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Used Models (used + models)
Selected AbstractsBounds on Parameters in Panel Dynamic Discrete Choice ModelsECONOMETRICA, Issue 3 2006Bo E. Honoré Identification of dynamic nonlinear panel data models is an important and delicate problem in econometrics. In this paper we provide insights that shed light on the identification of parameters of some commonly used models. Using these insights, we are able to show through simple calculations that point identification often fails in these models. On the other hand, these calculations also suggest that the model restricts the parameter to lie in a region that is very small in many cases, and the failure of point identification may, therefore, be of little practical importance in those cases. Although the emphasis is on identification, our techniques are constructive in that they can easily form the basis for consistent estimates of the identified sets. [source] Cable model with frequency dependent parameters calculated by using subconductorsEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 1 2003L. Hofmann A cable model with frequency dependent parameters in the time domain for the simulation of transients in the short time range is presented. The model is based on the division of the transmission line into m short lossless delay line segments of length ,xk = l/m, which are connected to each other by damping-networks consisting of constant ohmic resistances and by distortion-networks simulating the frequency dependence of the series impedance per unit length. The damping-networks consider ohmic line losses as well as leakage losses by shunt conductances. The distortion-networks are represented by state space equations in the time domain. One advantage of the model in comparison to generally used models is the direct determination of the model parameters from the geometrical dimensions of the line-ground configuration. This model also provides a fast and accurate algorithm. [source] Multifunctional host defense peptides: antiparasitic activitiesFEBS JOURNAL, Issue 22 2009Amram Mor Whereas significant knowledge is accumulating on the antibacterial and antifungal properties of host defense peptides (HDPs) and their synthetic mimics, much less is known of their activities against parasites. A variety of in vitro and in vivo antiparasitic assays suggest that these notorious antimicrobial compounds could represent a powerful tool for the development of novel drugs to fight parasites in the vertebrate host or to complement current therapeutic strategies, albeit the fact that HDPs essentially act by nonspecific mechanisms casts serious doubt on their ability to exert sufficient selectivity to be considered ideal candidates for drug development. This minireview summarizes recent efforts to assess the antiparasitic properties of HDPs and their synthetic derivatives, focusing on two of the most used models ,Plasmodium and Leishmania species , for antiparasitic assays against the different development stages. [source] Simulating the response of a closed-basin lake to recent climate changes in tropical West Africa (Lake Bosumtwi, Ghana)HYDROLOGICAL PROCESSES, Issue 13 2007Timothy M. Shanahan Abstract Historical changes in the level of Lake Bosumtwi, Ghana, have been simulated using a catchment-scale hydrological model in order to assess the importance of changes in climate and land use on lake water balance on a monthly basis for the period 1939,2004. Several commonly used models for computing evaporation in data-sparse regions are compared, including the Penman, the energy budget, and the Priestley,Taylor methods. Based on a comparison with recorded lake level variations, the model with the energy-budget evaporation model subcomponent is most effective at reproducing observed lake level variations using regional climate records. A sensitivity analysis using this model indicates that Lake Bosumtwi is highly sensitive to changes in precipitation, cloudiness and temperature. However, the model is also sensitive to changes in runoff related to vegetation, and this factor needs to be considered in simulating lake level variations. Both interannual and longer-term changes in lake level over the last 65 years appear to have been caused primarily by changes in precipitation, though the model also suggests that the drop in lake level over the last few decades has been moderated by changes in cloudiness and temperature over that time. Based on its effectiveness at simulating the magnitude and rate of lake level response to changing climate over the historical record, this model offers a potential future opportunity to examine the palaeoclimatic factors causing past lake level fluctuations preserved in the geological record at Lake Bosumtwi. Copyright © 2006 John Wiley & Sons, Ltd. [source] Comparative analysis of colonic gene expression of three experimental colitis models mimicking inflammatory bowel diseaseINFLAMMATORY BOWEL DISEASES, Issue 3 2007Anje A. te Velde PhD Abstract Background: Mouse models of inflammatory bowel diseases (IBD) are used to unravel the pathophysiology of IBD and to study new treatment modalities, but their relationship to Crohn's disease (CD) or ulcerative colitis (UC) is speculative. Methods: Using Agilent mouse TOX oligonucleotide microarrays, we analyzed colonic gene expression profiles in three widely used models of experimental colitis. In 2 of the models (TNBS and DSS-induced colitis), exogenous agents induce the colitis. In the third model the colitis is induced after transfer of a T-cell population (CD4+CD45RBhigh T cells) that lacks regulatory cells into an immunodeficient host. Results: Compared with control mice, in DSS, TNBS, and the CD45RB transfer colitis mice, 387, 21, and 582 genes were more than 2-fold upregulated in the intestinal mucosa. Analyses of exclusively shared gene expression profiles between the different models revealed that DSS/transfer colitis share 69 concordantly upregulated genes, DSS/TNBS 6, and TNBS/transfer colitis 1. Seven genes were upregulated in all three models. The CD45RB transfer model expression profile included the most genes that are known to be upregulated in IBD. Of 32 genes that are known to change transcriptional activity in IBD (TNF, IFN -,, Lt,, IL - 6, IL - 16, IL - 18R1, IL - 22, CCR2, 7, CCL2, 3, 4, 5, 7, 11, 17, 20, CXCR3, CXCL1, 5, 10, Mmp3, 7,9, 14, Timp1, Reg3,, and Pap, S - 100a8, S - 100a9, Abcb1, and Ptgs2), 2/32 are upregulated in TNBS, 15/32 are upregulated or downregulated in DSS and 30/32 are upregulated or downregulated in the CD45RB transfer colitis. Conclusion: The pattern of gene expression in the CD45RB transfer model most closely reflects altered gene expression in IBD. (Inflamm Bowel Dis 2007) [source] A model for abrupt double heterojunction bipolar transistorsINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 1 2004Antonio J. García-Loureiro Abstract In this paper, we present a model for double heterojunction bipolar transistors (DHBTs) that takes into account Fermi,Dirac statistics as well as an arbitrary injection level. The most commonly used models in the literature for heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and PN junctions can be easily obtained as a particular case of the general model presented here. In order to illustrate its features, the model is applied to an InP/GaAsSb/InP DHBT and an InP/InGaAs HBT. Copyright © 2004 John Wiley & Sons, Ltd. [source] Modelling the long-term sustainability of indigenous hunting in Manu National Park, Peru: landscape-scale management implications for AmazoniaJOURNAL OF APPLIED ECOLOGY, Issue 4 2009Taal Levi Summary 1. ,Widespread hunting throughout Amazonia threatens the persistence of large primates and other vertebrates. Most studies have used models of limited validity and restricted spatial and temporal scales to assess the sustainability. 2. ,We use human-demographic, game-harvest and game-census data to parameterize a spatially explicit hunting model. We explore how population growth and spread, hunting technology and effort, and source,sink dynamics impact the density of black spider monkeys Ateles chamek over time and space in the rainforests of south-eastern Peru. 3. ,In all scenarios, spider monkey populations, which are vulnerable to hunting, persist in high numbers in much of Manu National Park over the next 50 years. Nonetheless, shotguns cause much more depletion than traditional bow hunting by Matsigenka (Machiguenga) indigenous people. 4. ,Maintenance of the current indigenous lifestyle (dispersed settlements, bow hunting) is unlikely to deplete spider monkeys and, by extension, other fauna, despite rapid human population growth. This helps explain why large, pre-Colombian human populations did not drive large primates to extinction. When guns are used, however, spider monkeys quickly become depleted around even small settlements, with depletion eventually reversing the short-term harvest advantage provided by shotgun hunting. Thus, our models show that when guns are used, limits on settlement numbers can reduce total depletion. 5. ,Synthesis and applications. Our framework lets us visualize the future effects of hunting, population growth, hunting technology and settlement spread in tropical forests. In Manu Park, the continued prohibition of firearms is important for ensuring long-term hunting sustainability. A complementary policy is to negotiate limits on new settlements in return for development aid in existing settlements. The advantage of the latter approach is that settlement numbers are more easily monitored than is hunting effort or technology. Similar policies could help to reduce landscape-scale depletion of prey species in human-occupied reserves and protected areas throughout the Amazon. [source] The continuous smooth hockey stick: a newly proposed spawner-recruitment modelJOURNAL OF APPLIED ICHTHYOLOGY, Issue 6 2008R. Froese Summary Spawner-recruit relationships are important components of fisheries management. The two most widely used models have been criticized for unsatisfactory fits and biologically unreasonable extrapolations. A simple hockey stick model has been shown to provide more robust predictions, however, this model is not widely used, possibly because the abrupt change from density-dependence to density-independence is unrealistic and the piecewise model is difficult to fit. Here I present a continuous two-parameter model that resembles a smoothed hockey stick and provides parameter estimates similar to the piecewise hockey stick. The new model is easily parameterized with regular curve-fitting routines. [source] Utilizing endocrine secretory pathways in salivary glands for systemic gene therapeutics,JOURNAL OF CELLULAR PHYSIOLOGY, Issue 1 2004Antonis Voutetakis Mammalian salivary glands are commonly used models of exocrine secretion. However, there is substantial experimental evidence showing the physiological existence of endocrine secretory pathways in these tissues. The use of gene transfer technology in vivo has allowed the unambiguous demonstration of these endocrine pathways. We and others have exploited such findings and evaluated salivary glands as possible target tissues for systemic applications of gene therapeutics. Salivary glands present numerous advantages for this purpose, including being well encapsulated, which limits extra-glandular vector dissemination, and having the luminal membranes of almost all parenchymal cells accessible via intraoral delivery of vectors through the main excretory ducts. Existing studies suggest that clinical benefits will result from salivary gland targeted systemic gene therapeutics. J. Cell. Physiol. 199: 1,7, 2004. Published 2003 Wiley-Liss, Inc. [source] A comparison of gut evacuation models for larval mackerel (Scomber scombrus) using serial photographyJOURNAL OF FISH BIOLOGY, Issue 4 2009R. Dunbrack A novel technique is described, using serial photography of the gut contents of transparent living larval fishes, to generate individual gut evacuation time series. This technique was applied to Atlantic mackerel Scomber scombrus larvae to compare three widely used models of gut evacuation: linear, exponential and square-root. Regression r2 for the exponential model exceeded those for the linear and square root models in 20 of 21 time series, strongly supporting the exponential model. At the initial gut fullness for each time series, total gut evacuation rates calculated with the exponential model averaged 2·2 and 1·3 times greater than those calculated with the linear and square-root models, respectively, and would produce correspondingly higher estimates of feeding rates for field-collected larvae with similar levels of gut fullness. The results highlight the importance of choosing the appropriate evacuation model in feeding studies, particularly those intended to examine short-term changes in larval fish feeding rates, a contributing factor to the highly variable yearly recruitment of many marine fish species. [source] Inhibition of localized thrombosis in P2Y1 -deficient mice and rodents treated with MRS2179, a P2Y1 receptor antagonistJOURNAL OF THROMBOSIS AND HAEMOSTASIS, Issue 6 2003N. Lenain Summary., Previous studies in experimental models revealed a role for the P2Y1 platelet ADP receptor in systemic vascular thromboembolism models. In the present work, we used models of localized arterial and venous thrombosis to assess the role of the P2Y1 receptor in these processes. Arterial thrombosis was induced in one mesenteric arteriole of a mouse using FeCl3, while venous thrombosis was studied in a Wessler model adapted to rats. P2Y1 -deficient mice and mice treated with the P2Y1 antagonist MRS2179 displayed significantly less arterial thrombosis than their respective controls. Combination of P2Y1 deficiency with P2Y12 inhibition led to a significant additive effect. Venous thrombosis was slightly but significantly inhibited in MRS2179-treated rats. These results demonstrate a role for the P2Y1 receptor in both arterial and venous thrombosis, further establishing this receptor as a potential target for antithrombotic drugs. [source] A brief overview of theories of PVC plasticization and methods used to evaluate PVC-plasticizer interaction,JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 4 2009Paul H. Daniels This paper reviews the most widely used models for explaining how plasticizers render PVC flexible. These models include the gel, lubricity, and free volume theories; kinetic theories; and mathematical models which predict on the basis of plasticizer structure how much a plasticizer will lower the polymer glass transition in a flexible PVC compound. Since plasticization results from interactions between plasticizer and polymer, methods which have been used to study either the strength or the permanence (or both) of those interactions are also briefly discussed. Tools which have often been used to study plasticizer-PVC interactions include infrared and nuclear magnetic resonance spectroscopy, compression and humid-aging tests, dynamic mechanical analysis, torque rheometer tests, plasticizer-resin clear point temperature measurements, plastisol gelation/fusion by hot stage measurements, and others. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers [source] Effects of molecular crowding by saccharides on ,-chymotrypsin dimerizationPROTEIN SCIENCE, Issue 5 2002Chetan N. Patel Abstract Given the importance of protein complexes as therapeutic targets, it is necessary to understand the physical chemistry of these interactions under the crowded conditions that exist in cells. We have used sedimentation equilibrium to quantify the enhancement of the reversible homodimerization of ,-chymotrypsin by high concentrations of the osmolytes glucose, sucrose, and raffinose. In an attempt to rationalize the osmolyte-mediated stabilization of the ,-chymotrypsin homodimer, we have used models based on binding interactions (transfer-free energy analysis) and steric interactions (excluded volume theory) to predict the stabilization. Although transfer-free energy analysis predicts reasonably well the relatively small stabilization observed for complex formation between cytochrome c and cytochrome c peroxidase, as well as that between bobtail quail lysozyme and a monoclonal Fab fragment, it underestimates the sugar-mediated stabilization of the ,-chymotrypsin dimer. Although predictions based on excluded volume theory overestimate the stabilization, it would seem that a major determinant in the observed stabilization of the ,-chymotrypsin homodimer is the thermodynamic nonideality arising from molecular crowding by the three small sugars. [source] Das Magnitude 8.8 Maule (Chile)-Erdbeben vom 27.BAUTECHNIK, Issue 8 2010Februar 2010, Ingenieuranalyse der Erdbebenschäden Das Maule (Chile)-Erdbeben vom 27. Februar 2010 gehört zu den stärksten, weltweit jemals registrierten Erdbeben. Die Bruchzone erstreckt sich über eine Länge von 500 km und eine Breite von 100 km, so dass acht Millionen Einwohner Chiles von dem Erdbeben mehr oder weniger direkt von den Schütterwirkungen betroffen waren. Bilder von spektakulären Schadensfällen aus der ca. 330 km entfernten Hauptstadt prägten die internationale Berichterstattung. Das seismische Ereignis löste einen Tsunami aus, der verheerende Schäden an der Küste Chiles verursachte und auch an den Küsten Hawaiis noch deutlich wahrgenommen werden konnte. Die seismischen Bodenbewegungen wurden bis ins Nachbarland Argentinien verspürt. Die Stärke des Bebens und ereignisspezifische Besonderheiten waren Motivation, im Rahmen einer Erkundungsmission der Ingenieurgruppe der Deutschen TaskForce Erdbeben im betroffenen Gebiet die Bauwerksschäden aufzunehmen und ihre regionale Verteilung zu dokumentieren. In fünf temporär, mit Starkbeben-Sensoren instrumentierten Gebäuden konnten mehrere Nachbeben aufgezeichnet werden, deren Beschleunigungsamplituden für allgemeine Hochbauten in deutschen Erdbebengebieten von Interesse bzw. maßgeblich wären. Die vorliegenden Messdaten ermöglichen die Interpretation der realen Gebäudereaktion und können in Folgeuntersuchungen zur Kalibrierung analytischer Modelle herangezogen werden. Der Beitrag vermittelt einen Eindruck von den erdbebenbedingten Schäden und soll das Verhalten der typischen Bauweisen unter diesen extremen Einwirkungen aufzeigen, das vor Ort festgestellte geringe Schadensausmaß durch die Umsetzung von Baunormen und darin verankerten Bemessungskonzepten erklären und letztlich die Übertragbarkeit dieser Beobachtungen auf andere Erdbebenregionen hinterfragen. In einem Folgebeitrag [1] werden die Schäden aus dem Tsunami einer Ingenieuranalyse unterzogen und die wesentlichen Wirkungsmechanismen bzw. einfachen baulichen Schutzmaßnahmen herausgearbeitet. The Magnitude 8.8 Maule (Chile) Earthquake of February 27, 2010 , Engineering analysis of earthquake damage. The Maule (Chile) February 27, 2010 Earthquake is regarded as one of the strongest earthquakes ever recorded world-wide. The rupture zone reached a length of about 500 km and a width of about 100 km; almost 8 million inhabitants were directly affected by the consequences of the earthquake. Photos from spectacular failure cases in the Capital (330 km away) were documented across the world and dominated the international reporting. The seismic event triggered a tsunami which caused serious damage alongside the coastal border; the waves were observed in the far-distant Hawaii Islands, too. The seismic ground motions were felt in the neighboring country Argentina, as well. The strength of the earthquake and the event-specific characteristics motivated the "Engineering Group of the German Task Force for earthquake" to analyze the building damage and to document their regional distribution. Five multi-storey RC structures were temporarily equipped with Strong-Motion sensors. Several aftershocks could be recorded; the peak acceleration amplitudes were in a level which was of interest for buildings in highest zone of German earthquake regions. In a first attempt, the measurements are used to interpret the response of real buildings in both horizontal directions; in ongoing studies data are used for the calibration of analytical models. The paper provides an overview of the earthquake induced damages in several building types and its variation within different structural systems. Reasons of low to moderate observed damage will be discussed in close relation to the code development and the preferred design concepts. The application of observed effects and derived lessons to other seismic regions is critically reviewed. In a subsequent paper [1] the damage caused by the tsunami is investigated in more detail. The engineering analysis will include the currently used models for the impact description and will elaborate simple, but quite efficient measures of protection. [source] Using Regression Models to Analyze Randomized Trials: Asymptotically Valid Hypothesis Tests Despite Incorrectly Specified ModelsBIOMETRICS, Issue 3 2009Michael Rosenblum Summary Regression models are often used to test for cause-effect relationships from data collected in randomized trials or experiments. This practice has deservedly come under heavy scrutiny, because commonly used models such as linear and logistic regression will often not capture the actual relationships between variables, and incorrectly specified models potentially lead to incorrect conclusions. In this article, we focus on hypothesis tests of whether the treatment given in a randomized trial has any effect on the mean of the primary outcome, within strata of baseline variables such as age, sex, and health status. Our primary concern is ensuring that such hypothesis tests have correct type I error for large samples. Our main result is that for a surprisingly large class of commonly used regression models, standard regression-based hypothesis tests (but using robust variance estimators) are guaranteed to have correct type I error for large samples, even when the models are incorrectly specified. To the best of our knowledge, this robustness of such model-based hypothesis tests to incorrectly specified models was previously unknown for Poisson regression models and for other commonly used models we consider. Our results have practical implications for understanding the reliability of commonly used, model-based tests for analyzing randomized trials. [source] Disease Mapping of Stage-Specific Cancer Incidence DataBIOMETRICS, Issue 3 2002Leonhard Knorr-Held Summary. We propose two approaches for the spatial analysis of cancer incidence data with additional information on the stage of the disease at time of diagnosis. The two formulations are extensions of commonly used models for multicategorical response data on an ordinal scale. We include spatial and age-group effects in both formulations, which we estimate in a nonparametric smooth way. More specifically, we adopt a fully Bayesian approach based on Gaussian pairwise difference priors where additional smoothing parameters are treated as unknown as well. We argue that the methods are useful in monitoring the effectiveness of mass cancer screening and illustrate this through an application to data on cervical cancer in the former German Democratic Republic. The results suggest that there are large spatial differences in the stage proportions, which indicate spatial variability with respect to the introduction and effectiveness of Pap smear screening programs. [source] Allometric Models for Tree Volume and Total Aboveground Biomass in a Tropical Humid Forest in Costa Rica1BIOTROPICA, Issue 1 2005Milena Segura ABSTRACT Allometric equations for the estimation of tree volume and aboveground biomass in a tropical humid forest were developed based on direct measurements of 19 individuals of seven tree species in Northern Costa Rica. The volume and the biomass of the stems represented about two-thirds of the total volume and total aboveground biomass, respectively. The average stem volume varied between 4 and 11 Mg/tree and the average total aboveground biomass ranged from 4 to 10 mg/tree. The mean specific gravity of the sampled trees was 0.62 ± 0.06 (g/cm3). The average biomass expansion factor was 1.6 ± 0.2. The best-fit equations for stem and total volume were of logarithmic form, with diameter at breast height (R2= 0.66 , 0.81) as an independent variable. The best-fit equations for total aboveground biomass that were based on combinations of diameter at breast height, and total and commercial height as independent variables had R2 values between 0.77 and 0.87. Models recommended for estimating total aboveground biomass are based on diameter at breast height, because the simplicity of these models is advantageous. This variable is easy to measure accurately in the field and is the most common variable recorded in forest inventories. Two widely used models in literature tend to underestimate aboveground biomass in large trees. In contrast, the models developed in this study accurately estimate the total aboveground biomass in these trees. RESUMEN Ecuaciones alométricas para la estimación de volumen y biomasa aérea de árboles en un bosque húmedo tropical fueron desarrollados basados en mediciones directas de 19 individuos de siete especies de árboles al norte de Costa Rica. El volumen y la biomasa del tronco representaron cerca de dos terceras partes del volumen total del árbol y de la biomasa aérea total. El volumen promedio del tronco varió entre 4 y 11 Mg/árbol y el promedio de la biomasa aérea total varió entre 4 y 10 mg/árbol. La gravedad específica promedio de los árboles muestreados fué de 0.62 ± 0.06 (g/cm3). El factor de expansión de biomasa promedio fué de 1.6 ± 0.2. Las ecuaciones de mejor ajuste para el volumen de tallo y total fueron de tipo logarítmico, con el diámetro a la altura de pecho (R2= 0.66 a 0.81) como variable independiente. Las ecuaciones de mejor ajuste para biomasa aérea total, las cuales fueron basadas en combinaciones de diámetro a la altura de pecho y altura total y comercial como variables independientes, presentaron valores de R2 entre 0.77 y 0.87. Los modelos recomendados para estimar biomasa aérea total están basados en diámetro a la altura de pecho, porque la simplicidad de estos modelos es ventajosa. Esta variable es de fácil medición en el campo y tiene mayor precision, además, es la más comúnmente registrada en inventarios forestales. Dos modelos ampliamente usados en la literature tienden a subestimar la biomasa aérea total en árboles grandes. En contraste, los modelos desarrollados en este estudio, estiman con mayor precisión la biomasa aérea total de estos árboles. [source] | ||||||||||||||||||||||