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2012

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Articles 241 - 251 of 251

Full-Text Articles in Physical Sciences and Mathematics

Using Ensemble-Based Methods For Directly Estimating Causal Effects: An Investigation Of Tree-Based G-Computation, Peter C. Austin Jan 2012

Using Ensemble-Based Methods For Directly Estimating Causal Effects: An Investigation Of Tree-Based G-Computation, Peter C. Austin

Peter Austin

Researchers are increasingly using observational or nonrandomized data to estimate causal treatment effects. Essential to the production of high-quality evidence is the ability to reduce or minimize the confounding that frequently occurs in observational studies. When using the potential outcome framework to define causal treatment effects, one requires the potential outcome under each possible treatment. However, only the outcome under the actual treatment received is observed, whereas the potential outcomes under the other treatments are considered missing data. Some authors have proposed that parametric regression models be used to estimate potential outcomes. In this study, we examined the use of …

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Regression Trees For Predicting Mortality In Patients With Cardiovascular Disease: What Improvement Is Achieved By Using Ensemble-Based Methods?, Peter C. Austin Jan 2012

Regression Trees For Predicting Mortality In Patients With Cardiovascular Disease: What Improvement Is Achieved By Using Ensemble-Based Methods?, Peter C. Austin

Peter Austin

In biomedical research, the logistic regression model is the most commonly used method for predicting the probability of a binary outcome. While many clinical researchers have expressed an enthusiasm for regression trees, this method may have limited accuracy for predicting health outcomes. We aimed to evaluate the improvement that is achieved by using ensemble-based methods, including bootstrap aggregation (bagging) of regression trees, random forests, and boosted regression trees. We analyzed 30-day mortality in two large cohorts of patients hospitalized with either acute myocardial infarction (N = 16,230) or congestive heart failure (N = 15,848) in two distinct eras (1991-2001 and …

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Generating Survival Times To Simulate Cox Proportional Hazards Models With Time-Varying Covariates., Peter C. Austin Jan 2012

Generating Survival Times To Simulate Cox Proportional Hazards Models With Time-Varying Covariates., Peter C. Austin

Peter Austin

Simulations and Monte Carlo methods serve an important role in modern statistical research. They allow for an examination of the performance of statistical procedures in settings in which analytic and mathematical derivations may not be feasible. A key element in any statistical simulation is the existence of an appropriate data-generating process: one must be able to simulate data from a specified statistical model. We describe data-generating processes for the Cox proportional hazards model with time-varying covariates when event times follow an exponential, Weibull, or Gompertz distribution. We consider three types of time-varying covariates: first, a dichotomous time-varying covariate that can …

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5x20 Matrix For Knowledge Management Lifecycle Based On The Five C’S Model And A Critical Review, Philadelphia University Jan 2012

5x20 Matrix For Knowledge Management Lifecycle Based On The Five C’S Model And A Critical Review, Philadelphia University

Philadelphia University, Jordan

No abstract provided.

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The Short Story In Jordan Its Origin And Development, Philadelphia University Jan 2012

The Short Story In Jordan Its Origin And Development, Philadelphia University

Philadelphia University, Jordan

No abstract provided.

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Oxidative Carbonylation Of 2-Propyn-1-Ol And 2-Methyl-3-Butyn-2-Ol In An Oscillatory Mode, Sergey N. Gorodsky Jan 2012

Oxidative Carbonylation Of 2-Propyn-1-Ol And 2-Methyl-3-Butyn-2-Ol In An Oscillatory Mode, Sergey N. Gorodsky

Sergey N. Gorodsky

No abstract provided.

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The Effects Of Neutral Gas Heating On H Mode Transition And Maintenance Currents In A 13.56 Mhz Planar Coil Inductively Coupled Plasma Reactor, Chin Oi Hoong Jan 2012

The Effects Of Neutral Gas Heating On H Mode Transition And Maintenance Currents In A 13.56 Mhz Planar Coil Inductively Coupled Plasma Reactor, Chin Oi Hoong

Chin Oi Hoong

The H mode transition and maintenance currents in a 13.56 MHz laboratory 6 turn planar coil inductively coupled plasma (ICP) reactor are simulated for low pressure argon discharge range of 0.02-0.3 mbar with neutral gas heating and at ambient temperature. An experimentally fitted 3D power evolution plot for 0.02 mbar argon pressure is also shown to visualize the effects of hysteresis in the system. Comparisons between simulation and experimental measurements show good agreement in the pressure range of 0.02-0.3 mbar for transition currents and 0.02-0.1 mbar for maintenance currents only when neutral gas heating is considered. This suggests that neutral …

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Determination Of Paleoearthquake Timing And Magnitudes On The Southern Segment Of The East Cache Fault, Utah, James P. Mccalpin Jan 2012

Determination Of Paleoearthquake Timing And Magnitudes On The Southern Segment Of The East Cache Fault, Utah, James P. Mccalpin

James P. McCalpin

We investigated the late Quaternary rupture history of the southern East Cache Fault zone [ECFZ], northern Utah with geologic mapping, paleoseismic logging of fault trenches, ground-penetrating radar, and optically stimulated luminescence dating. McCalpin (1989) indicated that the southern segment of the ECFZ consisted of three strands. We excavated four trenches across these strands, and evaluate the stratigraphy and structure of the sites. We conclude that the western fault strand of the ECFZ has had no late Quaternary displacement or does not exist; the central strand has not had any late Quaternary movement and may be a drape fold; and the …

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Instabilities And Patterns In Coupled Reaction-Diffusion Layers, Anne J. Catlla, Amelia Mcnamara, Chad M. Topaz Jan 2012

Instabilities And Patterns In Coupled Reaction-Diffusion Layers, Anne J. Catlla, Amelia Mcnamara, Chad M. Topaz

Chad M. Topaz

We study instabilities and pattern formation in reaction-diffusion layers that are diffusively coupled. For two-layer systems of identical two-component reactions, we analyze the stability of homogeneous steady states by exploiting the block symmetric structure of the linear problem. There are eight possible primary bifurcation scenarios, including a Turing-Turing bifurcation that involves two disparate length scales whose ratio may be tuned via the interlayer coupling. For systems of n-component layers and nonidentical layers, the linear problem’s block form allows approximate decomposition into lower-dimensional linear problems if the coupling is sufficiently weak. As an example, we apply these results to a two-layer …

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Observational Constraints On The Molecular Gas Content In Nearby Starburst Dwarf Galaxies, Kristen B.W. Mcquinn, Evan D. Skillman, Julianne J. Dalcanton, Andrew E. Dolphin, John M. Cannon, Jon Holtzman, Daniel R. Weisz, Benjamin F. Williams Jan 2012

Observational Constraints On The Molecular Gas Content In Nearby Starburst Dwarf Galaxies, Kristen B.W. Mcquinn, Evan D. Skillman, Julianne J. Dalcanton, Andrew E. Dolphin, John M. Cannon, Jon Holtzman, Daniel R. Weisz, Benjamin F. Williams

John Cannon

No abstract provided.

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Elements Of Proximal Formative Assessment In Learners’ Discourse About Energy, Benedikt W. Harrer, Rachel E. Scherr, Michael C. Wittmann, Hunter G. Close, Brian W. Frank Jan 2012

Elements Of Proximal Formative Assessment In Learners’ Discourse About Energy, Benedikt W. Harrer, Rachel E. Scherr, Michael C. Wittmann, Hunter G. Close, Brian W. Frank

Benedikt W. Harrer

Proximal formative assessment, the just-in-time elicitation of students' ideas that informs ongoing instruction, is usually associated with the instructor in a formal classroom setting. However, the elicitation, assessment, and subsequent instruction that characterize proximal formative assessment are also seen in discourse among peers. We present a case in which secondary teachers in a professional development course at SPU are discussing energy flow in refrigerators. In this episode, a peer is invited to share her thinking (elicitation). Her idea that refrigerators move heat from a relatively cold compartment to a hotter environment is inappropriately judged as incorrect (assessment). The "instruction" (peer …

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