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Poisson
Kinds of Poisson Terms modified by Poisson Selected AbstractsPOISSON VERSUS BINOMIAL: APPOINTMENT OF JUDGES TO THE U.S. SUPREME COURTAUSTRALIAN & NEW ZEALAND JOURNAL OF STATISTICS, Issue 3 2010Vassilly Voinov Summary The problem of discriminating between the Poisson and binomial models is discussed in the context of a detailed statistical analysis of the number of appointments of the U.S. Supreme Court justices from 1789 to 2004. Various new and existing tests are examined. The analysis shows that both simple Poisson and simple binomial models are equally appropriate for describing the data. No firm statistical evidence in favour of an exponential Poisson regression model was found. Two attendant results were obtained by simulation: firstly, that the likelihood ratio test is the most powerful of those considered when testing for the Poisson versus binomial and, secondly, that the classical variance test with an upper-tail critical region is biased. [source] ROMANTIC PARTNERS' INFLUENCE ON MEN'S LIKELIHOOD OF ARREST IN EARLY ADULTHOOD,CRIMINOLOGY, Issue 2 2008DEBORAH M. CAPALDI Female romantic partners' influence on official crime occurrence for men across a 12-year period in early adulthood was examined within a comprehensive dynamic prediction model, including both social learning and social control predictors. We hypothesized that relationship stability, rather than attachment to partner, would be associated with reduced likelihood of crime, whereas women's antisocial behavior would be a risk factor, along with deviant peer association. Models were tested on a sample of at-risk men [the Oregon Youth Study (OYS)] using zero-inflated Poisson (ZIP) modeling predicting 1) arrest persistence (class and count) and 2) arrest onset class. The findings indicated that women's antisocial behavior was predictive of both onset and persistence of arrests for men and that deviant peer association was predictive of persistence. Relationship stability was protective against persistence. [source] A probabilistic framework for quantification of aftershock ground-motion hazard in California: Methodology and parametric studyEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 1 2009Gee Liek Yeo Abstract This paper presents a proposed method of aftershock probabilistic seismic hazard analysis (APSHA) similar to conventional ,mainshock' PSHA in that it estimates the likelihoods of ground motion intensity (in terms of peak ground accelerations, spectral accelerations or other ground motion intensity measures) due to aftershocks following a mainshock occurrence. This proposed methodology differs from the conventional mainshock PSHA in that mainshock occurrence rates remain constant for a conventional (homogeneous Poisson) earthquake occurrence model, whereas aftershock occurrence rates decrease with increased elapsed time from the initial occurrence of the mainshock. In addition, the aftershock ground motion hazard at a site depends on the magnitude and location of the causative mainshock, and the location of aftershocks is limited to an aftershock zone, which is also dependent on the location and magnitude of the initial mainshock. APSHA is useful for post-earthquake safety evaluation where there is a need to quantify the rates of occurrence of ground motions caused by aftershocks following the initial rupture. This knowledge will permit, for example, more informed decisions to be made for building tagging and entry of damaged buildings for rescue, repair or normal occupancy. Copyright © 2008 John Wiley & Sons, Ltd. [source] Nonlinear Smoluchowski velocity for electroosmosis of Power-law fluids over a surface with arbitrary zeta potentialsELECTROPHORESIS, Issue 5 2010Cunlu Zhao Abstract Electroosmotic flow of Power-law fluids over a surface with arbitrary zeta potentials is analyzed. The governing equations including the nonlinear Poisson,Boltzmann equation, the Cauchy momentum equation and the continuity equation are solved to seek exact solutions for the electroosmotic velocity, shear stress, and dynamic viscosity distributions inside the electric double layer. Specifically, an expression for the general Smoluchowski velocity is obtained for electroosmosis of Power-law fluids in a fashion similar to the classic Smoluchowski velocity for Newtonian fluids. The existing Smoluchowski slip velocities under two special cases, (i) for Newtonian fluids with arbitrary zeta potentials and (ii) for Power-law fluids with small zeta potentials, can be recovered from our derived formula. It is interesting to note that the general Smoluchowski velocity for non-Newtonian Power-law fluids is a nonlinear function of the electric field strength and surface zeta potentials; this is due to the coupling electrostatics and non-Newtonian fluid behavior, which is different from its counterpart for Newtonian fluids. This general Smoluchowski velocity is of practical significance in determining the flow rates in microfluidic devices involving non-Newtonian Power-law fluids. [source] EOF using the Ritz method: Application to superelliptic microchannelsELECTROPHORESIS, Issue 18 2007Chang Yi Wang Abstract An efficient Ritz method is developed from the variational principle to solve the Poisson,Boltzmann equation under the Debye,Hückel approximation for studying the EOF in microchannels. The method is applied to the family of superelliptic cross sections which includes the elliptic channel and the rectangular channel as limiting cases. Several accurate tables presented are useful for design of electroosmotic channels, especially rectangular channels with rounded corners. It is shown how the flow rate Q is a sophisticated consequence of the nondimensional electrokinetic width K, the aspect ratio b as well as the superelliptic exponent n. [source] Characterization of Hprt mutations in cDNA and genomic DNA of T-cell mutants from control and 1,3-butadiene-exposed male B6C3F1 mice and F344 ratsENVIRONMENTAL AND MOLECULAR MUTAGENESIS, Issue 2 2004Quanxin Meng Abstract A multiplex PCR procedure for analysis of genomic DNA mutations in the mouse hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene was developed and then used with other established methods for the coincident identification of large- and small-scale genetic alterations in the Hprt gene of mutant T-cell isolates propagated from sham- and 1,3-butadiene (BD)-exposed mice and rats. The spectra data for RT-PCR/cDNA analysis and multiplex PCR of genomic DNA from Hprt mutants were combined, and statistical analyses of the mutant fractions for the classes of mutations identified in control versus exposed animals were conducted. Under the assumption that the mutant fractions are distributed as Poisson variates, BD exposure of mice significantly increased the frequencies of (1) nearly all types of base substitutions; (2) single-base deletions and insertions; and (3) all subcategories of deletions. Significantly elevated fractions of G:C,C:G and A:T,T:A transversions in the Hprt gene of BD-exposed mice were consistent with the occurrence of these substitutions as the predominant ras gene mutations in multiple tumor types increased in incidence in carcinogenicity studies of BD in mice. BD exposure of rats produced significant increases in (1) base substitutions only at A:T base pairs; (2) single-base insertions; (3) complex mutations; and (4) deletions (mainly 5, partial and complete gene deletions). Future coincident analyses of large- and small-scale mutations in rodents exposed to specific BD metabolites should help identify species differences in the sources of deletion mutations and other types of mutations induced by BD exposures in mice versus rats. Environ. Mol. Mutagen. 43:75,92, 2004. © 2004 Wiley-Liss, Inc. [source] Sampling from Dirichlet partitions: estimating the number of speciesENVIRONMETRICS, Issue 7 2009Thierry Huillet Abstract The Dirichlet partition of an interval can be viewed as the generalization of several classical models in ecological statistics. We recall the unordered Ewens sampling formulae -ESF) from finite Dirichlet partitions. As this is a key variable for estimation purposes, focus is on the number of distinct visited species in the sampling process. These are illustrated in specific cases. We use these preliminary statistical results on frequencies distribution to address the following sampling problem: what is the estimated number of species when sampling is from Dirichlet populations? The obtained results are in accordance with the ones found in sampling theory from random proportions with Poisson,Dirichlet -PD) distribution. To conclude with, we apply the different estimators suggested to two different sets of real data. Copyright © 2009 John Wiley & Sons, Ltd. [source] INAR(1) modeling of overdispersed count series with an environmental applicationENVIRONMETRICS, Issue 4 2008Harry Pavlopoulos Abstract This paper is concerned with a novel version of the INAR(1) model, a non-linear auto-regressive Markov chain on ,, with innovations following a finite mixture distribution of Poisson laws. For , the stationary marginal probability distribution of the chain is overdispersed relative to a Poisson, thus making INAR(1) suitable for modeling time series of counts with arbitrary overdispersion. The one-step transition probability function of the chain is also a finite mixture, of m Poisson-Binomial laws, facilitating likelihood-based inference for model parameters. An explicit EM-algorithm is devised for inference by maximization of a conditional likelihood. Alternative options for inference are discussed along with criteria for selecting m. Integer-valued prediction (IP) is developed by a parametric bootstrap approach to ,coherent' forecasting, and a certain test statistic based on predictions is introduced for assessing performance of the fitted model. The proposed model is fitted to time series of counts of pixels where spatially averaged rain rate exceeds a given threshold level, illustrating its capabilities in challenging cases of highly overdispersed count data. Copyright © 2007 John Wiley & Sons, Ltd. [source] Space,time zero-inflated count models of Harbor seals,ENVIRONMETRICS, Issue 7 2007Jay M. Ver Hoef Abstract Environmental data are spatial, temporal, and often come with many zeros. In this paper, we included space-time random effects in zero-inflated Poisson (ZIP) and ,hurdle' models to investigate haulout patterns of harbor seals on glacial ice. The data consisted of counts, for 18 dates on a lattice grid of samples, of harbor seals hauled out on glacial ice in Disenchantment Bay, near Yakutat, Alaska. A hurdle model is similar to a ZIP model except it does not mix zeros from the binary and count processes. Both models can be used for zero-inflated data, and we compared space-time ZIP and hurdle models in a Bayesian hierarchical model. Space-time ZIP and hurdle models were constructed by using spatial conditional autoregressive (CAR) models and temporal first-order autoregressive (AR(1)) models as random effects in ZIP and hurdle regression models. We created maps of smoothed predictions for harbor seal counts based on ice density, other covariates, and spatio-temporal random effects. For both models predictions around the edges appeared to be positively biased. The linex loss function is an asymmetric loss function that penalizes overprediction more than underprediction, and we used it to correct for prediction bias to get the best map for space-time ZIP and hurdle models. Published in 2007 by John Wiley & Sons, Ltd. [source] Statistical analysis of temperature impact on daily hospital admissions: analysis of data from Udine, ItalyENVIRONMETRICS, Issue 1 2006Francesco Pauli Abstract This article is devoted to the analysis of the relationship between the health status of an urban population and meteorological variables. The analysis considers daily number of hospital admissions, not due to surgery, regarding the population resident in the Municipality of Udine, aged 75 and over. Hourly records on temperature, humidity, rain, atmospheric pressure, solar radiation, wind velocity and direction recorded at an observation site located near the center of Udine are considered. The study also considers hourly measures of pollutant concentrations collected by six monitoring stations. All data are relative to the summer periods of years 1995,2003. Generalized additive models (GAM) are used in which the response variable is the number of hospital admissions and is assumed to be distributed as a Poisson whose rate varies as a possibly non-linear function of the meteorological variables and variables allowing for calendar effects and pollutant concentrations. The subsequent part of the analysis explores the distribution of temperature conditional on the number of daily admissions through quantile regression. A non-linear (N-shaped) relationship between hospital admissions and temperature is estimated; temperature at 07:00 is selected as a covariate, revealing that nighttime temperature is more relevant than daytime. The quantile regression analysis points out, as expected, that the distribution of temperature on days with more admissions has higher q -quantiles with q near unity, while a clear-cut conclusion is not reached for q quantiles with q near 0. Copyright © 2005 John Wiley & Sons, Ltd. [source] Traffic analysis in optical burst switching networks: a trace-based case studyEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 7 2009Ahmad Rostami Optical burst switching (OBS) appears as a promising technology for building dynamic optical transport networks. The main advantage of OBS is that it allows for dynamic allocation of resources at sub-wavelength granularity. Nevertheless, the burst contention problem, which occurs frequently inside the network, has to be addressed before OBS can be really deployed as the next generation optical transport network. Recently a lot of attention is devoted to different approaches for resolving contentions in OBS networks. Although performance analysis of these approaches is strongly dependent on the traffic characteristics in the network, the majority of the studies is so far based on very hypothetical traffic assumptions. In this study we use traces of real measurements in the Internet to derive realistic data about the traffic that is injected into the OBS network. Specifically, we investigate the marginal distributions of burst size, burst interdeparture time, assembly delay and number of packets per burst as well as the burstiness of the burst traces. We demonstrate that the performance of an OBS core node using the real traces is pretty similar to the results obtained when the traffic arriving to the core node is assumed to be Poisson. In fact, usage of the Poisson as the process of burst arrival to the core node leads in all the investigated cases to an upper bound on the burst drop rate at that node. Copyright © 2009 John Wiley & Sons, Ltd. [source] The Influence of Mass Transfer on a Porous Fuel Cell ElectrodeFUEL CELLS, Issue 1-2 2004Y.-P. Sun Abstract A one-dimensional model for a porous fuel cell electrode using a liquid electrolyte with dissolved reactant is presented. The model consists of a Poisson, second-order ordinary differential equation, describing the effect of the electric field and a one-dimensional; Fickian diffusion, second-order ordinary differential equation describing the concentration variation associated with diffusion. The model accounts for mass transport and heterogeneous electrochemical reaction. The solution of this model is by the approximate Adomian polynomial method and is used to determine lateral distributions of concentration, overpotential and current density and overall cell polarisation. The model is used to simulate the effects of important system and operating parameters, i.e. local diffusion rates, and mass transport coefficients and electrode polarisation behaviour. [source] Beyond Mule Kicks: The Poisson Distribution in Geographical AnalysisGEOGRAPHICAL ANALYSIS, Issue 2 2006Daniel A. Griffith The Poisson model, discovered nearly two centuries ago, is the basis for analyses of rare events. Its first applications included descriptions of deaths from mule kicks. More than half a century ago the Poisson model began being used in geographical analysis. Its initial descriptions of geographic distributions of points, disease maps, and spatial flows were accompanied by an assumption of independence. Today this unrealistic assumption is replaced by one allowing for the presence of spatial autocorrelation in georeferenced counts. Contemporary statistical theory has led to the creation of powerful Poisson-based modeling tools for geographically distributed count data. [source] The rapid spread of invasive Eurasian Collared Doves Streptopelia decaocto in the continental USA follows human-altered habitatsIBIS, Issue 3 2010IKUKO FUJISAKI Understanding factors related to the range expansion trajectory of a successful invasive species may provide insights into environmental variables that favour additional expansion or guide monitoring and survey efforts for this and other invasive species. We examined the relationship of presence and abundance of Eurasian Collared Doves Streptopelia decaocto to environmental factors using recent data from the North American Breeding Bird Survey to understand factors influencing its expansion into the continental USA. A zero-inflated Poisson (ZIP) model was used to account for excess zero observations because this species was not observed on the majority of survey routes, despite its large geographical range. Model fit was improved when we included environmental covariates as compared with the null model, which only included distance from the route where this species was first observed. Probability of zero count was positively related to the distance from the first route and road density and was inversely related to minimum temperature and distance to coast. Abundance of the species was positively related to road density and was inversely related to annual precipitation and distance to coast. Random intercept by land-cover type also improved model fit. Model fit was improved with the ZIP model over the standard Poisson model, suggesting that presence and abundance of this species are characterized by different environmental factors. However, overall low accuracy of model-predicted presence/absence and abundance with the independent validation dataset may indicate either that there are other explanatory factors or that there is great uncertainty in the species' colonization process. Our large-scale study provides additional evidence that the range expansion of this species tends to follow human-altered landscapes such as road and agricultural areas as well as responding to general geographical features such as coastlines or thermal clines. Such patterns may hold true for other invasive species and may provide guidelines for monitoring and assessment activities in other invasive taxa. [source] A three-scale computational model of reactive pollutant transport in smectitic claysINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 8 2006Márcio A. Murad Abstract A three-scale model of dual-porosity type is proposed to describe contaminant transport in swelling clays. The swelling medium is characterized by three separate length scales (nano, micro and macro) and two levels of porosity (micro- and macro-pores). At the nanoscale the medium is composed of charged clay particles saturated by a binary monovalent aqueous electrolyte solution occupying the micro-pores. At the intermediate (micro) scale the two-phase system is represented in a homogenized fashion with averaged microscopic equations governing the behaviour of the clay clusters (or aggregates) regarded a two-phase mixture composed of clay particles and electrolyte solution. At the macroscale, the microscale mixture of clay clusters is homogenized with the bulk solution containing non-electrolyte species lying in the macro-pore system. The resultant macroscopic picture appears governed by a dual-porosity model wherein the clay clusters act as sources/sinks of mass to the macro-pore system. Under a local equilibrium assumption between the clay clusters and macro-pores, a quasi-steady version of the dual-porosity model is derived. This framework combined with a three-scale picture of the colloidal system allows to provide nanoscopic representations for the retardation coefficient, governing the instantaneous adsorption/desorption of the ionic species in the micro-pores, in terms of the local behaviour of the electrical double layer potential which satisfies a Poisson,Boltzmann-type problem at the nanoscale. Copyright © 2006 John Wiley & Sons, Ltd. [source] Tests of radial basis functions in the 3D DRM-MDINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 1 2006Bruno Natalini Abstract Several global as well as compactly supported radial basis functions were tested in the boundary element dual reciprocity method-multi domain approach (DRM-MD) applied to 3D potential problems. Compactly supported radial basis functions (CS-RBFs) showed the highest accuracy, while the augmented thin plate splines (ATPS) showed the highest consistency in terms of accuracy and convergence for the two examples considered, the Poisson and the advection,diffusion equations. A problem for implementation of the CS-RBFs is the lack of guidelines in choosing the size of the support, which showed to have large influence on the accuracy and convergence when these types of RBFs are used. The ATPS showed satisfactory accuracy and since its use does not involve any extra parameters, at the moment it is this function that is the choice of the authors for use in the DRM-MD. Copyright © 2005 John Wiley & Sons, Ltd. [source] A robust a priori error estimate for the Fortin,Soulie finite element methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 7 2005David J. BlackerArticle first published online: 14 MAR 200 Abstract It is well known that conforming finite element schemes exhibit Poisson locking in the incompressible limit as the Poisson ratio , tends to 1/2. A remedy for this is to use a non-conforming method (Math. Comput. 1992; 59:321-328) in which an a priori error bound is proved for the Crouzeix,Raviart scheme. In this paper we derive a new a priori estimate for the error in energy for the Fortin,Soulie finite element method using a method similar to that used in Brenner and Sung. We then illustrate the new error bound by presenting some numerical examples. Copyright © 2005 John Wiley & Sons, Ltd. [source] Computation of a few smallest eigenvalues of elliptic operators using fast elliptic solversINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 8 2001Janne Martikainen Abstract The computation of a few smallest eigenvalues of generalized algebraic eigenvalue problems is studied. The considered problems are obtained by discretizing self-adjoint second-order elliptic partial differential eigenvalue problems in two- or three-dimensional domains. The standard Lanczos algorithm with the complete orthogonalization is used to compute some eigenvalues of the inverted eigenvalue problem. Under suitable assumptions, the number of Lanczos iterations is shown to be independent of the problem size. The arising linear problems are solved using some standard fast elliptic solver. Numerical experiments demonstrate that the inverted problem is much easier to solve with the Lanczos algorithm that the original problem. In these experiments, the underlying Poisson and elasticity problems are solved using a standard multigrid method. Copyright © 2001 John Wiley & Sons, Ltd. [source] On the optimum support size in meshfree methods: A variational adaptivity approach with maximum-entropy approximantsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 7 2010Adrian Rosolen Abstract We present a method for the automatic adaption of the support size of meshfree basis functions in the context of the numerical approximation of boundary value problems stemming from a minimum principle. The method is based on a variational approach, and the central idea is that the variational principle selects both the discretized physical fields and the discretization parameters, here those defining the support size of each basis function. We consider local maximum-entropy approximation schemes, which exhibit smooth basis functions with respect to both space and the discretization parameters (the node location and the locality parameters). We illustrate by the Poisson, linear and non-linear elasticity problems the effectivity of the method, which produces very accurate solutions with very coarse discretizations and finds unexpected patterns of the support size of the shape functions. Copyright © 2009 John Wiley & Sons, Ltd. [source] A two-dimensional stochastic algorithm for the solution of the non-linear Poisson,Boltzmann equation: validation with finite-difference benchmarks,INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 1 2006Kausik Chatterjee Abstract This paper presents a two-dimensional floating random walk (FRW) algorithm for the solution of the non-linear Poisson,Boltzmann (NPB) equation. In the past, the FRW method has not been applied to the solution of the NPB equation which can be attributed to the absence of analytical expressions for volumetric Green's functions. Previous studies using the FRW method have examined only the linearized Poisson,Boltzmann equation. No such linearization is needed for the present approach. Approximate volumetric Green's functions have been derived with the help of perturbation theory, and these expressions have been incorporated within the FRW framework. A unique advantage of this algorithm is that it requires no discretization of either the volume or the surface of the problem domains. Furthermore, each random walk is independent, so that the computational procedure is highly parallelizable. In our previous work, we have presented preliminary calculations for one-dimensional and quasi-one-dimensional benchmark problems. In this paper, we present the detailed formulation of a two-dimensional algorithm, along with extensive finite-difference validation on fully two-dimensional benchmark problems. The solution of the NPB equation has many interesting applications, including the modelling of plasma discharges, semiconductor device modelling and the modelling of biomolecular structures and dynamics. Copyright © 2005 John Wiley & Sons, Ltd. [source] On the application of forking nodes to product-form queueing networks,INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 2 2008Essia H. Elhafsi Abstract We define a ,forking node' as a service centre with one input feeding two outputs (each served by its own queue) under the control of an internal path-selection (PS) policy. We assume that both outputs lead to paths through which a packet reaches its final destination. However, the mean downstream delays on the two paths may be different and the PS policy should favour the path with the lower downstream delay. Using simulation, we compare the performance of this system under a variety of random, deterministic, state-dependent PS policies, including threshold-based and join-shortest-queue with bias (JSQ + b). We show that JSQ + b has better performance than the other alternatives. Moreover, if the input process to the forking node is Poisson, standard time series analysis techniques show that its two outputs are very close to being independent Poisson processes. Thus, if we find an accurate and efficient ,offline' analytical performance model for JSQ + b forking node, we can extend the applicability of product-form queueing networks to include such forking nodes. For this reason, we present several ways of modelling the performance of a JSQ + b node, using bounds, and compare their results on example networks. We establish a closed-form expression relating the bias b and the delays of the downstream paths. Copyright © 2007 John Wiley & Sons, Ltd. [source] Multilevel generalized linear models for modelling age-related gender difference in violent behaviour and associated factors in the general household populationINTERNATIONAL JOURNAL OF METHODS IN PSYCHIATRIC RESEARCH, Issue 3 2005Min Yang Senior Statistician Abstract It is preferable to use longitudinal data when studying patterns of violence and antisocial behaviour over the lifespan together with the associated risk factors in the general population. From the statistical modelling perspective, random samples of cross-sectional data, representative of the population, can be a reliable alternative. Sampling, weighting, and possible geographical clustering of the behaviour must be considered in the analysis together with correct choice of model as a function of age, although cohort effects and age effects are not separated from the analysis. This paper demonstrates the use of multilevel generalized linear models in the British National Survey of Psychiatric Morbidity in 2000. A multilevel logistic model as a special case of a generalized linear model with individual weightings was adapted for a dichotomous measure of violence and extended to Poisson and negative binomial outcomes. Three types of age function, discrete age effects, continuous age effects, and piecewise polynomial function of age intervals were evaluated for goodness of fit, and for their practical advantages and disadvantages. Models were developed for possible risk factors in relation to specific age groups of interest. Copyright © 2005 John Wiley & Sons, Ltd. Copyright © 2005 John Wiley & Sons, Ltd. [source] Hot-electron numerical modelling of short gate length pHEMTs applied to novel field plate structuresINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 1 2003Shahzad Hussain Abstract Hot-electron numerical simulations were carried out in order to simulate the DC parameters of pseudomorphic high electron mobility transistors (pHEMTs). The hot-electron effects were studied by simulating several HEMT device structures. Hot-carrier injection in the substrate and the formation of the peak of electric field in the channel were studied in detail. The inclusion of a field-plate contact in a multiple recessed pHEMT structure lowered the peak value of the electric field by 24% compared with the conventional pHEMT. These devices were modelled by solving the two-dimensional Poisson, current continuity and energy transport equations consistently with the time-independent Schrödinger wave equation. Appropriate Ohmic boundaries are discussed here and implemented in the simulations of pHEMT structures. A new integral approximation is used to calculate electron densities and electron energy densities for degenerate approximations. Copyright © 2002 John Wiley & Sons, Ltd. [source] Diverting with Benevolent Military Force: Reducing Risks and Rising above Strategic BehaviorINTERNATIONAL STUDIES QUARTERLY, Issue 2 2007EMIZET F. KISANGANI Research on the diversionary use of force has burgeoned in recent years, but the literature remains divided. This paper attempts to reconcile extant findings by advancing a new theoretical framework for diversionary force centered on the agenda-setting literature. It expands the conventional conception of diversionary behavior and distinguishes the benevolent use of force over low politics issues (which we term socioeconomic interventions, SEI) from belligerent force used over high politics issues (which we term politico-strategic interventions, PSI). This expansion also refines our understanding of strategic conflict avoidance (SCA). Using Zero-Inflated Poisson (ZIP) regression on 140 countries from 1950 to 1996, we find that democracies and mixed regimes tend to use SEI for diversion even though strategic conflict avoidance does not prevent them from using PSI. We further find that autocracies do not externalize their internal problems with either type of armed force and that, surprisingly, strategic conflict avoidance may constrain autocracies suffering economic decline. These outcomes suggest that our theory has utility and that research on both diversion and SCA would benefit from further theoretical refinement. [source] An investigation of incident frequency, duration and lanes blockage for determining traffic delayJOURNAL OF ADVANCED TRANSPORTATION, Issue 3 2009Yi (Grace) Qi Traffic delay caused by incidents is closely related to three variables: incident frequency, incident duration, and the number of lanes blocked by an incident that is directly related to the bottleneck capacity. Relatively, incident duration has been more extensively studied than incident frequency and the number of lanes blocked in an incident. In this study, we provide an investigation of the influencing factors for all of these three variables based on an incident data set that was collected in New York City (NYC). The information about the incidents derived from the identification can be used by incident management agencies in NYC for strategic policy decision making and daily incident management and traffic operation. In identifying the influencing factors for incident frequency, a set of models, including Poisson and Negative Binomial regression models and their zero-inflated models, were considered. An appropriate model was determined based on a model decision-making tree. The influencing factors for incident duration were identified based on hazard-based models where Exponential, Weibull, Log-logistic, and Log-normal distributions were considered for incident duration. For the number of lanes blocked in an incident, the identification of the influencing factors was based on an Ordered Probit model which can better capture the order inherent in the number of lanes blocked in an incident. As identified in this study, rain is the only factor that significantly influenced incident frequency. For incident duration and the number of lanes blocked in an incident, various factors had significant impact. As concluded in this study, there is a strong need to identify the influencing factors in terms of different types of incidents and the roadways where the incidents occured. [source] Fracture Prediction From Bone Mineral Density in Japanese Men and Women,JOURNAL OF BONE AND MINERAL RESEARCH, Issue 8 2003Saeko Fujiwara Abstract In a cohort of 2356 Japanese elderly, after adjusting for age and prevalent vertebral fracture, baseline BMD predicted the risk of spine and hip fracture with similar RR to that obtained from previous reports in whites. The RR per SD decrease in BMD for fracture declined with age. Introduction: Low bone mineral density (BMD) is one of the most important predictors of a future fracture. However, we are not aware of any reports among Japanese in Japan. Materials and Methods: We examined the association of BMD with risk of fracture of the spine or hip among a cohort of 2356 men and women aged 47,95 years, who were followed up by biennial health examinations. Follow-up averaged 4 years after baseline measurements of BMD that were taken with the use of DXA. Vertebral fracture was assessed using semiquantitative methods, and the diagnosis of hip fracture was based on medical records. Poisson and Cox regression analysis were used. Results: The incidence was twice as high in women as in men, after adjusting for age. After adjusting for baseline BMD and prevalent vertebral fracture, however, the gender difference was no longer significant. Age, baseline BMD of spine and femoral neck, and prior vertebral fracture predicted vertebral fracture and hip fracture. Loss of absolute BMD of the femoral neck predicted spine fracture, after adjusting for baseline BMD; rates of change in percent BMD, weight, height, body mass index, and age at menopause did not. The predictive value of baseline BMD for vertebral fracture risk was similar in men and women. The relative risk (RR) for vertebral fracture and hip fracture per SD decrease in BMD declined with age, after adjustment for prevalent vertebral fractures. Conclusions: Baseline BMD, loss of femoral neck BMD, and prior vertebral fracture predict the risk of spine and hip fracture in Japanese with similar RR to that obtained from previous reports in whites. The RR per SD decrease in BMD for fracture declined with age, suggesting that factors other than BMD might play a greater role in the elderly. [source] Can MM-PBSA calculations predict the specificities of protein kinase inhibitors?JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2006Christopher S. Page Abstract An application of the Molecular mechanics Poisson,Boltzmann surface area (MM-PBSA) protocol to the prediction of protein kinase inhibitor selectivity is presented. Six different inhibitors are placed in equivalent orientations in each of six different receptors. Fully solvated molecular dynamics is then run for 1 ns on each of the 36 complexes, and the resulting trajectories scored, using the implicit solvent model. The results show some correlation with experimentally-determined specificities; anomalies may be attributed to a variety of causes, including difficulties in quantifying induced fit penalties and variabilities in normal modes calculations. Decomposing interaction energies on a per-residue basis yields more useful insights into the natures of the binding modes and suggests that the real value of such calculations lies in understanding interactions rather than outright prediction. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] The parameterization and validation of generalized born models using the pairwise descreening approximationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2004Julien Michel Abstract Generalized Born Surface Area (GBSA) models for water using the Pairwise Descreening Approximation (PDA) have been parameterized by two different methods. The first method, similar to that used in previously reported parameterizations, optimizes all parameters against the experimental free energies of hydration of organic molecules. The second method optimizes the PDA parameters to compensate only for systematic errors of the PDA. The best models are compared to Poisson,Boltzmann calculations and applied to the computation of potentials of mean force (PMFs) for the association of various molecules. PMFs present a more rigorous test of the ability of a solvation model to correctly reproduce the screening of intermolecular interactions by the solvent, than its accuracy at predicting free energies of hydration of small molecules. Models derived with the first method are sometimes shown to fail to compute accurate potentials of mean force because of large errors in the computation of Born radii, while no such difficulties are observed with the second method. Furthermore, accurate computation of the Born radii appears to be more important than good agreement with experimental free energies of solvation. We discuss the source of errors in the potentials of mean force and suggest means to reduce them. Our findings suggest that Generalized Born models that use the Pairwise Descreening Approximation and that are derived solely by unconstrained optimization of parameters against free energies of hydration should be applied to the modeling of intermolecular interactions with caution. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1760,1770, 2004 [source] Converging free energy estimates: MM-PB(GB)SA studies on the protein,protein complex Ras,RafJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2004Holger Gohlke Abstract Estimating protein,protein interaction energies is a very challenging task for current simulation protocols. Here, absolute binding free energies are reported for the complex H-Ras/C-Raf1 using the MM-PB(GB)SA approach, testing the internal consistency and model dependence of the results. Averaging gas-phase energies (MM), solvation free energies as determined by Generalized Born models (GB/SA), and entropic contributions calculated by normal mode analysis for snapshots obtained from 10 ns explicit-solvent molecular dynamics in general results in an overestimation of the binding affinity when a solvent-accessible surface area-dependent model is used to estimate the nonpolar solvation contribution. Applying the sum of a cavity solvation free energy and explicitly modeled solute,solvent van der Waals interaction energies instead provides less negative estimates for the nonpolar solvation contribution. When the polar contribution to the solvation free energy is determined by solving the Poisson,Boltzmann equation (PB) instead, the calculated binding affinity strongly depends on the atomic radii set chosen. For three GB models investigated, different absolute deviations from PB energies were found for the unbound proteins and the complex. As an alternative to normal-mode calculations, quasiharmonic analyses have been performed to estimate entropic contributions due to changes of solute flexibility upon binding. However, such entropy estimates do not converge after 10 ns of simulation time, indicating that sampling issues may limit the applicability of this approach. Finally, binding free energies estimated from snapshots of the unbound proteins extracted from the complex trajectory result in an underestimate of binding affinity. This points to the need to exercise caution in applying the computationally cheaper "one-trajectory-alternative" to systems where there may be significant changes in flexibility and structure due to binding. The best estimate for the binding free energy of Ras,Raf obtained in this study of ,8.3 kcal mol,1 is in good agreement with the experimental result of ,9.6 kcal mol,1, however, further probing the transferability of the applied protocol that led to this result is necessary. © 2003 Wiley Periodicals, Inc. J Comput Chem 2: 238,250, 2003 [source] Computational alanine scanning of the 1:1 human growth hormone,receptor complexJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2002Shuanghong Huo Abstract The MM-PBSA (Molecular Mechanics,Poisson,Boltzmann surface area) method was applied to the human Growth Hormone (hGH) complexed with its receptor to assess both the validity and the limitations of the computational alanine scanning approach. A 400-ps dynamical trajectory of the fully solvated complex was simulated at 300 K in a 101 Å×81 Å×107 Å water box using periodic boundary conditions. Long-range electrostatic interactions were treated with the particle mesh Ewald (PME) summation method. Equally spaced snapshots along the trajectory were chosen to compute the binding free energy using a continuum solvation model to calculate the electrostatic desolvation free energy and a solvent-accessible surface area approach to treat the nonpolar solvation free energy. Computational alanine scanning was performed on the same set of snapshots by mutating the residues in the structural epitope of the hormone and the receptor to alanine and recomputing the ,Gbinding. To further investigate a particular structure, a 200-ps dynamical trajectory of an R43A hormone,receptor complex was simulated. By postprocessing a single trajectory of the wild-type complex, the average unsigned error of our calculated ,,Gbinding is ,1 kcal/mol for the alanine mutations of hydrophobic residues and polar/charged residues without buried salt bridges. When residues involved in buried salt bridges are mutated to alanine, it is demonstrated that a separate trajectory of the alanine mutant complex can lead to reasonable agreement with experimental results. Our approach can be extended to rapid screening of a variety of possible modifications to binding sites. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 15,27, 2002 [source] | |||||||||||||||||||||||||||||||||||||||||||