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Analyzing Longitudinal Data with Informative Observation and Terminal Event Times

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Acta Mathematicae Applicatae Sinica 2016年第4期32卷 1035-1052页

作者： Rui MIAO Xin CHEN liu-quan sun Institute of Applied Mathematics Academy of Mathematics and Systems ScienceChinese Academy of Sciences

Longitudinal data often arise when subjects are followed over a period of time, and in many situations, there may exist informative observation times and a dependent terminal event such as death that stops the follow-up. In this article, we propose joint modeling and analysis of longitudinal data with possibly informative observation times and a dependent terminal event in which a common subject-specific latent variable is used to characterize the correlations. A borrow-strength estimation procedure is developed for parameter estimation, and both large-sample and finite^sample properties of the proposed estimators are established. In addition, some goodness-of-fit methods for assessing the adequacy of the model are provided. An application to a bladder cancer study is illustrated.

关键词： borrow-strength method frailty model informative observation times joint modeling longitudi-nal data terminal event

Adjusted Log-rank Test with Double Inverse Weighting under Dependent Censoring

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Acta Mathematica Sinica,English Series 2021年第10期37卷 1573-1585页

作者： Yi Min GUO Jie ZHOU liu Quan sun Institute of Applied Mathematics Academy of Mathematics and Systems ScienceChinese Academy of SciencesBeijing 100190P.R.China University of Chinese Academy of Sciences Beijing 100049P.R.China School of Mathematics of Sciences Capital Normal UniversityBeijing 100048P.R.China

It is a common issue to compare treatment-specific survival and the weighted log-rank test is the most popular method for group comparison. However, in observational studies, treatments and censoring times are usually not independent, which invalidates the weighted log-rank tests. In this paper, we propose adjusted weighted log-rank tests in the presence of non-random treatment assignment and dependent censoring. A double-inverse weighted technique is developed to adjust the weighted log-rank tests. Specifically, inverse probabilities of treatment and censoring weighting are involved to balance the baseline treatment assignment and to overcome dependent censoring, respectively. We derive the asymptotic distribution of the proposed adjusted tests under the null hypothesis, and propose a method to obtain the critical values. Simulation studies show that the adjusted log-rank tests have correct sizes whereas the traditional weighted log-rank tests may fail in the presence of non-random treatment assignment and dependent censoring. An application to oropharyngeal carcinoma data from the Radiation Therapy Oncology Group is provided for illustration.

关键词： Adjusted log-rank test dependent censoring double inverse weighting survival data

Estimating Differences in Restricted Mean Lifetime Using Additive Hazards Models under Dependent Censoring

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Acta Mathematicae Applicatae Sinica 2021年第1期37卷 17-34页

作者： Qun LI Bao-xue ZHANG liu-quan sun School of Statistics Capital University of Economics and BusinessBeijing 100070China Institute of Applied Mathematics Academy of Mathematics and Systems ScienceChinese Academy of SciencesBeijing 100190China

In epidemiological and clinical studies,the restricted mean lifetime is often of direct interest *** differences of this quantity can be used as a basis of comparing several treatment groups with respect to their survival *** the factor of interest is not randomized and lifetimes are subject to both dependent and independent censoring,the imbalances in confounding factors need to be *** use the mixture of additive hazards model and inverse probability of censoring weighting method to estimate the differences of restricted mean *** average causal effect is then obtained by averaging the differences in fitted values based on the additive hazards *** asymptotic properties of the proposed method are also derived and simulation studies are conducted to demonstrate their finite-sample *** application to the primary biliary cirrhosis(PBC)data is illustrated.

关键词： additive hazards model dependent censoring inverse probability censoring weighting mean lifetime

Semiparametric Analysis of Longitudinal Data with Informative Observation Times

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Acta Mathematicae Applicatae Sinica 2011年第1期27卷 29-42页

作者： liu-quan sun Xiao-yun Mu Zhi-hua sun Xing-wei Tong Academy of Mathematics and Systems Science Chinese Academy of Sciences Beijing 100190 China School of mathematical Sciences Beijing Normal University Beijing 100875 China

In many longitudinal studies, observation times as well as censoring times may be correlated with longitudinal responses. This paper considers a multiplicative random effects model for the longitudinal response where these correlations may exist and a joint modeling approach is proposed via a shared latent variable. For inference about regression parameters, estimating equation approaches are developed and asymptotic properties of the proposed estimators are established. The finite sample behavior of the methods is examined through simulation studies and an application to a data set from a bladder cancer study is provided for illustration.

关键词： Estimating equations Informative observation times Joint modeling Latent variables,Longitudinal data

Joint channel and carrier frequency offset estimation for multi-user MIMO-OFDM uplink transmissions

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The Journal of China Universities of Posts and Telecommunications 2008年第1期15卷 75-79页

作者： LI Jing liu Pei-zhi sun Hao Wireless Technology Innovation Institute and Key Laboratory of Universal Wireless Communications Ministry of Education Beijing University of Posts and Telecommunications Beijing 100876 China

This article proposes a new algorithm of joint channel and carrier frequency-offset OCCFO） estimation for multi-user multi-input and multi-output orthogonal frequency division multiplexing （MIMO-OFDM） systems. A least square （LS） channel estimation and a carrier frequency offset （CFO） correlation estimation are combined in this contribution. CFOs are generally estimated using training sequences in a special synchronization timeslot. In this contribution, CFO estimation is further improved by taking advantages of channel estimation based on pilot symbols in traffic timeslots. The CFOs can be first obtained from the primary channel estimation. And then, with the knowledge of the CFOs estimated, channel estimation can be enhanced greatly. Computer simulation results indicate that the proposed JCCFO scheme is of good performance. Besides, the computational complexity is low.

关键词： multi-user MIMO-OFDM joint channel and carrier frequency offset estimation least square correlation

Structure and Performance Changes of Ni-Co-Al Shape Memory Alloys in Relation to Co/Al Atomic Ratio

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Journal of Iron and Steel Research International 2015年第7期22卷 652-656页

作者： Jia JU Feng XUE liu-xia sun School of Materials Science and Engineering Southeast University

As a potential ferromagnetic shape memory alloy, Ni-Co-Al has excellent mechanical properties, large ma gentic-field-induced strain and high martensitic transformation temperature. The relationship between microstructure and performance （mechanical and magnetic properties） of Ni-Co-Al with different Co/Al atomic ratios （RCo/Al） was investigated. Samples exhibit β and γ dual-phase structure. The γ phase grows coarse and the volume fraction of γ phase increases with the rise of RCo/Al. Besides, sample with high amount of γ phase content has smaller βgrains ow- ing to the pinning effect of γ phase. The martensite, transformed from β phase, is tetragonal L10 structure with a （111） twinning plane. The martensitic transformation temperature of samples ascends with increasing R co/Al owing to more Co embedded into the cell, which makes the valence electron concentration （e/a） of system rise. The satura- tion magnetization （Ms） of samples increases as Rco/Al rises because Co-rich 7 phase has excellent magnetic property. Meanwhile, both compressive and micro-hardness tests reveal that the samples containing more γ phase have excel- lent ductility due to the intrinsic good ductility nature of γ phase.

关键词： microstructure Co/Al atomic ratio martensitic transformation magnetic property Ni-Co-Al

A Class of Weighted Estimators for Additive Hazards Model in Case-cohort Studies

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Acta Mathematicae Applicatae Sinica 2014年第4期30卷 1153-1168页

作者： Cai-lin DONG Jie ZHOU liu-quan sun School of Mathematics and Statistics Huazhong Normal University Academy of Mathematics and Systems Science Chinese Academy of Sciences School of Mathematics Capital Normal University

Case-cohort sampling is a commonly used and efficient method for studying large cohorts. In many situations, some covariates are easily measured on all cohort subjects, and surrogate measurements of the expensive covariates also may be observed. In this paper, to make full use of the covariate data collected outside the case-cohort sample, we propose＇a class of weighted estimators with general time-varying weights for the additive hazards model, and the estimators are shown to be consistent and asymptotically normal. We also identify the estimator within this class that maximizes efficiency, and simulation studies show that the efficiency gains of the proposed estimator over the existing ones can be substantial in practical situations. A real example is provided.

关键词： additive hazards case-cohort study stratified sampling survival data two-phase design

Stable Ultrahigh Specific Capacitance of NiO Nanorod Arrays

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Nano Research 2011年第7期4卷 658-665页

作者： Zhiyi Lu Zheng Chang Junfeng liu Xiaoming sun State Key Laboratory of Chemical Resource Engineering P. O. Box 98 Beijing University of Chemical Technology Beijing 100029China

Previously reported examples of electrochemical pseudocapacitors based on cheap metal oxides have suffered from the need to compromise between specific capacitance, rate capacitance, and reversibility. Here we show that NiO nanorod arrays on Ni foam have a combination of ultrahigh specific capacitance （2018 F/g at 2.27 A/g）, high power density （1536 F/g at 22.7 A/g）, and good cycling stability （only 8% of capacitance was lost in the first 100 cycles with no further change in the subsequent 400 cycles）. This resulted in an improvement in the reversible capacitance record for NiO by 50% or more, reaching 80% of the theoretical value, and demonstrated that a three-dimensional regular porous array structure can afford all of these virtues in a supercapacitor. The excellent performance can be attributed to the slim （〈 20 nm） rod morphology, high crystallinity, regularly aligned array structure and strong bonding of the nanorods to the metallic Ni substrate, as revealed by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction （XRD）.

关键词： Pseudocapacitance cycling stability NiO array nanorod

Photocatalytic Oxidative Desulfurization of Gasoline by TiO_2 in[BMIm]Cu_2Cl_3 Ionic Liquid

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China Petroleum Processing & Petrochemical Technology 2008年第4期10卷 53-57页

作者： Li Fatang liu Ruihong sun Zhimin College of Science Hebei University of Science and Technology Shijiazhuang 050018

Photocatalytic oxidative desulfurization of gasoline in [BMIm]Cu2Cl3 ionic liquid was studied. A 500-W high-pressure mercury lamp was used as the light source for irradiation, nano-TiO2 was used as the photocatalyst and air was introduced by a gas pump to supply O2 as the oxidant. Influence of the ratio of V（ionic liquid） to V（oil） and the TiO2 addition on the desulfurization rate of gasoline was investigated. An oxidative kinetics equation was founded. The results showed that the [BMIm]CUECl3 ionic liquid was an effective extractant for the desulfurization of gasoline. The appropriate TiO2 addition was 0.05 g in 50 mL of reaction mixture. The yield of desulfurized gasoline could reach 98.2% after being subjected to reaction for 2 h under the conditions of adopting a ratio of V（ionic liquid）： V（oil）=1：4, an air flow of 100 mL/min and a TiO2 addition dosage of 0.05 g. The kinetics reaction for photo-oxidation of gasoline was a first-order reaction with an apparent rate constant of 1.9664 h^-1 and a half-time of 0.3525 h.

关键词： desulfrization [BMIm]Cu2Cl3 ionic liquid gasoline photo-oxidation TiO2 kinetics