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Modeling Approach (modeling + approach)
Kinds of Modeling Approach Selected AbstractsAPPLICATION OF DISCRETE MODELING APPROACH TO FLUIDIZED BED YEAST DRYINGJOURNAL OF FOOD PROCESS ENGINEERING, Issue 2010F. DEBASTE ABSTRACT Yeast drying is widely used to ease transport and conservation. In this work, baker's yeast drying in fluidized bed is modeled using a pore network model. Classical balanced equations at the reactor scale are coupled with the pore network for the grain, which takes into account diffusion in the gas phase, transport by liquid film in partially saturated region and pressure gradient effects in the liquid phase. The porous structure to be applied in the model is obtained using environmental scanning electron microscopy. Simulations are validated on a thermogravimetric analysis experiment. The model is then applied to fluidized bed drying for which experimental results obtained on a laboratory pilot are available. Finally, the model results are compared to those of a simplified receding front model. PRACTICAL APPLICATIONS The presented model allows simulation of Saccharomyces cerevisiae fluidized bed drying. Taking into account transport phenomena in the grain offers the opportunity to predict drying rate without the use of a desorption isotherm. Moreover, the model predicts roughly the critical humidity. Therefore, the model can be used for scale-up, design and optimization of dryer including the effect of changes in yeast granulation. [source] THE IMPACT OF RENT CONTROLS IN NON-WALRASIAN MARKETS: AN AGENT-BASED MODELING APPROACHJOURNAL OF REGIONAL SCIENCE, Issue 3 2006Ralph Bradburd ABSTRACT We use agent-based models to consider rent ceilings in non-Walrasian housing markets, where bargaining between landlord and tenant leads to exchange at a range of prices. In the non-Walrasian setting agents who would be extramarginal in the Walrasian setting frequently are successful in renting, and actually account for a significant share of the units rented. This has several implications. First, rent ceilings above the Walrasian equilibrium price (WEP) can affect the market outcome. Second, rent ceilings that reduce the number of units rented do not necessarily reduce total market surplus. Finally, the distributional impact of rent controls differs from the Walrasian setting. [source] Inverse Modeling Approach to Allogenic Karst System CharacterizationGROUND WATER, Issue 3 2009N. Dörfliger Allogenic karst systems function in a particular way that is influenced by the type of water infiltrating through river water losses, by karstification processes, and by water quality. Management of this system requires a good knowledge of its structure and functioning, for which a new methodology based on an inverse modeling approach appears to be well suited. This approach requires both spring and river inflow discharge measurements and a continuous record of chemical parameters in the river and at the spring. The inverse model calculates unit hydrographs and the impulse responses of fluxes from rainfall hydraulic head at the spring or rainfall flux data, the purpose of which is hydrograph separation. Hydrograph reconstruction is done using rainfall and river inflow data as model input and enables definition at each time step of the ratio of each component. Using chemical data, representing event and pre-event water, as input, it is possible to determine the origin of spring water (either fast flow through the epikarstic zone or slow flow through the saturated zone). This study made it possible to improve a conceptual model of allogenic karst system functioning. The methodology is used to study the Bas-Agly and the Cent Font karst systems, two allogenic karst systems in Southern France. [source] Vorgehensmodell zur Abbildung und Analyse des Lebenszykluserfolges von Werkstoffen , Konzeption und beispielhafte Veranschaulichung.MATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 6 2010Modeling approach for the life cycle profit of materials, case study, conceptual design Life Cycle Costs; Life Cycle Profit; Modeling Approach; Cost Management Abstract Eine langfristig erfolgreiche Entwicklung, Herstellung und Verwendung von (neuen) Werkstoffen setzt voraus, dass die daran beteiligten, auf wirtschaftlichen Erfolg angewiesenen Unternehmen mit diesen Werkstoffen einen positiven monetären Lebenszykluserfolg erzielen. Die Ermittlung eines solchen Lebenszykluserfolgs ist allerdings eine komplexe Aufgabenstellung, wird dieser doch durch eine Vielzahl verschiedenartiger unternehmensexterner und -interner Größen beeinflusst. Gerade bei derartigen komplexen Problemstellungen bietet es sich an, Vorgehensmodelle für eine systematische und strukturierte Problemanalyse und -zerlegung sowie die darauf basierende Bildung und Auswertung von monetären Lebenszyklusmodellen zu nutzen. Im vorliegenden Beitrag soll daher ein Vorgehensmodell zur Ermittlung und Analyse des Lebenszykluserfolgs von Werkstoffen zunächst konzipiert und anschließend anhand eines Fallbeispiels veranschaulicht werden. Long-term successful development, production and use of (new) materials presuppose that involved companies achieve a positive monetary life cycle benefit with these materials. However, the appraisal of this life cycle profit is a demanding task, since the profit is influenced by a huge number of different internal and external variables. Especially in case of such complex problems procedure models can support the systematic and structured analysis and decomposition of the problems as well as the creation and evaluation of monetary life cycle models basing on that. Therefore, this paper offers a suitable modeling approach for the life cycle profit of materials, which is illustrated by a case study. [source] Evaluating the Effect of Conservation Policies on Agricultural Land Use: A Site-specific Modeling ApproachCANADIAN JOURNAL OF AGRICULTURAL ECONOMICS, Issue 3 2004Katsuya Tanaka This study evaluates quantitatively the effect of three policies (payments for cropland retirement, fertilizer use taxes and payments for crop rotations) on agricultural land use in the upper Mississippi River basin. This is done by estimating two logit models of land use decisions using data from the 1982, 1987,1992 and 1997 Natural Resource Inventories. The models predict farmers' crop choice, crop rotation and participation in the Conservation Reserve Program (CRP) at more than 48,000 Natural Resource Inventories sites under each of the three policies. Results suggest that an increase in the CRP rental rates would significantly increase the CRP acreage, but most of the acreage increase would come initially from less fertilizer-intensive crops. In contrast, a fertilizer use tax would significantly reduce acreage planted to more fertilizer-intensive crops, and thus would likely be cost effective for reducing agricultural chemical use and pollution. Although an incentive payment for a corn-soybean rotation would raise acreage of this rotation and reduce the acreage of continuous corn, the acreage response is in general quite inelastic. Cette étude évalue quantitativement les effets rovoqués par les trois politiques (paiements pour le retrait des terres cultivables, taxes sur l'utilisation d'engrais et paiements pour l'alternance des cultures) sur les terres agricoles du bassin supérieur du Mississipi. Ceci est obtenu en évaluant deux modéles logit des décisions sur l'utilisation des terres provenant des données des «Natural Resource Inventories» de 1982, 1987, 1992 et 1997. Les modéles prédisent le choix des cultures des agriculteurs, l'alternance des cultures et la participation du «Conservation Reserve Program (CRP)» dans plus de 48 000 Natural Resource Inventories dans le cadre de chacune des trois politiques. Les résultats suggérent qu'une augmentation des taux de location du CRP accroisse de maniére significative la surface de CRP, mais la majeure partie de cet accroissement de surface provenaient initialement de cultures moins intensives sans engrais. Cependant, l'utilisation d'une taxe sur l'utilisation d'engrais pouvait réduire de maniére significative la surface plantée avec des récoltes intensives utilisant plus d'engrais, et ainsi ce serait sans doute plus économique pour réduire la pollution et l'utilisation de produits chimiques en agriculture. Bien que des paiements incitatifs à l'alternance maïs-soja réduisent la surface d'une culture continue de maïs et augmentaient la surface de l'alternance maïs-soja, les résultats aux transformations des surfaces des terres seraient tout à fait rigides. [source] Conformational Analysis of R207910, a New Drug Candidate for the Treatment of Tuberculosis, by a Combined NMR and Molecular Modeling ApproachCHEMICAL BIOLOGY & DRUG DESIGN, Issue 2 2006Sandrine Gaurrand R207910 is an enantiomeric compound from a new class of antimycobacterial agents, the diarylquinolines [Science; 307:223 (2005)]. As enantiospecific interaction is required for biologic activity, we have undertaken a combined nuclear magnetic resonance and molecular modeling study to gain new insights into its conformation in solution and its absolute configuration. A conformational analysis using a Monte-Carlo method has been performed on each of the four possible stereomers of this compound leading to the identification of their most stable conformation. Additional ab initio calculation was performed with emphasis on the strength of the observed intramolecular hydrogen bond. Simultaneously, a complete structural identification has been carried out by a set of monodimensional and bidimensional 1H- 13C-NMR experiments. Determination of inter-proton distances has been achieved by a series of 1H- 1H ROESY NMR experiments with different mixing times followed by a volume quantification of the correlations peaks. These experimental data were compared with the theoretical distances obtained from the conformational analysis. The remarkable match shows that R207910 adopts one of the low-energy conformations predicted by molecular modeling and belongs to the (RS, SR) couple of diastereoisomers. A posteriori validation of our approach has been performed by X-ray structure determination that concluded for the RS configuration. [source] Latent Class Modeling Approaches for Assessing Diagnostic Error without a Gold Standard: With Applications to p53 Immunohistochemical Assays in Bladder TumorsBIOMETRICS, Issue 2 2001Paul S. Albert Summary. Improved characterization of tumors for purposes of guiding treatment decisions for cancer patients will require that accurate and reproducible assays be developed for a variety of tumor markers. No gold standards exist for most tumor marker assays. Therefore, estimates of assay sensitivity and specificity cannot be obtained unless a latent class model-based approach is used. Our goal in this article is to estimate sensitivity and specificity for p53 immunohistochemical assays of bladder tumors using data from a reproducibility study conducted by the National Cancer Institute Bladder Tumor Marker Network. We review latent class modeling approaches proposed by previous authors, and we find that many of these approaches impose assumptions about specimen heterogeneity that are not consistent with the biology of bladder tumors. We present flexible mixture model alternatives that are biologically plausible for our example, and we use them to estimate sensitivity and specificity for our p53 assay example. These mixture models are shown to offer an improvement over other methods in a variety of settings, but we caution that, in general, care must be taken in applying latent class models. [source] Consumers' taste for rarity drives sturgeons to extinctionCONSERVATION LETTERS, Issue 5 2008Agnès Gault Abstract The international market for luxury goods puts pressure on many wildlife species, with potentially irreversible consequences for many of them. Although classical economic theory suggests that trade alone would not drive a rare species to extinction, in practice numerous species are being threatened by overexploitation. This is for example the case for sturgeons, exploited for their caviar, of which all 27 species are threatened with extinction. We performed a caviar-tasting experiment, combined with a modeling approach merging ecological theory and psychosociology. This allowed us to demonstrate that the human predisposition to place exaggerated value on rarity drives sturgeons' overexploitation, despite caviar's ever-increasing price and the imminent loss of these species. These findings suggest that this mechanism probably drives the entire market for wildlife based luxury goods. [source] Multiple Conceptualizations of Small Business Web Use and Benefit*DECISION SCIENCES, Issue 3 2003Kurt A. Pflughoeft ABSTRACT Small businesses play an important role in the U.S. economy and there is anecdotal evidence that use of the Web is beneficial to such businesses. There is, however, little systematic analysis of the conditions that lead to successful use of and thereby benefits from the Web for small businesses. Based on the innovation adoption, organizations, and information systems (IS) implementation literature, we identify a set of variables that are related to adoption, use, and benefits of information technology (IT), with particular emphasis on small businesses. These variables are reflective of an organization's contextual characteristics, its IT infrastructure, Web use, and Web benefits. Since the extant research does not suggest a single theoretical model for Web use and benefits in the context of small businesses, we adopt a modeling approach and explore the relationships between "context-IT-use-benefit" (CIUB) through three models,partial-mediator, reduced partial-mediator, and mediator. These models posit that the extent of Web use by small businesses and the associated benefits are driven by organizations' contextual characteristics and their IT infrastructure. They differ in the endogeneity/exogeneity of the extent of IT sophistication, and in the direct/mediated effects of organizational context. We examine whether the relationships between variables identified in the literature hold within the context of these models using two samples of small businesses with national coverage, including various sizes, and representing several industry sectors. The results show that the evidence for patterns of relationships is similar across the two independent samples for two of these models. We highlight the relationships within the reduced partial-mediator and mediator models for which conclusive evidence are given by both samples. Implications for small business managers and providers of Web-based technologies are discussed. [source] Advantages of mixed effects models over traditional ANOVA models in developmental studies: A worked example in a mouse model of fetal alcohol syndromeDEVELOPMENTAL PSYCHOBIOLOGY, Issue 7 2007Patricia E. Wainwright Abstract Developmental studies in animals often violate the assumption of statistical independence of observations due to the hierarchical nature of the data (i.e., pups cluster by litter, correlation of individual observations over time). Mixed effect modeling (MEM) provides a robust analytical approach for addressing problems associated with hierarchical data. This article compares the application of MEM to traditional ANOVA models within the context of a developmental study of prenatal ethanol exposure in mice. The results of the MEM analyses supported the ANOVA results in showing that a large proportion of the variability in both behavioral score and brain weight could be explained by ethanol. The MEM also identified that there were significant interactions between ethanol and litter size in relation to behavioral scores and brain weight. In addition, the longitudinal modeling approach using linear MEM allowed us to model for flexible weight gain over time, as well as to provide precise estimates of these effects, which would be difficult in repeated measures ANOVA. © 2007 Wiley Periodicals, Inc. Dev Psychobiol 49: 664,674, 2007. [source] A spatial model of bird abundance as adjusted for detection probabilityECOGRAPHY, Issue 2 2009P. Marcos Gorresen Modeling the spatial distribution of animals can be complicated by spatial and temporal effects (i.e. spatial autocorrelation and trends in abundance over time) and other factors such as imperfect detection probabilities and observation-related nuisance variables. Recent advances in modeling have demonstrated various approaches that handle most of these factors but which require a degree of sampling effort (e.g. replication) not available to many field studies. We present a two-step approach that addresses these challenges to spatially model species abundance. Habitat, spatial and temporal variables were handled with a Bayesian approach which facilitated modeling hierarchically structured data. Predicted abundance was subsequently adjusted to account for imperfect detection and the area effectively sampled for each species. We provide examples of our modeling approach for two endemic Hawaiian nectarivorous honeycreepers: ,i,iwi Vestiaria coccinea and ,apapane Himatione sanguinea. [source] The Impact of Vertical Scaling Decisions on Growth InterpretationsEDUCATIONAL MEASUREMENT: ISSUES AND PRACTICE, Issue 4 2009Derek C. Briggs Most growth models implicitly assume that test scores have been vertically scaled. What may not be widely appreciated are the different choices that must be made when creating a vertical score scale. In this paper empirical patterns of growth in student achievement are compared as a function of different approaches to creating a vertical scale. Longitudinal item-level data from a standardized reading test are analyzed for two cohorts of students between Grades 3 and 6 and Grades 4 and 7 for the entire state of Colorado from 2003 to 2006. Eight different vertical scales were established on the basis of choices made for three key variables: Item Response Theory modeling approach, linking approach, and ability estimation approach. It is shown that interpretations of empirical growth patterns appear to depend upon the extent to which a vertical scale has been effectively "stretched" or "compressed" by the psychometric decisions made to establish it. While all of the vertical scales considered show patterns of decelerating growth across grade levels, there is little evidence of scale shrinkage. [source] The effect of pH and ionic strength on the sorption of sulfachloropyridazine, tylosin, and oxytetracycline to soilENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2006Thomas L. ter Laak Abstract Antimicrobial agents are the most heavily used pharmaceuticals in intensive husbandry. Their usual discharge pathway is application to agricultural land as constituents of animal manure, which is used as fertilizer. Many of these compounds undergo pH-dependent speciation and, therefore, might occur as charged species in the soil environment. Hence, pH and ionic strength of the soil suspension can affect the sorption behavior of these compounds to soil. Consequently, the soil sorption of three antimicrobial agents,sulfachloropyridazine (SCP), tylosin (TYL), and oxytetracycline (OTC),was investigated. Their respective sorption coefficients in two agricultural soils ranged from 1.5 to 1,800 L/kg. Sorption coefficients were greater under acidic conditions. Addition of an electrolyte to the solution led to decreased sorption of TYL and OTC by a factor of 3 to 20, but it did not influence the sorption of SCP. This behavior was analyzed by accounting for the pH-dependent speciation of TYL and OTC and considering the presence of OTC-calcium complexes. It appears that the decreased sorption of TYL and OTC with increasing ionic strength results from competition of the electrolyte cations with the positively charged TYL species and the positively charged OTC complexes. A model linking sorbate speciation with species-specific sorption coefficients can describe the pH dependence of the apparent sorption coefficients. This modeling approach is proposed for implementation in the assessment of sorption of ionizable compounds. [source] A modeling approach to link food availability, growth, emergence, and reproduction for the midge Chironomus ripariusENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2002Alexandre R. R. Péry Abstract We present models to link feeding with growth, emergence, and reproduction of the midge Chironomus riparius. These models are based on assumptions about the biology of this species and distinguish between males and females. The assumptions are the isomorphism of the chironomidae, the fact that much more energy is used for growth than for maintenance, and the existence of a maximum length for male and female larvae that does not depend on food availability. We supported our assumptions by experimental data and estimated the parameters of the model. We then successfully predicted the length pattern of 2-d-old larvae exposed in an artificial sediment to different feeding levels with different starting densities and also linked emergence time and growth pattern. We found our model to be consistent with data from another study and another species (Chironomus plumosus). As for reproduction, the mean number of eggs per mass was described as a linear function of feeding quantity. Our models could be used in sediment risk assessment to choose feeding level, to build effects models, or to predict the effects of toxicants at the population level. [source] Predicting the probability of detecting organochlorine pesticides and polychlorinated biphenyls in stream systems on the basis of land use in the Pacific Northwest, USA,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2000Robert W. Black Abstract We analyzed streambed sediment and fish tissue (Cottus sp.) at 30 sites in the Puget Sound and Willamette basins in Washington and Oregon, USA, respectively, for organochlorine pesticides and polychlorinated biphenyls (PCBs). The study was designed to determine the concentrations of organochlorine pesticides and PCBs in fish tissue and sediment by land use within these basins and to develop an empirical relation between land use and the probability of detecting these compounds in fish tissue or sediment. We identified 14 organochlorines in fish tissue and sediment; three compounds were unique to either fish tissue or sediment samples. The highest number of organochlorines detected in both fish tissue and streambed sediment was at those sites located in watersheds dominated by urban land uses. Using logistic regression, we found a significant relation between percentage agriculture and urban land use and organochlorines in fish tissue. The results of this study indicate that organochlorine pesticides and PCBs are still found in fish tissues and bed sediments in these two basins. In addition, we produced statistically significant models capable of predicting the probability of detecting specific organochlorines in fish on the basis of land use. Although the presented models are specific to the two study basins, the modeling approach could be applied to other basins as well. [source] A steady-state modeling approach to validate an in vivo mechanism of the GAL regulatory network in Saccharomyces cerevisiaeFEBS JOURNAL, Issue 20 2004Malkhey Verma Cellular regulation is a result of complex interactions arising from DNA,protein and protein,protein binding, autoregulation, and compartmentalization and shuttling of regulatory proteins. Experiments in molecular biology have identified these mechanisms recruited by a regulatory network. Mathematical models may be used to complement the knowledge-base provided by in vitro experimental methods. Interactions identified by in vitro experiments can lead to the hypothesis of multiple candidate models explaining the in vivo mechanism. The equilibrium dissociation constants for the various interactions and the total component concentration constitute constraints on the candidate models. In this work, we identify the most plausible in vivo network by comparing the output response to the experimental data. We demonstrate the methodology using the GAL system of Saccharomyces cerevisiae for which the steady-state analysis reveals that Gal3p neither dimerizes nor shuttles between the cytoplasm and the nucleus. [source] Bacterivorous grazers facilitate organic matter decomposition: a stoichiometric modeling approachFEMS MICROBIOLOGY ECOLOGY, Issue 2 2009Hao Wang Abstract There is widespread empirical evidence that protist grazing on bacteria reduces bacterial abundances but increases bacteria-mediated decomposition of organic matter. This paradox has been noted repeatedly in the microbiology literature but lacks a generally accepted mechanistic explanation. To explain this paradox quantitatively, we develop a bacteria-grazer model of organic matter decomposition that incorporates protozoa-driven nutrient recycling and stoichiometry. Unlike previous efforts, the current model includes explicit limitation, via Liebig's law of minimum, by two possible factors, nutrient and carbon densities, as well as their relative ratios in bacteria and grazers. Our model shows two principal results: (1) when the environment is carbon limiting, organic matter can always be decomposed completely, regardless of the presence/absence of grazers; (2) when the environment is nutrient (such as nitrogen) limiting, it is possible for organic matter to be completely decomposed in the presence, but not absence, of grazers. Grazers facilitate decomposition by releasing nutrients back into the environment, which would otherwise be limiting, while preying upon bacteria. Model analysis reveals that facilitation of organic matter decomposition by grazers is positively related to the stoichiometric difference between bacteria and grazers. In addition, we predict the existence of an optimal density range of introduced grazers, which maximally facilitate the decomposition of organic matter in a fixed time period. This optimal range reflects a trade-off between grazer-induced nutrient recycling and grazer-induced mortality of bacteria. [source] Preparation of High-Performance Conductive Polymer Fibers through Morphological Control of Networks Formed by NanofillersADVANCED FUNCTIONAL MATERIALS, Issue 9 2010Hua Deng Abstract A general method is described to prepare high-performance conductive polymer fibers or tapes. In this method, bicomponent tapes/fibers containing two layers of conductive polymer composites (CPCs) filled with multiwall carbon nanotubes (MWNT) or carbon black (CB) based on a lower-melting-temperature polymer and an unfilled polymer core with higher melting temperature are fabricated by a melt-based process. Morphological control of the conductive network formed by nanofillers is realized by solid-state drawing and annealing. Information on the morphological and electrical change of the highly oriented conductive nanofiller network in CPC bicomponent tapes during relaxation, melting, and crystallization of the polymer matrix is reported for the first time. The conductivity of these polypropylene tapes can be as high as 275,S,m,1 with tensile strengths of around 500,MPa. To the best of the authors' knowledge, it is the most conductive, high-strength polymer fiber produced by melt-processing reported in literature, despite the fact that only ,5,wt.% of MWNTs are used in the outer layers of the tape and the overall MWNT content in the bicomponent tape can be much lower (typically ,0.5,wt.%). Their applications could include sensing, smart textiles, electrodes for flexible solar cells, and electromagnetic interference (EMI) shielding. Furthermore, a modeling approach was used to study the relaxation process of highly oriented conductive networks formed by carbon nanofillers. [source] Efficiency Enhancement in Organic Photovoltaic Cells: Consequences of Optimizing Series ResistanceADVANCED FUNCTIONAL MATERIALS, Issue 1 2010Jonathan D. Servaites Abstract Here, means to enhance power conversion efficiency (PCE or ,) in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells by optimizing the series resistance (Rs),also known as the cell internal resistance,are studied. It is shown that current state-of-the-art BHJ OPVs are approaching the limit for which efficiency can be improved via Rs reduction alone. This evaluation addresses OPVs based on a poly(3-hexylthiophene):6,6-phenyl C61 -butyric acid methyl ester (P3HT:PCBM) active layer, as well as future high-efficiency OPVs (,,>,10%). A diode-based modeling approach is used to assess changes in Rs. Given that typical published P3HT:PCBM test cells have relatively small areas (,0.1,cm2), the analysis is extended to consider efficiency losses for larger area cells and shows that the transparent anode conductivity is then the dominant materials parameter affecting Rs efficiency losses. A model is developed that uses cell sizes and anode conductivities to predict current,voltage response as a function of resistive losses. The results show that the losses due to Rs remain minimal until relatively large cell areas (>0.1,cm2) are employed. Finally, Rs effects on a projected high-efficiency OPV scenario are assessed, based on the goal of cell efficiencies >10%. Here, Rs optimization effects remain modest; however, there are now more pronounced losses due to cell size, and it is shown how these losses can be mitigated by using higher conductivity anodes. [source] Using species distribution models to identify suitable areas for biofuel feedstock productionGCB BIOENERGY, Issue 2 2010JASON M. EVANS Abstract The 2007 Energy Independence and Security Act mandates a five-fold increase in US biofuel production by 2022. Given this ambitious policy target, there is a need for spatially explicit estimates of landscape suitability for growing biofuel feedstocks. We developed a suitability modeling approach for two major US biofuel crops, corn (Zea mays) and switchgrass (Panicum virgatum), based upon the use of two presence-only species distribution models (SDMs): maximum entropy (Maxent) and support vector machines (SVM). SDMs are commonly used for modeling animal and plant distributions in natural environments, but have rarely been used to develop landscape models for cultivated crops. AUC, Kappa, and correlation measures derived from test data indicate that SVM slightly outperformed Maxent in modeling US corn production, although both models produced significantly accurate results. When compared with results from a mechanistic switchgrass model recently developed by Oak Ridge National Laboratory (ORNL), SVM results showed higher correlation than Maxent results with models fit using county-scale point inputs of switchgrass production derived from expert opinion estimates. However, Maxent results for an alternative switchgrass model developed with point inputs from research trial sites showed higher correlation to the ORNL model than the corresponding results obtained from SVM. Further analysis indicates that both modeling approaches were effective in predicting county-scale increases in corn production from 2006 to 2007, a time period in which US corn production increased by 24%. We conclude that presence-only methods are a powerful first-cut tool for estimating relative land suitability across geographic regions in which candidate biofuel feedstocks can be grown, and may also provide important insight into potential land-use change patterns likely to be associated with increased biofuel demand. [source] Association and aggregation analysis using kin-cohort designs with applications to genotype and family history data from the Washington Ashkenazi StudyGENETIC EPIDEMIOLOGY, Issue 2 2001Nilanjan Chatterjee Abstract When a rare inherited mutation in a disease gene, such as BRCA1, is found through extensive study of high-risk families, it is critical to estimate not only age-specific penetrance of the disease associated with the mutation, but also the residual effect of family history once the mutation is taken into account. The kin-cohort design, a cross-sectional survey of a suitable population that collects DNA and family history data, provides an efficient alternative to cohort or case-control designs for estimating age-specific penetrance in a population not selected because of high familial risk. In this report, we develop a method for analyzing kin-cohort data that simultaneously estimate the age-specific cumulative risk of the disease among the carriers and non-carriers of the mutations and the gene-adjusted residual familial aggregation or correlation of the disease. We employ a semiparametric modeling approach, where the marginal cumulative risks corresponding to the carriers and non-carriers are treated non-parametrically and the residual familial aggregation is described parametrically by a class of bivariate failure time models known as copula models. A simple and robust two-stage method is developed for estimation. We apply the method to data from the Washington Ashkenazi Study [Struewing et al., 1997, N Engl J Med 336:1401,1408] to study the residual effect of family history on the risk of breast cancer among non-carriers and carriers of specific BRCA1/BRCA2 germline mutations. We find that positive history of a single first-degree relative significantly increases risk of the non-carriers (RR = 2.0, 95% CI = 1.6,2.6) but has little or no effect on the carriers. Genet. Epidemiol. 21:123,138, 2001. © 2001 Wiley-Liss, Inc. [source] A Combined Cluster and Interaction Model: The Hierarchical Assignment ProblemGEOGRAPHICAL ANALYSIS, Issue 3 2005Mark W. Horner This article presents a new spatial modeling approach that deals with interactions between individual geographic entities. The developed model represents a generalization of the transportation problem and the classical assignment problem and is termed the hierarchical assignment problem (HAP). The HAP optimizes the spatial flow pattern between individual origin and destination locations, given that some grouping, or aggregation of individual origins and destinations is permitted to occur. The level of aggregation is user specified, and the aggregation step is endogenous to the model itself. This allows for the direct accounting of aggregation costs in pursuit of optimal problem solutions. The HAP is formulated and solved with several sample data sets using commercial optimization software. Trials illustrate how HAP solutions respond to changes in levels of aggregation, as well as reveal the diverse network designs and allocation schemes obtainable with the HAP. Connections between the HAP and the literature on the p-median problem, cluster analysis, and hub-and-spoke networks are discussed and suggestions for future research are made. [source] Multilevel analysis of effects of individual characteristics and household factors on self-rated health among older adults in rural VietnamGERIATRICS & GERONTOLOGY INTERNATIONAL, Issue 2 2010Hoang Van Minh Aim: This paper aims to describe self-rated health (SRH) status among older adults in a rural community of Vietnam, and examine individual and household-level factors associated with good health rating among the study populations. Methods: The study was carried out in the Bavi district, a rural community located 60 km west of Hanoi, the capital, within the Epidemiological Field Laboratory of Bavi (FilaBavi) in Vietnam in 2006. All people aged 50 years and over who lived within the district were surveyed. Face-to-face household interviews were conducted by trained surveyors using standard World Health Organization/INDEPTH network questionnaire,summary version. A logistic multilevel modeling approach was applied to analyze the association between SRH and both individual and household-level factors. Results: The proportion of people aged 50 years and older in FilaBavi reported having good/very good health and poor/very poor health was 15.1% and 24.8%, respectively. SRH status was reported to be better among: (i) men; (ii) younger people; (iii) people with higher education; (iv) people who were currently in marital a partnership; (v) those from wealthier households; and (vi) those who were living in riverside/island or highland areas compared to those of other categories of the same variable. Conclusion: The findings reveal that there exist problems of inequality in health among older adults in the study setting by sex, age, education, wealth status and place of residence. We also found a considerable contribution of the household-level factors to SRH of the study populations.. [source] Predicting population consequences of ocean climate change for an ecosystem sentinel, the seabird Cassin's aukletGLOBAL CHANGE BIOLOGY, Issue 7 2010SHAYE G. WOLF Abstract Forecasting the ecological effects of climate change on marine species is critical for informing greenhouse gas mitigation targets and developing marine conservation strategies that remain effective and increase species' resilience under changing climate conditions. Highly productive coastal upwelling systems are predicted to experience substantial effects from climate change, making them priorities for ecological forecasting. We used a population modeling approach to examine the consequences of ocean climate change in the California Current upwelling ecosystem on the population growth rate of the planktivorous seabird Cassin's auklet (Ptychoramphus aleuticus), a demographically sensitive indicator of marine climate change. We use future climate projections for sea surface temperature and upwelling intensity from a regional climate model to forecast changes in the population growth rate of the auklet population at the important Farallon Island colony in central California. Our study projected that the auklet population growth rate will experience an absolute decline of 11,45% by the end of the century, placing this population on a trajectory toward extinction. In addition, future changes in upwelling intensity and timing of peak upwelling are likely to vary across auklet foraging regions in the California Current Ecosystem (CCE), producing a mosaic of climate conditions and ecological impacts across the auklet range. Overall, the Farallon Island Cassin's auklet population has been declining during recent decades, and ocean climate change in this century under a mid-level emissions scenario is projected to accelerate this decline, leading toward population extinction. Because our study species has proven to be a sensitive indicator of oceanographic conditions in the CCE and a powerful predictor of the abundance of other important predators (i.e. salmon), the significant impacts we predicted for the Cassin's auklet provide insights into the consequences that ocean climate change may have for other plankton predators in this system. [source] Analysis of a Japanese Black Cattle-rearing system utilizing a bahiagrass (Paspalum notatum Flügge) pasture: 3.GRASSLAND SCIENCE, Issue 3 2006Intake from pasture Abstract A Japanese Black Cattle-rearing system utilizing a bahiagrass (Paspalum notatum Flügge) pasture in coastal Miyazaki, southern Japan, was analyzed in terms of intake from pasture. In a field approach, herbage intake by grazing cattle was measured for nine periods (1,5 days) during a grazing season (May,October) along with some other variables (e.g. air temperature, herbage mass, digestibility of grazed herbage), under varying levels of supplementary feeding. The intake from the pasture was most closely related to the intake of supplement, showing a negative linear response at a substitution rate of 0.736,0.750. The intake under no supplementation, i.e. a maximum intake from the pasture, was lower than the voluntary intake predicted with feeding standards. In a modeling approach, a semi-mechanistic model for predicting grazing intake was developed using information from the literature as well as the field data. The performance of the model was acceptable. The model showed similar substitution rates (0.64,0.69), and considerable intake restriction (nearly 30%) that is not attributable to limitations by herbage mass, herbage allowance, diet digestibility or air temperature. The results indicate that a low maximal intake by the animals due to low grazing motivation is a major characteristic of the system where supplementation is a usual management practice. [source] Inverse Modeling Approach to Allogenic Karst System CharacterizationGROUND WATER, Issue 3 2009N. Dörfliger Allogenic karst systems function in a particular way that is influenced by the type of water infiltrating through river water losses, by karstification processes, and by water quality. Management of this system requires a good knowledge of its structure and functioning, for which a new methodology based on an inverse modeling approach appears to be well suited. This approach requires both spring and river inflow discharge measurements and a continuous record of chemical parameters in the river and at the spring. The inverse model calculates unit hydrographs and the impulse responses of fluxes from rainfall hydraulic head at the spring or rainfall flux data, the purpose of which is hydrograph separation. Hydrograph reconstruction is done using rainfall and river inflow data as model input and enables definition at each time step of the ratio of each component. Using chemical data, representing event and pre-event water, as input, it is possible to determine the origin of spring water (either fast flow through the epikarstic zone or slow flow through the saturated zone). This study made it possible to improve a conceptual model of allogenic karst system functioning. The methodology is used to study the Bas-Agly and the Cent Font karst systems, two allogenic karst systems in Southern France. [source] Effect of Heterogeneity on Radionuclide Retardation in the Alluvial Aquifer Near Yucca Mountain, NevadaGROUND WATER, Issue 3 2001S. Painter The U.S. Department of Energy is currently studying Yucca Mountain, Nevada, as a potential site for a geological high-level waste repository. In the current conceptual models of radionuclide transport at Yucca Mountain, part of the transport path to pumping locations would be through an alluvial aquifer. Interactions with minerals in the alluvium are expected to retard the downstream migration of radionuclides, thereby delaying arrival times and reducing ground water concentrations. We evaluate the effectiveness of the alluvial aquifer as a transport barrier using the stochastic Lagrangian framework. A transport model is developed to account for physical and chemical heterogeneities and rate-limited mass transfer between mobile and immobile zones. The latter process is caused by small-scale heterogeneity and is thought to control the macroscopic-scale retardation in some field experiments. A geostatistical model for the spatially varying sorption parameters is developed from a site-specific database created from hydrochemical measurements and a calibrated modeling approach (Turner and Pabalan 1999). Transport of neptunium is considered as an example. The results are sensitive to the rate of transfer between mobile and immobile zones, and to spatial variability in the hydraulic conductivity. Chemical heterogeneity has only a small effect, as does correlation between hydraulic conductivity and the neptunium distribution coefficient. These results illustrate how general sensitivities can be explored with modest effort within the Lagrangian framework. Such studies complement and guide the application of more detailed numerical simulations. [source] Multilevel Analysis of the Chronic Care Model and 5A Services for Treating Tobacco Use in Urban Primary Care ClinicsHEALTH SERVICES RESEARCH, Issue 1 2009Dorothy Y. Hung Objective. To examine the chronic care model (CCM) as a framework for improving provider delivery of 5A tobacco cessation services. Methods. Cross-sectional surveys were used to obtain data from 497 health care providers in 60 primary care clinics serving low-income patients in New York City. A hierarchical generalized linear modeling approach to ordinal regression was used to estimate the probability of full 5A service delivery, adjusting for provider covariates and clustering effects. We examined associations between provider delivery of 5A services, clinic implementation of CCM elements tailored for treating tobacco use, and the degree of CCM integration in clinics. Principal Findings. Providers practicing in clinics with enhanced delivery system design, clinical information systems, and self-management support for cessation were 2.04,5.62 times more likely to perform all 5A services ( p<.05). CCM integration in clinics was also positively associated with 5As delivery. Compared with none, implementation of one to six CCM elements corresponded with a 3.69,30.9 increased odds of providers delivering the full spectrum of 5As ( p<.01). Conclusions. Findings suggest that the CCM facilitates provider adherence to the Public Health Service 5A clinical guideline. Achieving the full benefits of systems change may require synergistic adoption of all model components. [source] Hierarchical model of the population dynamics of hippocampal dentate granule cellsHIPPOCAMPUS, Issue 5 2002G.A. Chauvet Abstract A hierarchical modeling approach is used as the basis for a mathematical representation of the population activity of hippocampal dentate granule cells. Using neural field equations, the variation in time and space of dentate granule cell activity is derived from the summed synaptic potential and summed action potential responses of a population of granule cells evoked by monosynaptic excitatory input from entorhinal cortical afferents. In this formulation of the problem, we have considered a two-level hierarchy: the synapses of entorhinal cortical axons define the first level of organization, and dentate granule cells, which include these synapses, define the second, higher level of organization. The model is specified by two state field variables, for membrane potential and for synaptic efficacy, respectively, with both evolving according to different time scales. The two state field variables introduce new parameters, physiological and anatomical, which characterize the dentate from the point of view of neuronal and synaptic populations: (1) a set of geometrical constraints corresponding to the morphological properties of granule cells and anatomical characteristics of entorhinal-dentate connections; and (2) a set of neuronal parameters corresponding to physiological mechanisms. Assuming no interaction between granule cells, i.e., neither ephaptic nor synaptic coupling, the model is shown to be mathematically tractable and allows solution of the field equations leading to the determination of activity. This treatment leads to the definition of two state variables, volume of stimulated synapses and firing time, which describe observed activity. Numerical simulations are used to investigate the populational characterization of the dentate by individual parameters: (1) the relationship between the conditions of stimulation of active perforant path fibers, e.g., stimulating intensity, and activity in the granule cell layer; and (2) the influence of geometry on the generation of activity, i.e., the influence of neuron density and synaptic density-connectivity. As an example application of the model, the granule cell population spike is reconstructed and compared with experimental data. Hippocampus 2002;12:698,712. © 2002 Wiley-Liss, Inc. [source] An engineering approach to dynamic prediction of network performance from application logsINTERNATIONAL JOURNAL OF NETWORK MANAGEMENT, Issue 3 2005Zalal Uddin Mohammad Abusina Network measurement traces contain information regarding network behavior over the period of observation. Research carried out from different contexts shows predictions of network behavior can be made depending on network past history. Existing works on network performance prediction use a complicated stochastic modeling approach that extrapolates past data to yield a rough estimate of long-term future network performance. However, prediction of network performance in the immediate future is still an unresolved problem. In this paper, we address network performance prediction as an engineering problem. The main contribution of this paper is to predict network performance dynamically for the immediate future. Our proposal also considers the practical implication of prediction. Therefore, instead of following the conventional approach to predict one single value, we predict a range within which network performance may lie. This range is bounded by our two newly proposed indices, namely, Optimistic Network Performance Index (ONPI) and Robust Network Performance Index (RNPI). Experiments carried out using one-year-long traffic traces between several pairs of real-life networks validate the usefulness of our model.,Copyright © 2005 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||||||||||||||||||||