estimating medical costs with censored data incompleteness of follow up data is a common problem in estimating medical costs naive analysis using summary statistics on the collected data can result in severely misleading statistical inference this paper focuses on the problem of estimating the mean medical cost from a sample of individuals whose medical costs may be right censored a class of weighted estimators which account appropriately for censoring are introduced our estimators are shown to be consistent and asymptotically normal with easily estimated variances the efficiency of these estimators is studied with the goal of finding as efficient an estimator for the mean medical cost as is feasible extensive simulation studies are used to show that our estimators perform well in finite samples even with heavily censored data for a variety of circumstances the methods are applied to a set of cost data from a cardiology trial conducted by the duke university medical center extensions to other censored data problems are also discussed

tdgl and mkdv equations for jamming transition in the lattice models of traffic abstract the lattice models of traffic are proposed to describe the jamming transition in traffic flow on a highway in terms of thermodynamic terminology of phase transitions and critical phenomena they are the lattice versions of the hydrodynamic model of traffic two lattice models are presented one is described by the differential difference equation where time is a continuous variable and space is a discrete variable and the other is the difference equation in which both time and space variables are discrete we apply the linear stability theory and the nonlinear analysis to the lattice models it is shown that the time dependent ginzburg landau tdgl equation is derived to describe the traffic flow near the critical point a thermodynamic theory is formulated for describing the phase transitions and critical phenomena it is also shown that the perturbed modified korteweg de vries mkdv equation is derived to describe the traffic jam

utilizing propensity scores to estimate causal treatment effects with censored time lagged data observational studies frequently are conducted to compare long term effects of treatments without randomization patients receiving one treatment are not guaranteed to be prognostically comparable to those receiving another treatment furthermore the response of interest may be right censored because of incomplete follow up statistical methods that do not account for censoring and confounding may lead to biased estimates this article presents a method for estimating treatment effects in nonrandomized studies with right censored responses we review the assumptions required to estimate average causal effects and derive an estimator for comparing two treatments by applying inverse weights to the complete cases the weights are determined according to the estimated probability of receiving treatment conditional on covariates and the estimated treatment specific censoring distribution by utilizing martingale representations the estimator is shown to be asymptotically normal and an estimator for the asymptotic variance is derived simulation results are presented to evaluate the properties of the estimator these methods are applied to an observational data set of acute coronary syndrome patients from duke university medical center to estimate the effect of a treatment strategy on the mean 5 year medical cost

a simplified algorithm for the second order sound fields an analytical description for the second order field of the sum difference frequency and second harmonic components is derived this treatment is based on the fact that an arbitrary axially symmetric beam can be expressed as the linear superposition of a set of gaussian beams j j wen and m a breazeale j acoust soc am 83 1752 1756 1988 so that the three dimensional integral for the second order components is typically expressed as a combination of a set of interaction terms of the gaussian beams correspondingly the evaluation of the field distribution is reduced to the summation of exponential integral functions from some examples the present approach provides the results for the second order field which are in good agreement with those obtained directly by numerical integration

analysis of car following model considering driver s physical delay in sensing headway abstract an extended car following model is proposed by taking into account the delay of the driver s response in sensing headway the stability condition of this model is obtained by using the linear stability theory the results show that the stability region decreases when the driver s physical delay in sensing headway increases the kdv equation and mkdv equation near the neutral stability line and the critical point are respectively derived by applying the reductive perturbation method the traffic jams could be thus described by soliton solution and kink antikink soliton solution for the kdv equation and mkdv equation respectively the numerical results in the form of the space time evolution of headway show that the stabilization effect is weakened when the driver s physical delay increases it confirms the fact that the delay of driver s response in sensing headway plays an important role in jamming transition and the numerical results are in good agreement with the theoretical analysis

comments on distortion of finite amplitude ultrasound in lossy media by m e haran and b d cook j acoust soc am 73 774 779 1983 readers of the subject paper might conclude that haran and cook are the first to derive an alogorithm based on burgers equation which calculates the nonlinear propagation of a plane wave in a lossy medium with nonquadratic frequency dependent absorption coefficients in fact we presented such an algorithm in an earlier paper d h trivett and a l van buren propagation of plane cylindrical and spherical finite amplitude waves j acoust soc am 69 943 949 1981 and we feel that the haran and cook s reference to our work was insufficient although the derivation given by haran and cook is correct it contains an inconsistency in notation also their numerical results for propagation in fresh water correspond to different initial conditions than stated in the paper

stabilization effect of multiple drivers desired velocities in car following theory abstract in order to reveal the influence of driver s individual behavior on traffic flow more accurately a new car following model is proposed with consideration of multiple drives desired velocities the stability criterion of the new model is derived through linear stability theory and the results show that the current driver s desired velocity can stabilize traffic flow but the preceding driver s desired velocity can damage traffic stability through nonlinear analysis the traffic jamming transition characteristics near the critical point can be described by the kink antikink soliton of the mkdv equation numerical simulation confirms the analytical results which shows that the multiple drivers desired velocities play an important role in traffic evolution

an iterative method for the computation of nonlinear wide angle pulsed acoustic fields of medical diagnostic transducers the development and optimization of medical ultrasound transducers and imaging modalities require a computational method that accurately predicts the nonlinear acoustic pressure field a prospective method should provide the wide angle pulsed field emitted by an arbitrary planar source distribution and propagating in a three dimensional large scale domain holding a nonlinear acoustic medium in this paper a method is presented that is free of any assumed wavefield directionality the nonlinear acoustic wave equation is solved by treating the nonlinear term as a contrast source this formulation leads to an iterative scheme that involves the repetitive solution of a linear wave problem through green s function method it is shown that accurate field predictions may be obtained within a few iterations moreover by employing a dedicated numerical convolution technique the method allows for a discretization down to two points per wavelength or period of the highest frequency of interest the performance of the method is evaluated through a number of nonlinear field predictions for pulsed transducers with various geometries the results demonstrate the directional independence of the method moreover comparison with results from several existing methods shows that the method accurately predicts the nonlinear field for weak to moderate nonlinearity

analysis of the numerically implemented angular spectrum approach based on the evaluation of two dimensional acoustic fields part ii characteristics as a function of angular range the numerically implemented angular spectrum approach asa is investigated in terms of the characteristics of the asa as a function of angular range over which the angular spectrally decomposed plane waves are superimposed to construct an acoustic field radiated by a source the investigation is based on the evaluation of the 2 d fields radiated by striplike planar sources and 1 d linear phased arrays with different aperture sizes and on the comparison with those obtained by using the analytical solution derived with the point spread function method the convergence and the spatial aliasing of the asa s results are systematically dealt with a region well converging to the analytical solution as the angular range increases is found to exist before the aliasing comes up analytical formulas to determine the boundaries of this region are given based on the instantaneous frequency concept an analytical expression for optimal selection of the angular range used is derived hence the sampling frequency can be chosen to be optimal and adaptive to the discretization size of the source plane without worrying that the number of the samples for the discretization size should be chosen to be a power of two that the fast fourier transform fft usually needs an excellent agreement of the asa s results with the analytical solution is demonstrated when using the optimal selection of the angular range for the specified parameters the analysis made in this paper can be extended to the case of 3 d fields

second harmonic generation of practical bessel beams a fast gaussian expansion approach is used to investigate fundamental and second harmonic generation in practical bessel beams of finite aperture the analysis is based on the integral solutions of the kzk equation under the quasilinear approximation the influence of the medium s attenuation on the beam profile is considered analysis results show that the absorption parameter has a significant effect on the far field beam profile of the second harmonic under certain circumstances the second harmonic of a practical bessel beam still has the main properties of an ideal bessel beam of infinite aperture when it propagates within its depth of field

modeling and application analysis of car following model with predictive headway variation abstract this work proposes an improved microscopic model by taking the predictive driving behavior of the preceding car into account firstly we use the linear method to investigate performance of the proposed model deduce the critical linear stability conditions and obtain a neutral curve accounting for stability of the new model then using the nonlinear analysis technique we derive the equations of burgers the korteweg de vries kdv and the modified kdv mkdv respectively to describe the triangular shock wave soliton density wave and the kink antikink density wave finally numerical experiments unfold from two aspects one for verifying the theoretical analysis results and another for testing the impact of predictive driving behavior of the preceding car on traffic stability simulation results show that the new model can stabilize traffic flow compared with the fvd model and confirm that the existence of three kinds of density waves in different phase diagram regions

optimal selection of parameters for the angular spectrum approach to numerically evaluate acoustic fields optimal selection of parameters is presented for the angular spectrum approach asa to the numerical calculation of acoustic fields radiated by planar transducers and linear arrays with and without focusing the parameters include spatial sampling interval discretization size of a source plane in which a source is located and the angular range over which the plane waves decomposed from the source using the discrete fourier transform dft are chosen to superimpose and construct the fields the concept of instantaneous frequency is applied to the fourier transformed green s function to determine the angular range of the plane waves used for the construction with minimal spatial aliasing error based on the minimization of spatial frequency and spatial aliasing errors in a constructed field optimal selection of the parameters is worked out the asa with the optimal selection is then applied to computing the fields radiated by planar transducers and linear arrays into water and a layered oil water medi

an extended macro traffic flow model accounting for the driver s bounded rationality and numerical tests abstract in this paper we propose a macro traffic flow model to explore the effects of the driver s bounded rationality on the evolutions of traffic waves which include shock and rarefaction waves and small perturbation and on the fuel consumption and emissions that include co hc and no x during the evolution process the numerical results illustrate that considering the driver s bounded rationality can prominently smooth the wavefront of the traffic waves and improve the stability of traffic flow which shows that the driver s bounded rationality has positive impacts on traffic flow but considering the driver s bounded rationality reduces the fuel consumption and emissions only at the upstream of the rarefaction wave while enhances the fuel consumption and emissions under other situations which shows that the driver s bounded rationality has positive impacts on the fuel consumption and emissions only at the upstream of the rarefaction wave while negative effects on the fuel consumption and emissions under other situations in addition the numerical results show that the driver s bounded rationality has little prominent impact on the total fuel consumption and emissions during the whole evolution of small perturbation

analysis of social optimum for staggered shifts in a single entry traffic corridor with no late arrivals abstract this paper investigates the traffic flow dynamics under the social optimum so principle in a single entry traffic corridor with staggered shifts from the analytical and numerical perspectives the lwr lighthill whitham and richards model and the greenshield s velocity density function are utilized to describe the dynamic properties of traffic flow the closed form so solution is analytically derived and some numerical examples are used to further testify the analytical solution the optimum proportion of the numbers of commuters with different desired arrival times is further discussed where the analytical and numerical results both indicate that the cumulative outflow curve under the so principle is piecewise smooth

an asymmetric full velocity difference car following model abstract this paper presents a car following model that considers the asymmetric characteristic of the velocity differences of the vehicles in a traffic stream the problems of the prevalent general force gf model and the full velocity difference fvd model were solved furthermore the optimal velocity ov model the gf model and the fvd model are proved to be the special cases of the proposed asymmetric full velocity difference afvd model the mathematical model is presented followed by simulation analysis which demonstrates the properties of the afvd model

unmitigated numerical solution to the diffraction term in the parabolic nonlinear ultrasound wave equation various numerical algorithms have been developed to solve the khokhlov kuznetsov zabolotskaya kzk parabolic nonlinear wave equation in this work a generalized time domain numerical algorithm is proposed to solve the diffraction term of the kzk equation this algorithm solves the transverse laplacian operator of the kzk equation in three dimensional 3d cartesian coordinates using a finite difference method based on the five point implicit backward finite difference and the five point crank nicolson finite difference discretization techniques this leads to a more uniform discretization of the laplacian operator which in turn results in fewer calculation gridding nodes without compromising accuracy in the diffraction term in addition a new empirical algorithm based on the lu decomposition technique is proposed to solve the system of linear equations obtained from this discretization the proposed empirical algorithm improves the calculation speed and memory usage while the order of computational complexity remains linear in calculation of the diffraction term in the kzk equation for evaluating the accuracy of the proposed algorithm two previously published algorithms are used as comparison references the conventional 2d texas code and its generalization for 3d geometries the results show that the accuracy efficiency performance of the proposed algorithm is comparable with the established time domain methods

asymptotic solutions for a multi anticipative car following model abstract using a long wave expansion we study the asymptotic solutions near a critical point of a multi anticipative car following model a suitable rescaling of the variables then yields a modified korteweg de vries mkdv equation with higher order corrections an equation of this general form has already been investigated in detail in case of a simple model which allows us to use these results we are thus led to the conclusion that the characteristic features of the multi interaction model in the congested state can be obtained from the simple model by appropriately scaling the results

lattice hydrodynamic model with bidirectional pedestrian flow abstract the two dimensional lattice hydrodynamic model of traffic is extended to the two dimensional bidirectional pedestrian flow via taking four types of pedestrians into account the stability condition and the mkdv equation to describe the density wave of pedestrian congestion are obtained by linear stability and nonlinear analysis respectively in addition there exist three phase transitions among the freely moving phase the coexisting phase and the uniformly congested phase in the phase diagram it can also be found that the critical point a c refers to not only the fraction c 1 of the eastbound and westbound pedestrians but also the fraction c 2 of the northbound and southbound pedestrians however the critical point a c could not appear in the phase diagram and congested crowd at any time when two fractions are equal to same value of 0 5 c 1 c 2 0 5 furthermore numerical simulation is carried out to examine the performance of such a model and the results show coincidence with the theory analysis results

analysis of optimal velocity model with explicit delay we analyze the optimal velocity model ovm with explicit delay the properties of congestion and the delay time of car motion are investigated by analytical and numerical methods it is shown that the small explicit delay time has almost no effects in the case of the large explicit delay time a new phase of congestion pattern of ovm seems to appear

a speed feedback control strategy for car following model abstract a speed feedback control mechanism was introduced into the system to improve the dynamical performance of the traffic flow the modern control theory was used to analyze the stability of the system it is found that the stability region varies with the feedback coefficient proportionally in addition the unit step responses in time domain and phase frequency curves in frequency domain were given with different feedback coefficients in step response diagram and bode diagram respectively the overshoot and phase margins are inversely proportional to the speed feedback coefficients in an underdamped condition the simulations were conducted to verify the validity of the improvement the conclusion can be drawn that the analytical result and the simulation result are in good agreement with each other

a filtered convolution method for the computation of acoustic wave fields in very large spatiotemporal domains the full wave computation of transient acoustic fields with sizes in the order of 100x100x100 wavelengths by 100 periods requires a numerical method that is extremely efficient in terms of storage and computation iterative integral equation methods offer a good performance on these points provided that the recurring spatiotemporal convolutions are computed with a coarse sampling and relatively few computational operations this paper describes a method for the numerical evaluation of very large scale four dimensional convolutions that employs a fast fourier transformation and that uses a sampling rate close to or at the limit of two points per wavelength and per period to achieve this the functions involved are systematically filtered windowed and zero padded with respect to all relevant coordinates prior to sampling the method is developed in the context of the neumann iterative solution of the acoustic contrast source problem for an inhomogeneous medium the implementation of the method on a parallel computer is discussed the obtained numerical results have a relative root mean square error of a few percent when sampling at two points per wavelength and per period further the results prove that the method enables the computation of transient fields in the order of the indicated size

kink soliton characterizing traffic congestion we study traffic congestion by analyzing a one dimensional traffic flow model developing an asymptotic method to investigate the long time behavior near a critical point we derive the modified korteweg de vries mkdv equation as the lowest order model there is an infinite number of kink solitons to the mkdv equation while it has been found by numerical simulations that the kink pattern arising in traffic congestion is uniquely determined irrespective of initial conditions in order to resolve this selection problem we consider higher order corrections to the mkdv equation and find that there is a kink soliton that can deform continuously with the perturbation represented by the addition of these corrections with numerical confirmation we show that this continuously deformable kink soliton characterizes traffic congestion we also discuss the relationship between traffic congestion and the slugging phenomenon in granular flow

derivation properties and simulation of a gas kinetic based nonlocal traffic model we derive macroscopic traffic equations from specific gas kinetic equations dropping some of the assumptions and approximations made in previous papers the resulting partial differential equations for the vehicle density and average velocity contain a non local interaction term which is very favorable for a fast and robust numerical integration so that several thousand freeway kilometers can be simulated in real time the model parameters can be easily calibrated by means of empirical data they are directly related to the quantities characterizing individual driver vehicle behavior and their optimal values have the expected order of magnitude therefore they allow to investigate the influences of varying street and weather conditions or freeway control measures simulation results for realistic model parameters are in good agreement with the diverse non linear dynamical phenomena observed in freeway traffic

nonlinear effects in the farfield of a directive sound source nonlinear propagation of a periodic sound beam in a dissipative fluid is considered using fourier series expansion and numerical methods to solve the governing equation of motion in the parabolic approximation the nearfield was considered in a previous paper aanonsen et al j acoust soc am 75 749 768 1984 the analysis is now extended to the farfield numerical and asymptotic results are derived and used to explain the development of the fundamental and harmonic components from the nearfield into the farfield a discussion is also given of some earlier models for the farfield of directional waves emphasis is put on the importance of imposing the proper matching conditions between the nearfield solution and the spherical solution in the farfield in order to obtain a good approximation propagation and saturation curves are calculated as well as beam patterns for various harmonic components the results are compared with available experimental observations nonlinear effects although generated

proportional differential effects in traffic car following model system abstract traffic flow system s stability analysis can be seen as a typical control system problem which has three kinds of damping ratio properties from the viewpoint of control theory to discover the damping ratio characteristics of traffic flow system under sudden outer impulse firstly we proposed a new method to build the traffic flow model based on state space theory secondly we deduced one form of state transfer matrix from system closed loop transfer function reversely lastly we fully investigated the proportional differential effects in the traffic car following model system analytically and numerically for unsteady traffic flow system with the proportional differential pd term in order to suppress the traffic jam we derived a kind of decoupled delay feedback control algorithm which used h norm and the numerical simulation method to find the feedback gain k and delay time the traffic flow system s transient performance formula was presented with and without pd term and we found out the important impact of pd term on traffic flow system stability the numerical simulation results substantiate our theoretical and analytical results

metastable states in a microscopic model of traffic flow it is a well known fact that metastable states of very high throughput and hysteresis effects exist in traffic flow which the simple cellular automaton model of traffic flow and its continuous generalization fail to reproduce it is shown that the model can be generalized to give a one parametric family of models a part of which reproduces the metastable states and the hysteresis the models that have that property and those that do not that are separated by a transition that can be clearly identified

from association to causation in observational studies the role of tests of strongly ignorable treatment assignment abstract if treatment assignment is strongly ignorable then adjustment for observed covariates is sufficient to produce consistent estimates of treatment effects in observational studies a general approach to testing this critical assumption is developed and applied to a study of the effects of nuclear fallout on the risk of childhood leukemia r a fisher s advice on the interpretation of observational studies was make your theories elaborate formally make causal theories sufficiently detailed that under the theory strongly ignorable assignment has testable consequences

asymmetric optimal velocity car following model abstract taking the asymmetric characteristic of the velocity differences of vehicles into account we present an asymmetric optimal velocity model for a car following theory the asymmetry between the acceleration and the deceleration is represented by the exponential function with an asymmetrical factor which agrees with the published experiment this model avoids the disadvantage of the unrealistically high acceleration appearing in previous models when the velocity difference becomes large this model is simple and only has two independent parameters the linear stability condition is derived and the phase transition of the traffic flow appears beyond the critical density the strength of interaction between clusters is shown to increase with the asymmetry factor in our model

scaling behaviour in discrete traffic models we investigate the noisy discrete single lane highway traffic model proposed by nagel and schreckenberg 1992 and demonstrate by computer simulations that as a function of the car density there is a dynamical transition from laminar flow to jammed traffic in the system related to the divergence of the relaxation time of average car velocity the critical density is close to the position of the maximum in the fundamental diagram we give estimates of the critical exponents related to this jamming transition the critical density of the geometrical 1 1 dimensional percolation transition of jammed regions can be rather far from the jamming point depending on the strength of the noise which indicates that cooperativity leading to the jamming transition can be long range in character

kdv burgers equation in a new continuum model based on full velocity difference model considering anticipation effect abstract in this paper a new continuum model based on full velocity difference car following model is developed with the consideration of driver s anticipation effect by applying the linear stability theory the new model s linear stability is obtained through nonlinear analysis the kdv burgers equation is derived to describe the propagating behavior of traffic density wave near the neutral stability line numerical simulation shows that the new model possesses the local cluster and it is capable of explaining some particular traffic phenomena numerical results show that when considering the effects of anticipation the traffic jams can be suppressed efficiently the key improvement of this new model is that the anticipation effect can improve the stability of traffic flow

modified kdv equation for jamming transition in the continuum models of traffic abstract continuum models of traffic are proposed to describe the jamming transition in traffic flow on a highway they are the simplified versions of the hydrodynamic model of traffic two continuum models are presented one is described by the partial differential equations and the other is the discrete lattice version the linear stability theory and the nonlinear analysis are applied to the continuum models the modified korteweg de vries equation kdv near the critical point is derived using the reduction perturbation method it is shown that the jamming transition and the density wave in the congested traffic flow are described by the modified kdv equation the solutions of the kdv equations obtained from the two models are compared with that of the optimal velocity model car following model

exact solutions for a discrete system arising in traffic flow this paper concerns wave propagation in a discrete nonlinear system of equations proposed and studied by g f newell as a model for car following in traffic flow in particular newell found exact solutions for shock waves and related phenomena here exact solutions representing periodic waves and solitary waves are obtained the method relates travelling wave solutions to the toda and kac van moerbeke discrete systems in this and other ways much of the interest is in the general phenomena possible in discrete systems here including also a time lag rather than in just the specific traffic flow setting

modeling of nonlinear ultrasound propagation in tissue from array transducers a computationally efficient model capable of simulating finite amplitude ultrasound beam propagation in water and in tissue from phased linear arrays and other transducers of arbitrary quasiplanar geometry is described it is based on a second order operator splitting approach tavakkoli et al j acoust soc am 104 2061 2072 1998 with a fractional step marching scheme whereby the effects of diffraction attenuation and nonlinearity can be computed independently over incremental steps this approach is an extension to that of christopher and parker j acoust soc am 90 507 521 90 488 499 1991 wherein linear and nonlinear effects are propagated separately over incremental steps and the computation of the diffractive substeps are based on an angular spectrum technique with a modified sampling scheme for accurate and efficient implementation of diffractive propagation from nonradially symmetric sources results of the model are compared with published data predicted field profiles for nonlinear propagation in tissue from realistic array transducers using the pulse inversion method are presented

stabilization effect of traffic flow in an extended car following model based on an intelligent transportation system application an extended car following model is proposed by incorporating an intelligent transportation system in traffic the stability condition of this model is obtained by using the linear stability theory the results show that anticipating the behavior of more vehicles ahead leads to the stabilization of traffic systems the modified korteweg de vries equation the mkdv equation for short near the critical point is derived by applying the reductive perturbation method the traffic jam could be thus described by the kink antikink soliton solution for the mkdv equation from the simulation of space time evolution of the vehicle headway it is shown that the traffic jam is suppressed efficiently with taking into account the information about the motion of more vehicles in front and the analytical result is consonant with the simulation one

phase transition in a difference equation model of traffic flow a difference equation is presented to describe traffic flow on a highway the difference equation model is derived from the optimal velocity models formulated in terms of the differential equations it is compared with the differential equation models we investigate phase transitions among the freely moving phase the coexisting phase in which jams appear and the uniform congested phase the linear stability theory is applied and the neutral stability line is obtained we find the critical point below which no jams appear to derive the modified korteweg de vries equation near the critical point we apply the reductive perturbation method we also compare the nonlinear analysis result with that of the optimal velocity model it is shown that the critical point and the amplitude of the jam are different from those of the optimal velocity model

full wave modeling of therapeutic ultrasound efficient time domain implementation of the frequency power law attenuation for the simulation of therapeutic ultrasound applications a method including frequency dependent attenuation effects directly in the time domain is highly desirable this paper describes an efficient numerical time domain implementation of the power law attenuation model presented by szabo szabo j acoust soc am 96 491 500 1994 simulations of therapeutic ultrasound applications are feasible in conjunction with a previously presented finite differences time domain fdtd algorithm for nonlinear ultrasound propagation ginter et al j acoust soc am 111 2049 2059 2002 szabo implemented the empirical frequency power law attenuation using a causal convolutional operator directly in the time domain equation though a variety of time domain models has been published in recent years no efficient numerical implementation has been presented so far for frequency power law attenuation models solving a convolutional integral with standard time domain techniques requires enormous computational ef

regression with a right censored predictor using inverse probability weighting methods in a longitudinal study measures of key variables might be incomplete or partially recorded due to drop out loss to follow up or early termination of the study occurring before the advent of the event of interest in this paper we focus primarily on the implementation of a regression model with a randomly censored predictor we examine particularly the use of inverse probability weighting methods in a generalized linear model glm when the predictor of interest is right censored to adjust for censoring to improve the performance of the complete case analysis and prevent selection bias we consider three different weighting schemes inverse censoring probability weights kaplan meier weights and cox proportional hazards weights we use monte carlo simulation studies to evaluate and compare the empirical properties of different weighting estimation methods finally we apply these methods to the framingham heart study data as an illustrative example to estimate the relationship between age of onset of a clinically diagnosed cardiovascular event and low density lipoprotein among cigarette smokers

extension of the angular spectrum approach to curved radiators the angular spectrum approach asa is conventionally applied to the evaluation of acoustic fields from planar radiators because it is usually based on the 2 d fourier transform or the zero order hankel transform in the axisymmetrical case which is implemented only in a plane the present paper is intended to extend the asa to more general cases where radiators have curved surfaces for this purpose two approaches are developed the first one is the extended asa and is derived in a general way from this approach the angular spectrum of a curved radiator is given by a double integral that does not take the 2 d fourier transform form and thus cannot be implemented using 2 d fast fourier transform fft but by numerical integration the second approach is the indirect asa that gives the angular spectrum via 2 d fourier transforming an initial field pre calculated in a plane the method for calculating the initial field is proposed based on the method developed by ocheltree and frizzell for planar sources an example is given of a linear array with a cylindrically concave surface and in this case the angular spectrum the double integral from the extended asa reduces to a single integral the angular spectra of the array are calculated using both approaches and compared the comparison has shown that the angular spectra obtained from both approaches are in excellent agreement the accuracy and efficiency of the two approaches are studied in the numerical implementation in this example the extended asa has been shown to be more efficient and more accurate than the latter approach both approaches can be applied to arbitrarily curved transducers in the general case where the double integral cannot be reduced to a single integral the latter approach can be more efficient

new approaches to the linear propagation of acoustic fields new algorithms are described that provide insight into linear field propagation and offer significant reductions in computational complexity the developments presented here include the usage of a recently developed discrete hankel transform to implement two single step planar propagation algorithms for baffled radially symmetric acoustic pressure or velocity fields an update on the single step approaches that reduce computational complexity through geometrically determined spatial frequency limitations and algorithms for extending to multistep propagation two equivalent means of introducing arbitrary medium attenuation into the above schemes are presented finally a planar boundary crossing algorithm that accounts for refraction and reflection but not multiple reflections is added to one of the multistep propagating algorithms the resulting algorithm is then used to examine the differences between the corresponding fields of a focused piston source operating in water and in a layered fat liver biomedical imaging medium the results yield computationally efficient algorithms that can be used for linear propagation of focused or unfocused beams in attenuating multilayer media and also provide the basis for a novel nonlinear propagation algorithm

nonlinear analysis for a modified continuum model considering electronic throttle et and backward looking effect abstract in this research a new car following model is developed by considering the influences of electronic throttle et dynamics and backward looking effect the improved car following model is transformed into a new continuum model through the relationship between the microscopic variables and macroscopic variables the linear stability condition of the extended continuum model can be obtained according to the linear stability theory the neutral stability curve shows that backward looking effect will improve the stability of traffic flow the kdv burgers equation is derived by the nonlinear analysis which can be used to describe the spreading behavior of traffic flow density wave in traffic congestion nearby the neutral stability line numerical experiments have been carried out including the shock waves rarefaction waves and energy consumption the numerical simulation results indicate that the traffic flow stability can be enhanced by considering et dynamics and backward looking effect in addition the above two factors can also effectively reduce energy consumption

the central role of the propensity score in observational studies for causal effects abstract the results of observational studies are often disputed because of nonrandom treatment assignment for example patients at greater risk may be overrepresented in some treatment group this paper discusses the central role of propensity scores and balancing scores in the analysis of observational studies the propensity score is the estimated conditional probability of assignment to a particular treatment given a vector of observed covariates both large and small sample theory show that adjustment for the scalar propensity score is sufficient to remove bias due to all observed covariates applications include matched sampling on the univariate propensity score which is equal percent bias reducing under more general conditions than required for discriminant matching multivariate adjustment by subclassification on balancing scores where the same subclasses are used to estimate treatment effects for all outcome variables and in all subpopulations and visual representation of multivariate adjustment by a two dimensional plot author

stabilization and enhancement of traffic flow by the next nearest neighbor interaction the car following model of traffic is extended to take into account the car interaction before the next car ahead the next nearest neighbor interaction the traffic behavior of the extended car following model is investigated numerically and analytically it is shown that the next nearest neighbor interaction stabilizes the traffic flow the jamming transition between the freely moving and jammed phases occurs at a higher density than the threshold of the original car following model by increasing the maximal velocity the traffic current is enhanced without jam by the stabilization effect the jamming transition is analyzed with the use of the linear stability and nonlinear perturbation methods the traffic jam is described by the kink solution of the modified korteweg char21 de vries equation the theoretical coexisting curve is in good agreement with the simulation result

stabilization analysis and modified korteweg de vries equation in a cooperative driving system two lattice traffic models are proposed by incorporating a cooperative driving system the lattice versions of the hydrodynamic model of traffic flow are described by the differential difference equation and difference difference equation respectively the stability conditions for the two models are obtained using the linear stability theory the results show that considering more than one site ahead in vehicle motion leads to the stabilization of the system the modified korteweg de vries equation the mkdv equation for short near the critical point is derived by using the reductive perturbation method to show the traffic jam which is proved to be described by kink anti kink soliton solutions obtained from the mkdv equations

an extended car following model considering the self stabilizing driving behavior of headway abstract considering the self stabilizing driving behavior of headway an extended car following model called the self stabilizing optimal velocity ssov model is presented in this paper to show the effect of the self stabilizing driving behavior of headway the ssov model is investigated by the analytical and numerical methods by using the linear analysis method the stability condition of the ssov model is obtained and then by using the nonlinear analysis method the burgers kdv and modified kdv equations are respectively derived to describe the propagating behavior of traffic density wave in the stable unstable regions and near the critical point the numerical simulations are presented to show the valid of the analytical results and the effect of the self stabilizing driving behavior the analytical and numerical results all show that the self stabilizing driving behavior plays an important role on the stability and flux of the traffic flow

traffic jams granular flow and soliton selection the flow of traffic on a long section of road without entrances or exits can be modeled by continuum equations similar to those describing fluid flow in a certain range of traffic density steady flow becomes unstable against the growth of a cluster or phantom traffic jam which moves at a slower speed than the otherwise homogeneous flow we show that near the onset of this instability traffic flow is described by a perturbed korteweg de vries kdv equation the traffic jam can be identified with a soliton solution of the kdv equation the perturbation terms select a unique member of the continuous family of kdv solitons these results may also apply to the dynamics of granular relaxation

a numerical method for general finite amplitude wave propagation in two dimensions and its application to spark pulses the general equations of finite amplitude acoustics including classical absorption effects and second order nonlinear effects are written in a form suitable for two dimensional numerical solution a finite difference scheme then is applied to numerically solve the equations to demonstrate the method examples are given of spherical free field propagation normal plane reflection from a hard surface and oblique spherical reflection from a hard surface for spark pulses this method has an advantage over burgers equation methods one way wave equation methods and pestorius type algorithms in that it can predict the interaction of multiple finite amplitude acoustic waves at arbitrary propagation angles

a new lattice model of traffic flow with the consideration of the traffic interruption probability abstract in this paper we present a new lattice model which involves the effects of traffic interruption probability to describe the traffic flow on single lane freeways the stability condition of the new model is obtained by the linear stability analysis and the modified korteweg de vries kdv equation is derived through nonlinear analysis thus the space will be divided into three regions stable metastable and unstable the simulation results also show that the traffic interruption probability could stabilize traffic flow

the effect of headway variation tendency on traffic flow modeling and stabilization abstract advanced communications technology enables us to obtain the headway variation tendency hvt of the next moment in a car following scenario to maximize the benefits of the new technology we developed a new traffic flow model that takes into consideration the effect of hvt first linear analysis method is used to derive the linear stability condition that determines whether the hvt can improve traffic system stability then using the reductive perturbation method a modified kdv mkdv equation is obtained and the kink antikink solutions are derived the phase diagram of driver sensitivity against equilibrium headway is also plotted by using the neutral stability line derived from linear analysis and the coexisting line derived from nonlinear analysis this diagram demonstrates that hvt does have the ability to improve the stability of traffic stream finally the effect of hvt is validated through numerical simulations where it is evident that the information of hvt can smooth the traffic stream and prevent the formation of traffic congestion

an optimal velocity estimation ove model based on non empirical formula abstract in many research of optimal velocity model the optimal velocity is estimated by empirical formulas there are several aspects of this approach that need to be improved on the one hand the physical significance of such empirical formulas is not clear enough on the other hand the empirical formula of macro results is not detailed enough at the micro level in this paper we derived an equation to estimate the optimal velocity from two basic assumptions based on this equation the optimal velocity estimation ove model was established to verify the rationality of ove model we conducted two single file pedestrian experiments from the results of experiments we find that pedestrians pursue a consistent velocity and a comfortable distance between pedestrians in single file movement and the comfortable distance is proportional to the square of the steady velocity these conclusions provide support for our assumptions in addition we simulated the experiments with the ovm model and a traditional ov model by comparison the ovm model can reproduce macro results very well further the ovm model has a better performance than traditional ov model in micro level in view of the clear physical meanings our ovm model has an important significance in studying the influencing factors of pedestrian movement

an improved car following model considering the immediately ahead car s velocity difference abstract the field car following data at a signalized intersection of jinan in china are collected for data mining an improved car following model considering the immediately ahead car s velocity difference on a single lane road was proposed calibrated and verified based on full velocity difference model the results of some numerical simulations indicate that the immediately ahead car s velocity difference has significant effects on the following car s motion that the improved car following model fits the measured data well and can qualitatively describe the impacts of the immediately ahead car s velocity difference on traffic flow and that modeling the car following behavior considering the immediately ahead car s velocity difference can improve the stability of the simulated traffic flow

discrete stochastic models for traffic flow we investigate a probabilistic cellular automaton model which has been introduced recently this model describes single lane traffic flow on a ring and generalizes the asymmetric exclusion process models we study the equilibrium properties and calculate the so called fundamental diagrams flow versus density for parallel dynamics this is done numerically by computer simulations of the model and by means of an improved mean field approximation which takes into account short range correlations for cars with a maximum velocity of 1 the simplest nontrivial approximation gives the exact result for higher velocities the analytical results obtained by iterated application of the approximation scheme are in excellent agreement with the numerical simulations

reducing bias in observational studies using subclassification on the propensity score abstract the propensity score is the conditional probability of assignment to a particular treatment given a vector of observed covariates previous theoretical arguments have shown that subclassification on the propensity score will balance all observed covariates subclassification on an estimated propensity score is illustrated using observational data on treatments for coronary artery disease five subclasses defined by the estimated propensity score are constructed that balance 74 covariates and thereby provide estimates of treatment effects using direct adjustment these subclasses are applied within sub populations and model based adjustments are then used to provide estimates of treatment effects within these sub populations two appendixes address theoretical issues related to the application the effectiveness of subclassification on the propensity score in removing bias and balancing properties of propensity scores with incomplete data

a new car following model with the consideration of anticipation optimal velocity abstract in this paper a new anticipation optimal velocity model aovm is proposed by considering anticipation effect on the basis of the full velocity difference model fvdm for car following theory on single lane the linear stability condition is derived from linear stability analysis starting and braking process is investigated for the car motion under a traffic signal which shows that the results accord with empirical traffic values especially aovm can avoid the disadvantage of the unrealistically high deceleration appearing in fvdm furthermore numerical simulation shows that aovm might avoid the disadvantage of negative velocity and headway that occur at small sensitivity coefficients in the fvdm since the anticipation effect is taken into account in aovm which means that collision disappears with the consideration of an appropriate anticipation parameter

a car following model considering the effect of electronic throttle opening angle over the curved road abstract this paper proposes an extended car following model by taking the effect of electronic throttle dynamics into account on the curved road in which the electronic throttle opening angle difference from multiple preceding vehicles at the previous moment is considered as a delay feedback control signal then the stability condition of the proposed model is gained using the control theory method considering the effects of friction coefficient radius of curved road and control signal the numerical simulations are designed to study the change of traffic flow stability under different parameters results show that the radius and friction coefficient have a negative effect on stabilizing traffic flow while the stability increases with an increase in the delay feedback control coefficient in addition the numerical simulation also verifies that the control signal can reduce the fuel consumption of traffic flow

distortion of finite amplitude ultrasound in lossy media a form of burgers equation is used to derive an algorithm for calculating harmonic generation by a continuous plane wave of ultrasound propagating in a nonlinear lossy nondispersive medium the algorithm accounts for attenuation that is not quadratically related to the frequency of the wave attenuation strongly affects the rate of harmonic production the effect of variations of the relationship between attenuation and frequency is shown biological tissue is an example of a highly lossy medium where the attenuation does not increase with the square of the frequency calculations for several types of tissue and biological fluids are presented that show for certain conditions finite amplitude distortion is possible

kdv burgers equation in the modified continuum model considering anticipation effect abstract the new continuum model mentioned in this paper is developed based on optimal velocity car following model which takes the drivers anticipation effect into account the critical condition for traffic flow is derived and nonlinear analysis shows density waves occur in traffic flow because of the small disturbance near the neutral stability line the kdv burgers equation is derived and one of the solutions is given numerical simulation is carried out to show the local cluster described by the model

social optimum for evening commute in a single entry traffic corridor with no early departures abstract in this paper we investigate the evening commute behaviors on the social optimum so state in a single entry traffic corridor with no early departures differing from the previous studies on evening commute the dynamic properties of traffic flow are analyzed with the lwr lighthill whitham richards model the properties of optimum cumulative inflow curve with general desired departure time distribution curve are deduced and then the analytic solutions for common desired departure time in so are obtained three numerical examples are carried out to capture the characteristics of evening commuting behaviors under different values of time the analytic and numerical results both indicate that the rarefaction wave originating from the first entry point influences the whole or part of the outflow curve no shock wave exists through the commuting process in addition the cost curves show that the trip cost increases and the departure delay cost decreases with departure time whereas the travel time cost first increases then decreases with departure time under the so principle

derivation and empirical validation of a refined traffic flow model abstract the gas kinetic foundation of fluid dynamic traffic equations suggested in previous papers d helbing physica a 219 1995 375 and 391 is further refined by applying the theory of dense gases and granular materials to the boltzmann like traffic model by paveri fontana it is shown that despite the phenomenologically similar behaviour or ordinary and granular fluids the relations for these cannot directly be transferred to vehicular traffic the dissipative and anisotropic interactions of vehicles as well as their velocity dependent space requirements lead to a considerably different structure of the macroscopic traffic equations also in comparison with the previously suggested traffic flow models as a consequence the instability mechanisms of emergent density waves are different crucial assumptions are validated by empirical traffic data and essential results are illustrated by figures

model based direct adjustment abstract direct adjustment or standardization applies population weights to subclass means in an effort to estimate population quantities from a sample that is not representative of the population direct adjustment has several attractive features but when there are many subclasses it can attach large weights to small quantities of data often in a fairly erratic manner in the extreme direct adjustment can attach infinite weight to nonexistent data a noticeable inconvenience in practice this article proposes a method of model based direct adjustment that preserves the attractive features of conventional direct adjustment while stabilizing the weights attached to small subclasses the sample mean and conventional direct adjustment are both special cases of model based direct adjustment under two different extreme models for the subclass specific selection probabilities the discussion of this method provides some insights into the behavior of true and estimated propensity scores the estimated scores

double robust estimator of average causal treatment effect for censored medical cost data in observational studies estimation of average causal treatment effect on a patient s response should adjust for confounders that are associated with both treatment exposure and response in addition the response such as medical cost may have incomplete follow up in this article a double robust estimator is proposed for average causal treatment effect for right censored medical cost data the estimator is double robust in the sense that it remains consistent when either the model for the treatment assignment or the regression model for the response is correctly specified double robust estimators increase the likelihood the results will represent a valid inference asymptotic normality is obtained for the proposed estimator and an estimator for the asymptotic variance is also derived simulation studies show good finite sample performance of the proposed estimator and a real data analysis using the proposed method is provided as illustration copyright 2016 john wiley sons ltd

diffusion in the presence of a pole from the continuous gaussian to a discrete lattice model abstract the continuous gaussian model with delta function potentials which has been used to describe the effects that interacting centers poles bring to the diffusion of a random walker provides an exact solution only in dimension d 1 its application in dimension d 2 is in need of introduction of cutoffs coming from the finite sizes of poles we therefore formulate an analogous discrete model in the d dimensional lattice space where no cutoffs or their regularization are necessary the discrete model is amenable to numerical treatment and the green s function its first moments and properties derived from them can be determined numerically to all dimensionalities in the one dimensional 1 d case in the presence of a single center the outcomes of the gaussian model are compared with the corresponding results of the discrete lattice model the possibility of extension of the lattice model to higher dimension permits the study of the two dimensional 2 d case too the intensities of interactions considered before to obtain positive values are extended to negative values in order to describe attractions also

stability of the car following model on two lanes in the case of two lane traffic vehicle drivers always worry about the lane changing actions from neighbor lane this paper studies the stability of a car following model on two lanes which incorporates the lateral effects in traffic the stability condition of the model is obtained by using the linear stability theory the modified korteweg de vries equation is constructed and solved and three types of traffic flows in the headway sensitivity space stable metastable and unstable are classified both analytical and simulation results show that the anxiousness about lane changing from neighbor lane indeed has influence upon people s driving behavior and the consideration of lateral effects could stabilize the traffic flows on both lanes

an extended car following model considering driver s desire for smooth driving on the curved road abstract in real traffic many driver s desire to lower the fuel consumption by driving smoothly in this paper an extended car following model considering the driver s desire for smooth driving on a curved road is proposed the desire for smooth driving can be considered as a control signal including the velocity difference between the steady and history velocity the stability conditions are obtained by the control theory and the modified korteweg de vries mkdv equation is derived via the non linear analysis method the numerical simulations are carried out to analyze the control signal the friction coefficient and radius of curved road effects on traffic flow the results show that the control signal has a positive effect on improving traffic flow stability while the stability decreases with the increase of the two other parameters

the influence of continuous historical velocity difference information on micro cooperative driving stability abstract in this paper a new micro cooperative driving car following model is proposed to investigate the effect of continuous historical velocity difference information on traffic stability the linear stability criterion of the new model is derived with linear stability theory and the results show that the unstable region in the headway sensitivity space will be shrunk by taking the continuous historical velocity difference information into account through nonlinear analysis the mkdv equation is derived to describe the traffic evolution behavior of the new model near the critical point via numerical simulations the theoretical analysis results are verified and the results indicate that the continuous historical velocity difference information can enhance the stability of traffic flow in the micro cooperative driving process

computation of nonlinear ultrasound fields using a linearized contrast source method nonlinear ultrasound is important in medical diagnostics because imaging of the higher harmonics improves resolution and reduces scattering artifacts second harmonic imaging is currently standard and higher harmonic imaging is under investigation the efficient development of novel imaging modalities and equipment requires accurate simulations of nonlinear wave fields in large volumes of realistic lossy inhomogeneous media the iterative nonlinear contrast source incs method has been developed to deal with spatiotemporal domains measuring hundreds of wavelengths and periods this full wave method considers the nonlinear term of the westervelt equation as a nonlinear contrast source and solves the equivalent integral equation via the neumann iterative solution recently the method has been extended with a contrast source that accounts for spatially varying attenuation the current paper addresses the problem that the neumann iterative solution converges badly for strong contrast sources the remedy is linearization of the nonlinear contrast source combined with application of more advanced methods for solving the resulting integral equation numerical results show that linearization in combination with a bi conjugate gradient stabilized method allows the incs method to deal with fairly strong inhomogeneous attenuation while the error due to the linearization can be eliminated by restarting the iterative scheme

analysis of the numerically implemented angular spectrum approach based on the evaluation of two dimensional acoustic fields part i errors due to the discrete fourier transform and discretization the numerically implemented angular spectrum approach asa is investigated based on the evaluation of the 2 d fields radiated by striplike planar sources and 1 d focusing linear phased arrays with different aperture sizes and on the comparison with those obtained by using the analytical solution derived with the point spread function method since the discrete fourier transform dft of a finite size source produces an angular spectrum with spatial frequency aliasing and discretization of the source in even numbered samples causes a half sample length phase shift error in the angular spectrum the effects of the aliasing and the phase shift on calculation accuracy are dealt with the results show that the frequency aliasing causes an overestimation of the near field and the phase shift distorts the results a numerical algorithm for eliminating the aliasing is proposed and correction of the phase shift is confirmed to be necessary the algorithm proposed can completely remove the errors and obtain exact

research on preceding vehicle s taillight effect and energy consumption in an extended macro traffic model abstract in this paper a modified continuum model considering the vehicle s taillight effect is presented the high order fluid dynamics model of taillight effect is derived by means of the transformation from micro to macro combined with the continuous conservation equation of traffic flow the linear and nonlinear analysis of the new macroscopic hydrodynamic model is carried out the linear analysis proves that the model has the characteristics of heterogeneity and the stability condition of traffic flow is obtained kdv burgers equations describing the qualitative characteristics of density waves are obtained by nonlinear analysis and the corresponding density wave solutions are obtained numerical simulation results show that the unstable state region of the macro model with taillight effect is shrinking in addition the energy consumption of the model is simulated numerically and the results show that the energy consumption of the vehicle is reduced by considering the taillight effect therefore we believe that the taillight effect is beneficial to traffic flow stability and reduce energy consumption

nonlinear analysis for a modified continuum model considering driver s memory and backward looking effect abstract account for the integral form of optimal velocity changes with memory and backward looking effect an extended car following model is proposed in this paper through the relation between macro and micro variables the extended car following is transformed to a new continuum model according to the linear stability theory the linear stability s condition of the presented model is obtained the modified kdv burgers equation is established through non linear analysis to describe the spreading behavior of the traffic density wave near the neutral stability line the numerical simulation results show that the backward looking effect can improve the traffic flow s stability and the integral form of optimal velocity changes with memory can relieve the traffic congestion

propensity score matching with time dependent covariates summary in observational studies with a time dependent treatment and time dependent covariates it is desirable to balance the distribution of the covariates at every time point a time dependent propensity score based on the cox proportional hazards model is proposed and used in risk set matching matching on this propensity score is shown to achieve a balanced distribution of the covariates in both treated and control groups optimal matching with various designs is conducted and compared in a study of a surgical treatment cystoscopy and hydrodistention given in response to a chronic bladder disease interstitial cystitis simulation studies also suggest that the statistical analysis after matching outperforms the analysis without matching in terms of both point and interval estimations

full velocity difference model for a car following theory in this paper we present a full velocity difference model for a car following theory based on the previous models in the literature to our knowledge the model is an improvement over the previous ones theoretically because it considers more aspects in car following process than others this point is verified by numerical simulation then we investigate the property of the model using both analytic and numerical methods and find that the model can describe the phase transition of traffic flow and estimate the evolution of traffic congestion

gas kinetic derivation of navier stokes like traffic equations macroscopic traffic models have recently been severely criticized to base on lax analogies only and to have a number of deficiencies therefore this paper shows how to construct a logically consistent fluid dynamic traffic model from basic laws for the acceleration and interaction of vehicles these considerations lead to the gas kinetic traffic equation of paveri fontana its stationary and spatially homogeneous solution implies equilibrium relations for the fundamental diagram the variance density relation and other quantities which are partly difficult to determine empirically paveri fontana s traffic equation allows the derivation of macroscopic moment equations which build a system of non closed equations this system can be closed by the well proved method of chapman and enskog which leads to euler like traffic equations in zeroth order approximation and to navier stokes like traffic equations in first order approximation the latter are finally corrected for the finite space requirements of vehicles it is shown that the resulting model is able to withstand the above mentioned criticism

delay effect on phase transitions in traffic dynamics a dynamical model of traffic is proposed to take into account the effect of acceleration delay in the limit of no delay the model reproduces the optimal velocity model of traffic when the delay is small it is shown that the phase transition among the freely moving phase the coexisting phase and the uniform congested phase occurs below the critical point above the critical point no phase transition occurs the value a c of the critical point increases with increasing delay time 1 b where a is the friction coefficient or sensitivity parameter when the delay time is longer than frac 1 2 the critical point disappears and the phase transition always occurs the linear stability theory and nonlinear analysis are applied the critical point predicted by the linear stability theory agrees with the simulation result the modified korteweg de vries kdv equation is obtained from the nonlinear analysis near the critical point the phase separation line obtained from the modified kdv equation is consistent with the simulation result