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Modeling Strategies (modeling + strategy)
Selected AbstractsRegional Climate Models for Hydrological Impact Studies at the Catchment Scale: A Review of Recent Modeling StrategiesGEOGRAPHY COMPASS (ELECTRONIC), Issue 7 2010Claudia Teutschbein This article reviews recent applications of regional climate model (RCM) output for hydrological impact studies. Traditionally, simulations of global climate models (GCMs) have been the basis of impact studies in hydrology. Progress in regional climate modeling has recently made the use of RCM data more attractive, although the application of RCM simulations is challenging due to often considerable biases. The main modeling strategies used in recent studies can be classified into (i) very simple constructed modeling chains with a single RCM (S-RCM approach) and (ii) highly complex and computing-power intensive model systems based on RCM ensembles (E-RCM approach). In the literature many examples for S-RCM can be found, while comprehensive E-RCM studies with consideration of several sources of uncertainties such as different greenhouse gas emission scenarios, GCMs, RCMs and hydrological models are less common. Based on a case study using control-run simulations of fourteen different RCMs for five Swedish catchments, the biases of and the variability between different RCMs are demonstrated. We provide a short overview of possible bias-correction methods and show that inter-RCM variability also has substantial consequences for hydrological impact studies in addition to other sources of uncertainties in the modeling chain. We propose that due to model bias and inter-model variability, the S-RCM approach is not advised and ensembles of RCM simulations (E-RCM) should be used. The application of bias-correction methods is recommended, although one should also be aware that the need for bias corrections adds significantly to uncertainties in modeling climate change impacts. [source] Correlative and mechanistic models of species distribution provide congruent forecasts under climate changeCONSERVATION LETTERS, Issue 3 2010Michael R. Kearney Abstract Good forecasts of climate change impacts on extinction risks are critical for effective conservation management responses. Species distribution models (SDMs) are central to extinction risk analyses. The reliability of predictions of SDMs has been questioned because models often lack a mechanistic underpinning and rely on assumptions that are untenable under climate change. We show how integrating predictions from fundamentally different modeling strategies produces robust forecasts of climate change impacts on habitat and population parameters. We illustrate the principle by applying mechanistic (Niche Mapper) and correlative (Maxent, Bioclim) SDMs to predict current and future distributions and fertility of an Australian gliding possum. The two approaches make congruent, accurate predictions of current distribution and similar, dire predictions about the impact of a warming scenario, supporting previous correlative-only predictions for similar species. We argue that convergent lines of independent evidence provide a robust basis for predicting and managing extinctions risks under climate change. [source] Low-frequency noise conversion modeling in RF devices under forced nonlinear operationINTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 1 2006Gabriele Conte Abstract The article addresses the frequency conversion of low-frequency noise deriving from trap-assisted generation-recombination (GR) noise in RF devices under forced, nonlinear operation through a physics-based noise model. The superposition of the stationary (small-signal) GR spectra originating from noninteracting trap levels with properly distributed energies is shown, in simple yet significant device case studies, to yield a 1/f or 1/f -like behaviour over a prescribed frequency range. The same trap distribution is also exploited for large-signal, cyclostationary noise simulation in forced periodic conditions. In this case, low-frequency 1/f -like noise is shown to be upconverted from the baseband to all noise sidebands. Circuit-level compact modeling strategies for noise-frequency conversion based on the modulation of small-signal low-frequency noise are also investigated and compared to the fundamental approach. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2006. [source] Multivariate calibration stability: a comparison of methodsJOURNAL OF CHEMOMETRICS, Issue 3 2002Brian D. Marx Abstract In the multivariate calibration framework we revisit and investigate the prediction performance of three high-dimensional modeling strategies: partial least squares, principal component regression and P-spline signal regression. Specifically we are interested in comparing the stability and robustness of prediction under differing conditions, e.g. training the model under one temperature and using it to predict under differing temperatures. An example illustrates stability comparisons. Copyright © 2002 John Wiley & Sons, Ltd. [source] Complexation and chiral drug recognition of an amphiphilic phenothiazine derivative with , -cyclodextrinJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 4 2008Andrés Guerrero-Martínez Abstract Promethazine hydrochloride (PTZ) is an amphiphilic drug derived from the phenothiazine structure that possesses a charged aliphatic chain with a chiral carbon. In the presence of , -cyclodextrin (, -CD), this drug undergoes significant changes of its photophysical properties in aqueous solution. Fluorescence spectroscopy measurements show the formation of a 1:1 stoichiometry complex with quantum yield lower than that of the pure PTZ, and two fluorescence lifetimes, which can be assigned to the free and complexed forms of the drug. In addition, 1H NMR spectra, and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY) were used to characterize the drug and the complex, and to determine the effects of the complexation on the aggregation. For the drug binary system, a noncooperative association process is observed, and in the presence of macrocycle, the chemical shifts reveal a chiral resolution of the drug enantiomers, with different stability constants of the complexes. , -CD modifies the aggregation of PTZ in an extension that confirms the formation of a 1:1 complex. ROE enhancements and molecular modeling strategies show the most likely structure of the complex in solution, in which one of the phenyl rings is buried into the CD cavity, with a slight inclusion of the aliphatic part. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:1484,1498, 2008 [source] Comparison of analytical and numerical methods for homogenization of nanotube-reinforced polymersPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009Ina Schmidt Carbon nanotubes are increasingly getting impact as reinforcing material for polymer based nanocomposites. Hence, new modeling strategies are necessary to calculate the behavior of these materials. In the last years some attempts have been made using and developing classical micromechanical models. On the other hand numerical homogenization methods are available to tackle this problem. Examples for both types of modeling strategies are presented with focus on the nanotube geometry. The nanotubes are modeled as hollow tubes as well as as isotropic and transversely isotropic cylinders. As expected the results of numerical and analytical methods are identical for isotropic cylinder inclusions. Small deviations occur for transversely isotropic cylinders in transverse direction. In the case of hollow tube inclusions, the analytical models lead to lower stiffness values in transverse direction and for shear. The largest deviations occur for longitudinal shear with magnitudes smaller than 10%. In contrast the effort to get numerical results is enormous, so that the analytical models are still useful. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] DOES ELECTRICITY RESTRUCTURING WORK?THE JOURNAL OF INDUSTRIAL ECONOMICS, Issue 3 2007EVIDENCE FROM THE U.S. NUCLEAR ENERGY INDUSTRY This paper examines whether electricity restructuring improves the efficiency of U.S. nuclear power generation. Using a panel dataset consisting of the full sample of 73 investor-owned nuclear plants in the United States from 1992 to 1998, I estimate the plant-level cross-sectional and longitudinal efficiency changes associated with restructuring. Special attention is given to the potential policy endogeneity bias and different modeling strategies are presented to cope with the issue. Overall, I find a striking positive relationship between restructuring and cost reduction, and increased plant utilization. [source] Analytical approaches for transplant research, 2004AMERICAN JOURNAL OF TRANSPLANTATION, Issue 4p2 2005Douglas E. Schaubel This article provides detailed explanations of the methods frequently employed in outcomes analyses performed by the Scientific Registry of Transplant Recipients (SRTR). All aspects of the analytical process are discussed, including cohort selection, post-transplant follow-up analysis, outcome definition, ascertainment of events, censoring, and adjustments. The methods employed for descriptive analyses are described, such as unadjusted mortality rates and survival probabilities, and the estimation of covariant effects through regression modeling. A section on transplant waiting time focuses on the kidney and liver waiting lists, pointing out the different considerations each list requires and the larger questions that such analyses raise. Additionally, this article describes specialized modeling strategies recently designed by the SRTR and aimed at specific organ allocation issues. The article concludes with a description of simulated allocation modeling (SAM), which has been developed by the SRTR for three organ systems: liver, thoracic organs, and kidney-pancreas. SAMs are particularly useful for comparing outcomes for proposed national allocation policies. The use of SAMs has already helped in the development and implementation of a new policy for liver candidates with high MELD scores to be offered organs regionally before the organs are offered to candidates with low MELD scores locally. [source] Identification of possible kinetically significant anion-binding sites in human serum transferrin using molecular modeling strategiesBIOPOLYMERS, Issue 2 2004Elizabeth Ambrose Amin Abstract Certain anions have been shown experimentally to influence the rate of iron release from human serum transferrin (HST), implying the existence of one or more allosteric kinetically significant anion- binding (KISAB) sites on or near the surface of the protein. A rank-ordered selection of potential HST KISAB sites has been obtained using a novel three-stage molecular modeling strategy. The crystal structure of HST (1A8E.pdb) was first subjected to a heuristic analysis, in which positively charged and hydrogen-bonding residues on or near the surface of the protein were identified. In this stage, a preliminary electrostatic potential map was also calculated, yielding six preliminary sites. Next, energy-grid calculations were conducted in order to identify anion,protein interaction energy minima, which resulted in the inclusion of three additional sites. Finally, three anions already shown experimentally to demonstrate varied effects on HST iron-release kinetics were placed at each potential site; molecular dynamics and molecular mechanics calculations were performed in order to elucidate the hydrogen-bonding environment around each anion of the protein as well as to calculate anion,protein-binding energies. © 2003 Wiley Periodicals, Inc. Biopolymers 73: 205,215, 2004 [source] A comprehensive and novel predictive modeling technique using detailed pathology factors in men with localized prostate carcinomaCANCER, Issue 7 2002Louis Potters M.D. Abstract BACKGROUND The purpose of the current study was to evaluate modeling strategies using sextant core prostate biopsy specimen data that would best predict biochemical control in patients with localized prostate carcinoma treated with permanent prostate brachytherapy (PPB). METHODS One thousand four hundred seventy,seven patients underwent PPB between 1992 and 2000. The authors restricted analysis to those patients who had sextant biopsies (n = 1073). A central pathology review was undertaken on all specimens. Treatment consisted of PPB with either I-125 or Pd-103 prescribed to 144 Gy or 140 Gy, respectively. Two hundred twenty,eight patients (21%) received PPB in combination with external radiotherapy and 333 patients (31%) received neoadjuvant hormones. In addition to clinical stage, biopsy Gleason sum, and pretreatment prostate specific antigen (pretx-PSA), the following detailed biopsy variables were considered: mean percentage of cancer in an involved core; maximum percentage of cancer; mean primary and secondary Gleason grades; maximum Gleason grade (primary or secondary); percentage of cancer in the apex, mid, and base; percent of cores positive; maximum primary and secondary Gleason grades in apex, mid, and base; maximum percent cancer in apex, mid, and base; maximum Gleason grade in apex, mid, and base; maximum primary Gleason grade; and maximum secondary Gleason grade. In all, 23 biopsy variables were considered. Four modeling strategies were compared. As a base model, the authors considered the pretx-PSA, clinical stage, and biopsy Gleason sum as predictors. For the second model, the authors added percent of cores positive. The third modeling strategy was to use stepwise variable selection to select only those variables (from the total pool of 26) that were statistically significant. The fourth strategy was to apply principal components analysis, which has theoretical advantages over the other strategies. Principal components analysis creates component scores that account for maximum variance in the predictors. RESULTS The median followup of the study cohort was 36 months (range, 6,92), and the Kattan modification of the American Society for Therapeutic Radiology and Oncology (ASTRO) definition was used to define PSA freedom from recurrence (PSA-FFR). The four models were compared in their ability to predict PSA-FFR as measured by the Somers D rank correlation coefficient. The Somers D rank correlation coefficients were then corrected for optimism with use of bootstrapping. The results for the four models were 0.32, 0.34, 0.37, and 0.39, respectively. CONCLUSIONS The current study shows that the use of principal components analysis with additional pathology data is a more discriminating model in predicting outcome in prostate carcinoma than other conventional methods and can also be used to model outcome predictions for patients treated with radical prostatectomy and external beam. Cancer 2002;95:1451,6. © 2002 American Cancer Society. DOI 10.1002/cncr.10869 [source] DESTINATION EFFECTS: RESIDENTIAL MOBILITY AND TRAJECTORIES OF ADOLESCENT VIOLENCE IN A STRATIFIED METROPOLIS,CRIMINOLOGY, Issue 3 2010PATRICK SHARKEY Two landmark policy interventions to improve the lives of youth through neighborhood mobility,the Gautreaux program in Chicago and the Moving to Opportunity (MTO) experiments in five cities,have produced conflicting results and have created a puzzle with broad implications: Do residential moves between neighborhoods increase or decrease violence, or both? To address this question, we analyze data from a subsample of adolescents ages 9,12 years from the Project on Human Development in Chicago Neighborhoods, a longitudinal study of children and their families that began in Chicago,the site of the original Gautreaux program and one of the MTO experiments. We propose a dynamic modeling strategy to separate the effects of residential moving across three waves of the study from dimensions of neighborhood change and metropolitan location. The results reveal countervailing effects of mobility on trajectories of violence; whereas neighborhood moves within Chicago lead to an increased risk of violence, moves outside the city reduce violent offending and exposure to violence. The gap in violence between movers within and outside Chicago is explained not only by the racial and economic composition of the destination neighborhoods but also by the quality of school contexts, adolescents' perceived control over their new environment, and fear. These findings highlight the need to simultaneously consider residential mobility, mechanisms of neighborhood change, and the wider geography of structural opportunity. [source] How well can animals navigate?ENVIRONMETRICS, Issue 4 2006Estimating the circle of confusion from tracking data Abstract State-space models have recently been shown to effectively model animal movement. In this paper we illustrate how such models can be used to improve our knowledge of animal navigation ability, something which is poorly understood. This work is of great interest when modeling the behavior of animals that are migrating, often over tremendously large distances. We use the term circle of confusion, first proposed by Kendall (1974), to describe the general inability of an animal to know its location precisely. Our modeling strategy enables us to statistically describe the circle of confusion associated with any animal movements where departure and destination points are known. For illustration, we use ARGOS satellite telemetry of leatherback turtles migrating over a distance of approximately 4000,km in the Atlantic Ocean. Robust features of the model enable one to deal with outlying observations, highly characteristic of these types of data. Although specifically designed for data obtained using satellite telemetry, our approach is generalizable to other common kinds of movement data such as archival tag data. Copyright © 2005 John Wiley & Sons, Ltd. [source] Evaluating effectiveness of preoperative testing procedure: some notes on modelling strategies in multi-centre surveysJOURNAL OF EVALUATION IN CLINICAL PRACTICE, Issue 1 2008Dario Gregori PhD Abstract Rationale, In technology assessment in health-related fields the construction of a model for interpreting the economic implications of the introduction of a technology is only a part of the problem. The most important part is often the formulation of a model that can be used for selecting patients to submit to the new cost-saving procedure or medical strategy. The model is usually complicated by the fact that data are often non-homogeneous with respect to some uncontrolled variables and are correlated. The most typical example is the so-called hospital effect in multi-centre studies. Aims and objectives, We show the implications derived by different choices in modelling strategies when evaluating the usefulness of preoperative chest radiography, an exam performed before surgery, usually with the aim to detect unsuspected abnormalities that could influence the anaesthetic management and/or surgical plan. Method, We analyze the data from a multi-centre study including more than 7000 patients. We use about 6000 patients to fit regression models using both a population averaged and a subject-specific approach. We explore the limitations of these models when used for predictive purposes using a validation set of more than 1000 patients. Results, We show the importance of taking into account the heterogeneity among observations and the correlation structure of the data and propose an approach for integrating a population-averaged and subject specific approach into a single modeling strategy. We find that the hospital represents an important variable causing heterogeneity that influences the probability of a useful POCR. Conclusions, We find that starting with a marginal model, evaluating the shrinkage effect and eventually move to a more detailed model for the heterogeneity is preferable. This kind of flexible approach seems to be more informative at various phases of the model-building strategy. [source] A robust calibration modeling strategy for analysis of interference-subject spectral dataAICHE JOURNAL, Issue 1 2010Chunhui Zhao Abstract Preprocessing and correction of mixture spectra have been an important issue with regard to the removal of undesired systematic variation due to variations in environmental, instrumental, or sample conditions. In this article, a new robust calibration modeling strategy is proposed on the basis of independent component analysis (ICA). It aims at separating the interference-subject parasitic subspace from the interference-immune common subspace among all considered cases. The common subspace is further divided into two orthogonal parts according to their relationship with quality: one is quality-irrelevant and the other is quality-informative, in which, only the second part is employed for quality prediction. Focusing on each subspace, it identifies distinct types of underlying source components underlying different spectra subspaces, analyzes their characteristics and roles, and accordingly models them for different applications, respectively. This approach provides a comprehensive insight into the inherent nature of interference-subject mixture spectra. Furthermore, several model statistics are defined to give quantitative indication on the effectiveness of the correction strategy. The feasibility and performance of the proposed method are illustrated with data from laboratory experiments. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Processing of short-fiber reinforced polypropylene.POLYMER ENGINEERING & SCIENCE, Issue 1 2000An experimental investigation of the processing of glass fiber reinforced polypropylene is presented. Final fiber orientation distribution, fiber distribution in filament sections, rheological properties, final fiber length distribution and surface morphology were analyzed. This analysis was done taking into account the quantity of fibers and their interactions and flow conditions. The final fiber orientation increased when shear rate increased and fiber concentration decreased. Moreover, inhomogeneities in fiber distribution increased as the concentration of fibers decreased. The density profile showed a significant variation with fiber concentration, but it was not dependent on the shear rate applied. The viscosity showed a linear dependence with shear rate. The average fiber length and the breadth of this distribution decreased with the increasing fiber concentration and extrusion rate. The extruded filament surface showed minor roughness when the shear rate increased or when the fiber concentration decreased. The results of this experimental characterization give useful information to determine the influence of the processing variables on the final properties of short-fiber reinforced polypropylene and constitutes the first part of a more ambitious project that also includes the development of a modeling strategy of the processing behavior for short-fiber composites. [source] Contextual Factors and the Extreme Right Vote in Western Europe, 1980,2002AMERICAN JOURNAL OF POLITICAL SCIENCE, Issue 2 2009Kai Arzheimer Research on the voters of the extreme right in Western Europe has become a minor industry, but relatively little attention has been paid to the twin question of why support for these parties is often unstable, and why the extreme right is so weak in many countries. Moreover, the findings from different studies often contradict each other. This article aims at providing a more comprehensive and satisfactory answer to this research problem by employing a broader database and a more adequate modeling strategy. The main finding is that while immigration and unemployment rates are important, their interaction with other political factors is much more complex than suggested by previous research. Moreover, persistent country effects prevail even if a whole host of individual and contextual variables is controlled for. [source] Identification of possible kinetically significant anion-binding sites in human serum transferrin using molecular modeling strategiesBIOPOLYMERS, Issue 2 2004Elizabeth Ambrose Amin Abstract Certain anions have been shown experimentally to influence the rate of iron release from human serum transferrin (HST), implying the existence of one or more allosteric kinetically significant anion- binding (KISAB) sites on or near the surface of the protein. A rank-ordered selection of potential HST KISAB sites has been obtained using a novel three-stage molecular modeling strategy. The crystal structure of HST (1A8E.pdb) was first subjected to a heuristic analysis, in which positively charged and hydrogen-bonding residues on or near the surface of the protein were identified. In this stage, a preliminary electrostatic potential map was also calculated, yielding six preliminary sites. Next, energy-grid calculations were conducted in order to identify anion,protein interaction energy minima, which resulted in the inclusion of three additional sites. Finally, three anions already shown experimentally to demonstrate varied effects on HST iron-release kinetics were placed at each potential site; molecular dynamics and molecular mechanics calculations were performed in order to elucidate the hydrogen-bonding environment around each anion of the protein as well as to calculate anion,protein-binding energies. © 2003 Wiley Periodicals, Inc. Biopolymers 73: 205,215, 2004 [source] Multichromophores for Nonlinear Optics: Designing the Material Properties by Electrostatic Interactions,CHEMPHYSCHEM, Issue 17 2007Francesca Terenziani Dr. Abstract To fully exploit the promise of molecular materials for NLO applications, inter- and supramolecular interactions must be accounted for. We review our recent work on electrostatic interchromophore interactions in multichromophores for NLO applications. The discussion is based on a bottom-up modeling strategy: each chromophore is described in terms of an essential state model, validated and parameterized against spectroscopic data for solvated chromophores. The relevant information is then used to build a model for clusters of chromophores interacting through electrostatic forces. Exact NLO responses and spectra calculated within this model fully account for collective and cooperative interchromophore interactions, which can either amplify or suppress NLO responses; supramolecular engineering of multichromophores is a powerful tool for the design of NLO materials. Moreover, new features emerge in multichromophores with no counterpart at the single-chromophore level, offering new exciting opportunities for applications. [source] | ||||||||||||||||||||||