In numerical simulation of wave propagation,both viscoelastic materials and perfectly matched layers(PMLs)attenuate waves.The wave equations for both the viscoelastic model and the PML contain convolution operators.Ho...In numerical simulation of wave propagation,both viscoelastic materials and perfectly matched layers(PMLs)attenuate waves.The wave equations for both the viscoelastic model and the PML contain convolution operators.However,convolution operator is intractable in finite-difference time-domain(FDTD)method.A great deal of progress has been made in using time stepping instead of convolution in FDTD.To incorporate PML into viscoelastic media,more memory variables need to be introduced,which increases the code complexity and computation costs.By modifying the nonsplitting PML formulation,I propose a viscoelastic model,which can be used as a viscoelastic material and/or a PML just by adjusting the parameters.The proposed viscoelastic model is essentially equivalent to a Maxwell model.Compared with existing PML methods,the proposed method requires less memory and its implementation in existing finite-difference codes is much easier.The attenuation and phase velocity of P-and S-waves are frequency independent in the viscoelastic model if the related quality factors(Q)are greater than 10.The numerical examples show that the method is stable for materials with high absorption(Q=1),and for heterogeneous media with large contrast of acoustic impedance and large contrast of viscosity.展开更多
倾向得分匹配-双重差分模型(PSM⁃DID)是政策评估及因果推断中最为流行的方法之一.但是在实际应用中,该方法面临着控制变量在处理组样本和控制组样本之间非平衡性的挑战.传统基于均值差异t检验的平衡性检验容易产生片面和误导性的结论,...倾向得分匹配-双重差分模型(PSM⁃DID)是政策评估及因果推断中最为流行的方法之一.但是在实际应用中,该方法面临着控制变量在处理组样本和控制组样本之间非平衡性的挑战.传统基于均值差异t检验的平衡性检验容易产生片面和误导性的结论,使得后续因果推断产生偏误.为克服上述问题,本文对传统的平衡性检验提出以下改进:一是推荐更全面的多维度的平衡性测度指标,便于在匹配后更严谨地比较处理组和控制组的平衡性;二是提出了适用于非平衡样本的新估计方法:倾向得分匹配-逆概率加权-双重差分(PSM⁃IPW⁃DID),该方法结合了倾向得分匹配(PSM)克服样本自选择内生性及对非平衡样本稳健的优势和逆概率加权(inverse probability weighting,IPW)利用全样本信息的长处,在不进一步删除样本的情况下得到一种更稳健的双重差分估计方法.数据模拟和应用实例显示,本文提出的新方法能更全面、客观地评价宏观、微观政策的作用,得到更为可信的因果推断.展开更多
Background and Aims:Liver transplantation(LT)using ABO-incompatible(ABOi)grafts can extend the donor pool to a certain extent and hence reduce the waiting time for transplantation.However,concerns of the impending pro...Background and Aims:Liver transplantation(LT)using ABO-incompatible(ABOi)grafts can extend the donor pool to a certain extent and hence reduce the waiting time for transplantation.However,concerns of the impending prognosis associated with this option,especially for patients with liver failure and higher model for end-stage liver disease(MELD)scores,who tend to be more fragile during the waiting period before LT.Methods:Recipients undergoing LT for acute-onchronic liver failure or acute liver failure were retrospectively enrolled at four institutions.Overall survival was compared and a Cox regression analysis was performed.Propensity score matching was performed for further comparison.Patients were stratified by MELD score and cold ischemia time(CIT)to determine the subgroups with survival benefits.Results:Two hundred ten recipients who underwent ABOi LT and 1,829 who underwent ABO compatible(ABOc)LT were enrolled.The 5-year overall survival rate was significantly inferior in the ABOi group compared with the ABOc group after matching(50.6%vs.75.7%,p<0.05).For patients with MELD scores≤30,using ABOi grafts achieved a comparable overall survival rate as using ABOc grafts(p>0.05).Comparison of the survival rates revealed no statistically significant difference for patients with MELD scores≥40(p>0.05).For patients with MELD scores of 31-39,the overall survival rate was significantly inferior in the ABOi group compared with the ABOc group(p<0.001);however,the rate was increased when the liver graft CIT was<8 h.Conclusions:For recipients with MELD scores≤30,ABOi LT had a prognosis comparable to that of ABOc LT and can be regarded as a feasible option.For recipients with MELD scores≥40,ABOi should be adopted with caution in emergency cases.For recipients with MELD scores of 31-39,the ABOi LT prognosis was worse.However,those patients benefited from receiving ABOi grafts with a CIT of<8 h.展开更多
文摘In numerical simulation of wave propagation,both viscoelastic materials and perfectly matched layers(PMLs)attenuate waves.The wave equations for both the viscoelastic model and the PML contain convolution operators.However,convolution operator is intractable in finite-difference time-domain(FDTD)method.A great deal of progress has been made in using time stepping instead of convolution in FDTD.To incorporate PML into viscoelastic media,more memory variables need to be introduced,which increases the code complexity and computation costs.By modifying the nonsplitting PML formulation,I propose a viscoelastic model,which can be used as a viscoelastic material and/or a PML just by adjusting the parameters.The proposed viscoelastic model is essentially equivalent to a Maxwell model.Compared with existing PML methods,the proposed method requires less memory and its implementation in existing finite-difference codes is much easier.The attenuation and phase velocity of P-and S-waves are frequency independent in the viscoelastic model if the related quality factors(Q)are greater than 10.The numerical examples show that the method is stable for materials with high absorption(Q=1),and for heterogeneous media with large contrast of acoustic impedance and large contrast of viscosity.
文摘倾向得分匹配-双重差分模型(PSM⁃DID)是政策评估及因果推断中最为流行的方法之一.但是在实际应用中,该方法面临着控制变量在处理组样本和控制组样本之间非平衡性的挑战.传统基于均值差异t检验的平衡性检验容易产生片面和误导性的结论,使得后续因果推断产生偏误.为克服上述问题,本文对传统的平衡性检验提出以下改进:一是推荐更全面的多维度的平衡性测度指标,便于在匹配后更严谨地比较处理组和控制组的平衡性;二是提出了适用于非平衡样本的新估计方法:倾向得分匹配-逆概率加权-双重差分(PSM⁃IPW⁃DID),该方法结合了倾向得分匹配(PSM)克服样本自选择内生性及对非平衡样本稳健的优势和逆概率加权(inverse probability weighting,IPW)利用全样本信息的长处,在不进一步删除样本的情况下得到一种更稳健的双重差分估计方法.数据模拟和应用实例显示,本文提出的新方法能更全面、客观地评价宏观、微观政策的作用,得到更为可信的因果推断.
基金This research was partially supported by National Natural Science Funds for Distinguished Young Scholar of China,(No.81625003)Key Program,National Natural Science Foundation of China,(No.81930016,No.81570589,No.81702858)+1 种基金Youth Program of National Natural Science Foundation of Zhejiang Province(No.LQ17H160006)National S&T Major Project(No.2017ZX10203205).
文摘Background and Aims:Liver transplantation(LT)using ABO-incompatible(ABOi)grafts can extend the donor pool to a certain extent and hence reduce the waiting time for transplantation.However,concerns of the impending prognosis associated with this option,especially for patients with liver failure and higher model for end-stage liver disease(MELD)scores,who tend to be more fragile during the waiting period before LT.Methods:Recipients undergoing LT for acute-onchronic liver failure or acute liver failure were retrospectively enrolled at four institutions.Overall survival was compared and a Cox regression analysis was performed.Propensity score matching was performed for further comparison.Patients were stratified by MELD score and cold ischemia time(CIT)to determine the subgroups with survival benefits.Results:Two hundred ten recipients who underwent ABOi LT and 1,829 who underwent ABO compatible(ABOc)LT were enrolled.The 5-year overall survival rate was significantly inferior in the ABOi group compared with the ABOc group after matching(50.6%vs.75.7%,p<0.05).For patients with MELD scores≤30,using ABOi grafts achieved a comparable overall survival rate as using ABOc grafts(p>0.05).Comparison of the survival rates revealed no statistically significant difference for patients with MELD scores≥40(p>0.05).For patients with MELD scores of 31-39,the overall survival rate was significantly inferior in the ABOi group compared with the ABOc group(p<0.001);however,the rate was increased when the liver graft CIT was<8 h.Conclusions:For recipients with MELD scores≤30,ABOi LT had a prognosis comparable to that of ABOc LT and can be regarded as a feasible option.For recipients with MELD scores≥40,ABOi should be adopted with caution in emergency cases.For recipients with MELD scores of 31-39,the ABOi LT prognosis was worse.However,those patients benefited from receiving ABOi grafts with a CIT of<8 h.
