOs had consistently faster response times than novices. In line with similar ‘Go/No-Go’ paradigms, we found greater increases in pre-response frontal-midline theta when participants did not shoot versus when they did. Comparisons of EEG between AFOs and novices revealed greater pre-response increases in frontal-midline theta and central delta when they equipped a firearm. Greater differences in delta activity were also observed between different levels of threat in the AFO group.Together, these findings suggest that differences in performance between experts and novices may be due to their greater attention towards threat. Further investigation of expert decision making should build on our use of naturalistic stimuli and expert participants to ensure that findings are ecologically valid.With increasing accessibility of modern game engines and virtual reality technology, this approach will be beneficial to researchers in many fields where ecological validity is required

Polarization of magnetospheric ULF waves excited by an interplanetary shock on 27 February 2014

Funding: KT was supported by NASA Grants NNX17AD34G, 80NSSC19K0259, and 80NSSC21K0453. TE was partially funded by Leverhulme Early Career Fellowship ECF-2019-155. ANW was partially funded by the Science and Technology Facilities Council (STFC) Grant (ST/N000609/1). AWD was supported by NSFC Grants 42225405 and NAF\R1\19“1047”.Many previous studies have reported that magnetospheric ultralow frequency waves excited by interplanetary shocks exhibit a strong toroidal component, corresponding to azimuthal displacement of magnetic field lines. However, the toroidal oscillations excited by an interplanetary shock on 27 February 2014 and observed on the dayside by multiple spacecraft were accompanied by a strong poloidal component (radial field line displacement). The frequency of the toroidal oscillations changed with the radial distance of the spacecraft as expected for standing Alfvén waves. We run a 3-D linear numerical simulation of this wave event using a model magnetosphere with a realistic radial variation of the Alfvén velocity. The simulated wave fields, when sampled at a radial distance comparable to those of the observations in the real magnetosphere, exhibit polarization similar to the observations. In the simulation, the poloidal component comes from radially standing fast mode waves and the toroidal component results from a field line resonance driven by the fast mode waves. As a consequence, the relative amplitude and phase of the toroidal and poloidal components change with radial distance.Publisher PDFPeer reviewe

An Analysis of Small-Scale Wind Pump Design for Use in Developing Countries

The aim of this project was to design and construct a wind pump that is able to provide water to a rural third world village. This was achieved by using a Savonius-Darrieus turbine connected to a rope pump in order to create a system that was robust and easy to construct in a low-technology area. The pump proved to be a cost-effective, low technology method of pumping clean water

Pioneer Valley

The D-Term Mason Square Supermarket project was aimed at building on the work of the C-Term project and determining the feasibility of various aspects of a new market, including public image/perception, business and finance, green technology, security/loss prevention, and banking. The topics to be studied were distributed among the team members. A bulk of information was found, then used to analyze the subject for feasibility and impact. This report discusses the feasibility of various solutions in each area. Additionally, the D-Term team addressed questions posed by the C-Term team

Sustainable rice cultivation in the deep flooded zones of the Vietnamese Mekong Delta

This paper explains how the management of the full-dyke system in the deep flooded zones of the Vietnamese Mekong Delta affects rice cultivation, and outlines how alternative dyke management strategies could offer more sustainable adaptations in the face of future environmental threats. The current management of the ‘full-dyke’ network has been successful in promoting triple-cropping rice cultivation, but this practice has prevented sediment deposition on the land surface. River-borne sediments deposited on the delta land surface have high economic value because they are (i) rich in nutrients (potentially 26 million USD/yr of free fertiliser to An Giang Province) and can (ii) help to maintain the Mekong Delta land above sealevel. Without a continuing supply of sediment to the delta, triple-cropping paddies may not continue to be sustainable or profitable for the majority of rice farmers over the next 10 to 20 years. The economic value of sediment as a free fertiliser is particularly important to poor farmers, as without sediment, they run a significant risk of debt due to fluctuations in rice, fertiliser, and other input prices. With incoming loads now declining, sediment must be managed carefully as a resource. Our projections show that the best use of the remaining sediment resource can be achieved by allowing full paddy flooding only in years of high sediment potential, and this would greatly increase the sustainability of rice agriculture in the face of future environmental change. This recommended policy is an option with few regrets, in that its other benefits include maximising groundwater replenishment, ensuring freshwater availability during drought periods (including countering salt water intrusion), cleansing rice paddies of pests and disease, and tempering downstream flooding. If triple-rice-cropping continues to have priority, financial support will particularly be needed to provide help to poorer farmers coping with increases in artificial fertiliser prices

Environmental and Social Issues Concerned with Hybrid Cars

The future of transportation in the United States is on the brink of change. This project has three main objectives. First, verify if independence of foreign oil is truly a possibility and how to accomplish this feat. Second, identify the major motivators for the American consumers who purchase these vehicles and how that can be used to increase the sales of HEVs and PHEVs respectively. The last objective is to determine the future impact of the all electric vehicle (EV)