571 research outputs found
On inference processes
No full textNaeve P, Westerhoff T. On inference processes. In: Edwards D, Raun NE, eds. COMPSTAT 1988: proceedings in computational statistics. 8th Symposium held in Copenhagen 1988. Heidelberg: Physica-Verl.; 1988: 193-198
Why a simple herding model may generate the stylized facts of daily returns: Explanation and estimation
No full textThe paper proposes an elementary agent-based asset pricing model that, invoking the two trader types of fundamentalists and chartists, comprises four features: (i) price determination by excess demand; (ii) a herding mechanism that gives rise to a macroscopic adjustment equation for the market fractions of the two groups; (iii) a rush towards fundamentalism when the price misalignment becomes too large; and (iv) a stronger noise component in the demand per chartist trader than in the demand per fundamentalist trader, which implies a structural stochastic volatility in the returns. Combining analytical and numerical methods, the interaction between these elements is studied in the phase plane of the price and a majority index. In addition, the model is estimated by the method of simulated moments, where the choice of the moments reflects the basic stylized facts of the daily returns of a stock market index. A (parametric) bootstrap procedure serves to set up an econometric test to evaluate the model's goodness-of-fit, which proves to be highly satisfactory. The bootstrap also makes sure that the estimated structural parameters are well identified. --structural stochastic volatility,method of simulated moments,autocorrelation pattern,fat tails,bootstrapped p-values
Development of ground and space based instrumentation for middle and upper atmospheric studies
No full textThe first objective of this research effort is to investigate the use of a high power-aperture Rayleigh lidar to measure neutral density and temperature in the upper atmosphere, into the lower thermosphere (>90 km). The scientific interest with this system is to obtain measurements of atmospheric tides, planetary and gravity wave amplitudes and phase in the lower thermosphere. Measurement of waves into the lower thermosphere is accomplished using Rayleigh lidar methods with high power-aperture (PA) products using new technology lasers and large collecting apertures, resulting in PA of 65-540 Wm2. This dissertation describes the simulation of high PA Rayleigh lidar systems and estimates their capability to measure gravity waves and tides in the mesosphere and lower thermosphere. Both traditional backscatter and bistatic (imaging) methods are simulated. Simulations show that substantive measurements can be achieved for characterizing gravity waves at altitudes >90 km and atmospheric tides at altitudes >100 km for this lidar system. Measurements of density and temperature to 10% precision are possible up to 115-130 km. A research and development effort funded by the NSF was pursued at the University of Illinois to develop a high-power Rayleigh lidar and explore the capabilities and challenges of implementing a high power-aperture (PA) Rayleigh lidar capable of measuring the neutral atmosphere at altitudes >110 km. Simulations comparing the capabilities of monostatic and bistatic lidar configurations for a high-PA lidar system were first performed, followed by a development effort to test a high-PA Rayleigh lidar. This dissertation details the first-light results of this lidar experiment, which achieves neutral atmosphere measurements up to 93 km altitude, the highest measurement to date for a bistatic Rayleigh lidar, based on a review of the literature. Methodologies for significantly improving these results in future studies are also discussed. This research effort demonstrates the capability of a bistatic Rayleigh lidar configuration for middle and upper atmospheric studies, which enables the use of new high-power, high pulse repetition frequency and continuous wave lasers that are unsuited for monostatic lidar systems. The potential of high power-aperture Rayleigh lidar in exploring the middle and upper atmosphere is shown in Gardner (2012), which simulates the performance of such a system using a 325W laser and an 8-meter telescope for a power-aperture product of 16,336 Wm2. The second objective of this research effort is to design an instrument to measure atmospheric gravity waves in the mesosphere as part of the Lower Atmosphere/Ionosphere Coupling Experiment (LAICE) CubeSat mission. Measurements of atmospheric gravity waves are obtained by remote sensing of mesospheric airglows during the nighttime portion of the orbit. This instrument is designed to collect measurements to determine intrinsic wave parameters of observed atmospheric gravity waves. These derived wave parameters must be of sufficient quality for coupling studies that are the scientific goal of the LAICE mission. The instrument must also meet its performance requirements in the restrictive mass, volume, power, and communication limits of a CubeSat, which in this case is a 6U CubeSat to accommodate flying two scientific payloads. An in-situ measurement instrument payload was developed at the Virginia Polytechnic Institute, while the remote sensing photometer instrument payload and the CubeSat bus were developed at the University of Illinois. This dissertation details the design of the photometer instrument for the LAICE mission and the expected performance with respect to derivation of mesospheric gravity wave parameters from airglow measurements. The LAICE mission is a groundbreaking CubeSat mission in low Earth orbit focused on understanding how atmospheric gravity waves generated by weather systems in the lower atmosphere propagate and deliver energy and momentum into the mesosphere, lower thermosphere, and ionosphere. These waves are an important facet of atmospheric physics, but their effects in the thermosphere and ionosphere are under-explored. They strongly influence the dynamics of the media through which they travel via momentum and energy deposition at altitudes well above their source regions, and they can seed the development of plasma instabilities that scintillate and disrupt radio propagation. LAICE will focus on these waves and attempt to connect their causes and effects in three widely different altitude ranges, substantially adding to our knowledge of a critical coupling process between disparate atmospheric regions.Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2025-02-04 without embargo termsThe student, John Westerhoff, accepted the attached license on 2024-05-07 at 14:26.The student, John Westerhoff, submitted this Dissertation for approval on 2024-05-07 at 14:37.This Dissertation was approved for publication on 2024-05-14 at 16:26.DSpace SAF Submission Ingestion Package generated from Vireo submission #20782 on 2025-02-04 at 21:03:1
Structural stochastic volatility in asset pricing dynamics: Estimation and model contest
No full textIn the framework of small-scale agent-based financial market models, the paper starts out from the concept of structural stochastic volatility, which derives from different noise levels in the demand of fundamentalists and chartists and the time-varying market shares of the two groups. It advances several different specifications of the endogenous switching between the trading strategies and then estimates these models by the method of simulated moments (MSM), where the choice of the moments reflects the basic stylized facts of the daily returns of a stock market index. In addition to the standard version of MSM with a quadratic loss function, we also take into account how often a great number of Monte Carlo simulation runs happen to yield moments that are all contained within their empirical confidence intervals. The model contest along these lines reveals a strong role for a (tamed) herding component. The quantitative performance of the winner model is so good that it may provide a standard for future research. --Method of simulated moments,moment coverage ratio,herding,discrete choice approach,transition probability approach
Rhythm of the beta-cell oscillator is not governed by a single regulator: multiple systems contribute to oscillatory behavior
No full textPulsatile insulin output, paralleled by oscillations in intracellular Ca(2+), reflect oscillating metabolism within beta-cells in response to secretory fuels. Here we question whether oscillatory periodicity is conserved or varied from stimulation to stimulation, whether glycolysis is essential for the manifestation of an oscillatory response, and if an environment of nutrient oversupply affects oscillatory regularity. We have determined that a beta-cell oscillatory Ca(2+) pattern is independent of the type of applied secretory fuel (glucose, methyl-pyruvate, or alpha-ketoisocaproate). In addition, single cells respond with the same pattern when repeatedly stimulated, regardless of the type of stimulatory fuel. Presence of substimulatory glucose is not necessary to obtain an oscillatory responses to methyl-pyruvate or alpha-ketoisocaproate. Glucose-6-phosphate, as a measure of glycolytic flux, is not detectable under these conditions. These data suggest that multiple systems, rather than a single enzyme component, can contribute to the beta-cell oscillatory behavior. Prolonged exposure to high levels of palmitate impaired oscillatory regularity in the individual beta-cells. This supports the hypothesis that a high-fat environment might contribute to loss of regular oscillatory pattern in diabetic subjects, acting, at least in part, at the level of the single beta-cel
Dramatizing Iraq and Afghanistan: Veteran Voices from the War on Terror
No full textVeteran written and inspired drama exists in a unique place in theatrical history. Many of the earliest dramatic writers were veterans, and many of their plays dealt with social and political responses to war. Dramatizing war on theatrical stages gives the veteran an avenue of expression for his or her wartime experiences, and it opens a space for social and political dialogue about the nature of war and its effects on veterans. This thesis examines the social and political relevance of three plays that have emerged from veterans from the War on Terror. Previous scholarship has examined the dramatic contribution of other veteran groups. However, because only a short time period has elapsed between the beginning of the War on Terror and today, a gap in the field of significant scholarly contributions to plays from this generation of veterans exists. This thesis addresses this gap by analyzing three plays to emerge from the War on Terror. These plays are veteran written or veteran inspired and each comes from unique perspectives and experiences from the war. War on Terror veterans writing about their experiences find a unique place in the body of veteran inspired dramas. The experiences these dramatists present in their plays--LGBT service, gender discrimination, and military sexual trauma--should open up a dialogue about socially and politically relevant issues to this veteran cohort. The specific approaches to the texts vary from chapter to chapter, and are based in large part on the nature of the play the chapter examines.Theatr
In vivo Measurement of Shoulder Joint Loads with Instrumented Implants
No full textIn dieser Arbeit wurde die Belastung des Schultergelenkes mit Hilfe von instrumentierten Prothesen gemessen. Insgesamt wurden 8 dieser Implantate bei 8 Patienten eingesetzt. Durch die unterschiedlichen funktionellen Ergebnisse konnten allerdings nicht alle Patienten an allen Studien der verschiedenen Themengebiete teilnehmen. Insbesondere anspruchsvolle Übungen mit zusätzlichen Gewichten waren für einige Patienten nicht durchzuführen. Die Ergebnisse wurden in bisher 5 Publikationen in international anerkannten (peerreviewed) Zeitschriften veröffentlicht bzw. eingereicht und bilden die Grundlage der vorliegenden Arbeit. Die absolut größten Kräfte von mehr als 200% des Körpergewichtes (%BW) wurden gemessen bei körperlich sehr starken Patienten, die entweder ein externes Gewicht über den langen Helbelarm des ausgestreckten Armes hoben, oder aber willentlich eine maximale Muskelanspannung durchführten. Dabei war die Kraftrichtung trotz großer Bewegungsumfänge sehr konstant. Dies ist ein Aspekt, der auch schon bei Messungen an der Hüfte [1] und am Knie [2]beobachtet wurde. Ein Patient war sogar in der Lage, Liegestütze durchzuführen. Allerdings wurde bei dieser Übung der maximale Messbereich der Prothese durch eine Kombination von großer axialer Kraft (mehr als 240% des Körpergewichtes) und gleichzeitig großen wirkenden Biegemomenten (mehr als 9 Nm) überschritten. Die nicht unerheblichen Belastungen durch Biege- und Torsionsmomente wurden bisher in computergestützten Modellberechnungen nicht einkalkuliert und es zeigten sich individuell große Unterschiede zwischen den Patienten. Als ein grundlegendes Ergebnis zeigte sich, dass die Modellierung des Schultergelenkes als reibungsfreies Kugelgelenk den realen Bedingungen im prothetisch versorgten Schultergelenk nicht gerecht wird. Grund für die gemessenen Momente kann offensichtlich nicht nur in Reibung im Gelenk sein. Andernfalls müssten unwahrscheinlich hohe Reibungskoeffizienten bei einigen Patienten angenommen werden. Vielmehr erscheinen exzentrische Krafteinwirkung und/oder zusätzliche Kraftangriffspunkte z.B. am Acromion für die gemessenen Momente verantwortlich zu sein. Dies soll zukünftig mit zusätzlichen flouroskopischen Messungen untersucht werden. Ein weiterer zukünftiger Aspekt ist die Umrechnung der Belastungen auf das Glenoid mit Hilfe von simultaner Bewegungsanalyse. Erste Ergebnisse dieser Zusammenarbeit mit einer Forschungsgruppe aus den Niederlanden werden als Ausblick am Ende der Arbeit dargestellt.Content of this thesis is the in vivo measurement of shoulder joint contact loads. For this purpose an instrumented shoulder implant was developed. Eight of these devices were implanted in 8 patients. Because of the individual different functional outcome, not all patients could take part in all studies. The results of particulat topics are published or submitted as scientific papers to international peerreviewed journals. In general the highest should joint loads of more than 200 percentage of Bodyweight (%BW) were measured in strong and athletic patients with a good muscular constitution during maximum voluntary muscle contractions. Also high loads occurred whenever the patients moved the straight arm with an external weight and a long lever arm. The load direction during these high demanding tasks was very constant throughout the motion. Similar effects were also seen in studies measuring the hip [1] and knee joint load [2]. One patient was even able to do push-ups. Unfortunately the maximum measurement range of the implant was exceeded during this exercise with a combination of high axial forces (more than 240%BW) together with bending moments of more than 9Nm. Reason for the measured moments could be friction on the one hand, but also an eccentric shift of the resultant force caused by additional points of load application for example. These points could be the acromion or the processus coracoideus. In previous model calculations of shoulder joint loads, bending and torsional moments were mostly not taken into account at all. After measuring these moments in vivo, modeling of the shoulder joint as a frictionless ball and socket joint seems not to be appropriate, at least in case of a endoprosthetic replacement. To further investigate the reasons for the measured moments fluoroscopic measurements are planned for the future. However, the biggest issue regarding shoulder implants is still the glenoid component. Therefore, combined in vivo measurements and motion analysis were done with the group of Prof. Frans van der Helm in the Netherlands to convert the measured humeral loads on to the glenoid
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