Integrated Testlets and the Immediate Feedback Assessment Technique

The increased use of multiple-choice (MC) questions in introductory-level physics final exams is largely hindered by reservations about its ability to test the broad cognitive domain that is routinely accessed with typical constructed-response (CR) questions. Thus, there is a need to explore ways in which MC questions can be utilized pedagogically more like CR questions while maintaining their attendant procedural advantages. we describe how an answer-until-correct MC response format allows for the construction of multiple-choice examinations designed to operate much as a hybrid between standard MC and CR testing. With this tool - the immediate feedback assessment technique (IF-AT) - students gain complete knowledge of the correct answer for each question during the examination, and can use such information for solving subsequent test items. This feature allows for the creation of a new type of context-dependent item sets; the "integrated testlet". In an integrated testlet certain items are purposefully inter-dependent and are thus presented in a particular order. Such integrated testlets represent a proxy of typical CR questions, but with a straightforward and uniform marking scheme that also allows for granting partial credit for proximal knowledge. We present a case study of an IF-AT-scored midterm and final examination for an introductory physics course, and discuss specific testlets with varying degrees of integration. In total, the items are found to allow for excellent discrimination, with a mean item-total correlation measure for the combined 45 items of the two examinations of $\bar{r}'=0.41\pm 0.13$ (mean $\pm$ standard deviation) and a final examination test reliability of $\alpha=0.82$ ($n=25$ items). Furthermore, partial credit is shown to be allocated in a discriminating and valid manner in these examinations.Comment: 13 pages. 7 figures. Accepted to the American Journal of Physics (August 2013

Deformation spaces of Kleinian surface groups are not locally connected

For any closed surface $S$ of genus $g \geq 2$, we show that the deformation space of marked hyperbolic 3-manifolds homotopy equivalent to $S$, $AH(S \times I)$, is not locally connected. This proves a conjecture of Bromberg who recently proved that the space of Kleinian punctured torus groups is not locally connected. Playing an essential role in our proof is a new version of the filling theorem that is based on the theory of cone-manifold deformations developed by Hodgson, Kerckhoff, and Bromberg

Forecasting in the Presence of Level Shifts

This article addresses the problem of forecasting time series that are subject to level shifts. Processes with level shifts possess a nonlinear dependence structure. Using the stochastic permanent breaks (STOPBREAK) model, I model this nonlinearity in a direct and flexible way that avoids imposing a discrete regime structure. I apply this model to the rate of price inflation in the United States, which I show is subject to level shifts. These shifts significantly affect the accuracy of out-of-sample forecasts, causing models that assume covariance stationarity to be substantially biased. Models that do not assume covariance stationarity, such as the random walk, are unbiased but lack precision in periods without shifts. I show that the STOPBREAK model outperforms several alternative models in an out-of-sample inflation forecasting experiment.Financial Economics, Research Methods/ Statistical Methods,

Representing Network Trust and Using It to Improve Anonymous Communication

Motivated by the effectiveness of correlation attacks against Tor, the censorship arms race, and observations of malicious relays in Tor, we propose that Tor users capture their trust in network elements using probability distributions over the sets of elements observed by network adversaries. We present a modular system that allows users to efficiently and conveniently create such distributions and use them to improve their security. The major components of this system are (i) an ontology of network-element types that represents the main threats to and vulnerabilities of anonymous communication over Tor, (ii) a formal language that allows users to naturally express trust beliefs about network elements, and (iii) a conversion procedure that takes the ontology, public information about the network, and user beliefs written in the trust language and produce a Bayesian Belief Network that represents the probability distribution in a way that is concise and easily sampleable. We also present preliminary experimental results that show the distribution produced by our system can improve security when employed by users; further improvement is seen when the system is employed by both users and services.Comment: 24 pages; talk to be presented at HotPETs 201

Orbit Characterization, Stabilization and Composition on 3D Underactuated Bipedal Walking via Hybrid Passive Linear Inverted Pendulum Model

A Hybrid passive Linear Inverted Pendulum (H-LIP) model is proposed for characterizing, stabilizing and composing periodic orbits for 3D underactuated bipedal walking. Specifically, Period-l (P1) and Period -2 (P2) orbits are geometrically characterized in the state space of the H-LIP. Stepping controllers are designed for global stabilization of the orbits. Valid ranges of the gains and their optimality are derived. The optimal stepping controller is used to create and stabilize the walking of bipedal robots. An actuated Spring-loaded Inverted Pendulum (aSLIP) model and the underactuated robot Cassie are used for illustration. Both the aSLIP walking with PI or P2 orbits and the Cassie walking with all 3D compositions of the PI and P2 orbits can be smoothly generated and stabilized from a stepping-in-place motion. This approach provides a perspective and a methodology towards continuous gait generation and stabilization for 3D underactuated walking robots