Importance Tempering

Simulated tempering (ST) is an established Markov chain Monte Carlo (MCMC) method for sampling from a multimodal density $\pi(\theta)$. Typically, ST involves introducing an auxiliary variable $k$ taking values in a finite subset of $[0,1]$ and indexing a set of tempered distributions, say $\pi_k(\theta) \propto \pi(\theta)^k$. In this case, small values of $k$ encourage better mixing, but samples from $\pi$ are only obtained when the joint chain for $(\theta,k)$ reaches $k=1$. However, the entire chain can be used to estimate expectations under $\pi$ of functions of interest, provided that importance sampling (IS) weights are calculated. Unfortunately this method, which we call importance tempering (IT), can disappoint. This is partly because the most immediately obvious implementation is na\"ive and can lead to high variance estimators. We derive a new optimal method for combining multiple IS estimators and prove that the resulting estimator has a highly desirable property related to the notion of effective sample size. We briefly report on the success of the optimal combination in two modelling scenarios requiring reversible-jump MCMC, where the na\"ive approach fails.Comment: 16 pages, 2 tables, significantly shortened from version 4 in response to referee comments, to appear in Statistics and Computin

Design, Implementation, and Evaluation of a Distributed Real-Time Kernel for Distributed Robotics (Dissertation Proposal)

Modern robotics applications are becoming more complex due to greater numbers of sensors and actuators. The control of such systems may require multiple processors to meet the computational demands and to support the physical topology of the sensors and actuators. A distributed real-time system is needed to perform the required communication and processing while meeting application-specified timing constraints. We are designing and implementing a real-time kernel for distributed robotics applications. The kernel\u27s salient features are consistent, user-definable scheduling, explicit dynamic timing constraints, and a two-tiered interrupt approach. The kernel wi1l be evaluated by implementing a two-arm robot control example. Its goal is to locate and manipulate cylindrical objects with spillable contents. Using the application and the kernel, we will investigate the effects of time granularity, network type and protocol, and the handling of external events using interrupts versus polling. Our research will enhance understanding of real-time kernels for distributed robotics control

Bridget O\u27Flynn : Where\u27ve Ya Been?

Contains advertisements and/or short musical examples of pieces being sold by publisher.https://digitalcommons.library.umaine.edu/mmb-vp/7055/thumbnail.jp

Addressing Intimate Partner Violence Among Female Clients Accessing HIV Testing and Counseling Services: Pilot Testing Tools in Rakai, Uganda.

The World Health Organization recommends that HIV counseling and testing (HCT) programs implement strategies to address how intimate partner violence (IPV) influences women's ability to protect themselves from and seek care and treatment for HIV infection. We discuss the process used to adapt a screening and brief intervention (SBI) for female clients of HCT services in Rakai, Uganda-a setting with high prevalence of both HIV and IPV. By outlining our collaborative process for adapting and implementing the SBI in Rakai and training counselors for its use, we hope other HCT programs will consider replicating the approach in their settings

Mass Flow Control in a Magnesium Hall-effect Thruster

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83626/1/AIAA-2010-6861-269.pd

Cooling of cryogenic electron bilayers via the Coulomb interaction

Heat dissipation in current-carrying cryogenic nanostructures is problematic because the phonon density of states decreases strongly as energy decreases. We show that the Coulomb interaction can prove a valuable resource for carrier cooling via coupling to a nearby, cold electron reservoir. Specifically, we consider the geometry of an electron bilayer in a silicon-based heterostructure, and analyze the power transfer. We show that across a range of temperatures, separations, and sheet densities, the electron-electron interaction dominates the phonon heat-dissipation modes as the main cooling mechanism. Coulomb cooling is most effective at low densities, when phonon cooling is least effective in silicon, making it especially relevant for experiments attempting to perform coherent manipulations of single spins.Comment: 9 pages, 5 figure

Basal mechanics of ice streams: Insights from the stick-slip motion of Whillans Ice Stream, West Antarctica

The downstream portion of Whillans Ice Stream, West Antarctica, moves primarily by stick-slip motion. The observation of stick-slip motion suggests that the bed is governed by velocity-weakening physics and that the basal physics is more unstable than suggested by laboratory studies. The stick-slip cycle of Whillans Ice Plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude. To understand this variability, we modeled the forces acting on the ice stream during the stick phase of the stick-slip cycle. The ocean tides introduce changes in the rate at which stress is applied to the ice plain. Increased loading rates promote earlier failure and vice versa. Results show that the bed of Whillans Ice Stream strengthens over time (healing) during the quiescent intervals in the stick-slip cycle, with the bed weakening during slip events. The time-dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0.35 kPa during the course of a single day

Clinical significance of perioperative Q-wave myocardial infarction: The Emory Angioplasty versus Surgery Trial

AbstractObjective: The primary end point of the Emory Angioplasty versus Surgery Trial was a composite of three events: death, Q-wave infarction, and a new large defect on 3-year postoperative thallium scan. This study examines the clinical significance of Q-wave infarction in the surgical cohort (194 patients) of the Emory trial. Methods: Twenty patients (10.3%) with Q-wave infarctions were identified: 13 patients had inferior Q-wave infarctions and seven patients had anterior, lateral, septal, or posterior Q-wave infarctions (termed anterior Q-wave infarctions). Results: In the inferior Q-wave infarction group, postoperative cardiac catheterization (at 1 year or 3 years) in 11 patients revealed normal ejection fraction (ejection fraction >55%) in 10 (91%), no wall motion abnormalities in 10 (91%), and all grafts patent in 10 (91%). In the anterior Q-wave infarction group, postoperative catheterizatiOn in six patients revealed normal ejection fractions in five (83%), no wall motion abnormalities in three (50%), and all grafts patent in three (50%). Average peak postoperative creatine kinase MB levels were as follows: no Q-wave infarction (n = 174) 37 ± 43 IU/L, inferior Q-wave infarction 40 ± 27 IU/L, and anterior Q-wave infarction 58 ± 38 IU/L. Mortality in the 20 patients with Q-wave infarctions was 5% (1/20) at 3 years; in patients without a Q-wave infarction it was 6.3% (11/174) (p = 0.64). Of 17 patients with a Q-wave infarction who underwent postoperative catheterization, 11 (65%) had a normal ejection fraction, normal wall motion, and all grafts patent with an uneventful 3-year postoperative course. Conclusions: The core laboratory screening of postoperative electrocardiograms, particularly in the case of inferior Q-wave infarctions, appears to identify a number of patients as having a Q-wave infarction with minimal clinical significance. Q-wave infarction identified in the postoperative period seems to be a weak end point with little prognostic significance and therefore not valuable for future randomized trials. (J Thorac Cardiovasc Surg 1996;112:1447-54