Lefschetz Center for Dynamical Systems Seminar
Brown University
Joint Materials/Solid Mechanics Seminar Series
Charlotte Maer Patton Centennial Fellowship in Engineering Aerospace Engineering and Engineering Mechanics | |
Abstract: Many bacteria and sperm propel themselves by spinning or beating flagella; interesting bifurcations have been observed in their motion that can be understood in terms of the theory of slender rods. Whirling of strings and slender rods has been studied for many centuries, with contributions beginning with Bernolli and Euler. Issues of bifurcations and stability of solutions have been considered over the past fifty years. We examine this problem through scaled experiments that reveal the underlying instabilities, and mimic flagellar motion.
Brown University Center for Statistical Sciences Seminar
1st Floor - Conference Room 106 (Coffee at 3:45 p.m.) |
Abstract: Researchers frequently elect to evaluate new therapies on the basis of patient survival. For example, clinicians might consider five-year survival when investigating drugs developed for use in childhood cancer, or 28-day survival when investigating the treatment of sepsis in patients suffering traumatic injury. Both of these examples focus on patient responses over a fixed period of time. However, for ethical reasons clinical trial designers must periodically analyze accruing data. In the case of censored survival data, testing is typically based on a logrank statistic while the development of group sequential methods has produced multiple criteria that are used to guide the decision of whether a trial should be stopped early given the data observed. Examples of such criteria include estimates of treatment effect at the time of analysis or measures of stochastic curtailment such as conditional power. As currently implemented, these criteria typically assume proportional hazards treatment effects and are defined for settings where the average effect of treatment up to the interim analysis is the same as that which would be observed if the trial continued on to maximum duration. In this talk, we discuss general issues associated with the use of weighted logrank statistics in such settings and address the uncertainty of future observations under potentially nonproportional hazards alternatives. We propose a method of imputation of future treatment effects based on random walks, which assumes minimally informative Bayesian prior distributions on the smoothness of survival of each comparison group. Imputation of future survival differences is carried out using standard Bayesian predictive distributions, thereby allowing for estimation of measures of stochastic curtailment. The proposed method is illustrated via simulation.
Cognitive & Linguistic Sciences Colloquium Series
Refreshments will be served at 3:45 in MRB Room 124-125 |
Abstract: Catching and hitting are interceptive actions that are interesting to study in terms of motor control because they require coordination between the end-effector (hand or bat) and a dynamic event in the environment (i.e. an approaching ball). The control of interceptive actions has been studied since the late 1960s, first from an information-processing perspective and later from the perspective of ecological psychology. Insights from this earlier work are combined with recent neurophysiological and new behavioral observations in the development of dynamical neural networks for the planning of interceptive movements. As it turns out, the models in question account well for the movement kinematics observed in both catching and hitting. In particular, they suggest that both catching and hitting are controlled prospectively without predicting in advance the location and time of interception.
<--- 2004 Index