Two-stage hierarchical modeling for analysis of subpopulations in conditional distributions

In this work, we develop modeling and estimation approach for the analysis of cross-sectional clustered data with multimodal conditional distributions where the main interest is in analysis of subpopulations. It is proposed to model such data in a hierarchical model with conditional distributions viewed as finite mixtures of normal components. With a large number of observations in the lowest level clusters, a two-stage estimation approach is used. In the first stage, the normal mixture parameters in each lowest level cluster are estimated using robust methods. Robust alternatives to the maximum likelihood estimation are used to provide stable results even for data with conditional distributions such that their components may not quite meet normality assumptions. Then the lowest level cluster-specific means and standard deviations are modeled in a mixed effects model in the second stage. A small simulation study was conducted to compare performance of finite normal mixture population parameter estimates based on robust and maximum likelihood estimation in stage 1. The proposed modeling approach is illustrated through the analysis of mice tendon fibril diameters data. Analyses results address genotype differences between corresponding components in the mixtures and demonstrate advantages of robust estimation in stage 1.     

Modelling human immunodeficiency virus ribonucleic acid levels with finite mixtures for censored longitudinal data

The measurement of human immunodeficiency virus ribonucleic acid levels over time leads to censored longitudinal data. Suitable models for dynamic modelling of these levels need to take this data characteristic into account. If groups of patients with different developments of the levels over time are suspected the model class of finite mixtures of mixed effects models with censored data is required. We describe the model specification and derive the estimation with a suitable expectation-maximization algorithm. We propose a convenient implementation using closed form formulae for the expected mean and variance of the truncated multivariate distribution. Only efficient evaluation of the cumulative multivariate normal distribution function is required. Model selection as well as methods for inference are discussed. The application is demonstrated on the clinical trial ACTG 315 data.     

新兴工业化中等收入国家的环境执法创新

本文考察了中国、巴西、印尼和墨西哥这几个新兴工业化中等收入国家为了改善环境法律执行状况而在近期采用的五种创新手段。在这五个案例中,当环境执法机关与其他国家机关、民间主体或市场主体联合起来形成执法联盟时,执法机关的监管能力得以扩大。此外,不同类型的主体参与环境执法对于执法的持续性效果具有一定的影响。更重要的是,当被监管企业通过自我检查、自我报告以及标签分类方案而参与环境监管时,监管机关对被监管企业遵守法律的设计方案与守法决心更具有依赖性。研究发现,由于中等收入国家的环境违法行为较为普遍,加上公众和市场对企业和产品的标签分类方案响应能力不足,所以,以降低监管自主性为特点的执法创新在中等收入国家的应用具有更大的风险。     

美国的卡尔梅克人

卡尔梅克人具有史诗《奥德赛》般的历史。早在17纪前的20年间,这个部落的一部分(即士尔扈特)就开始离开祖先的营地向西迁徙,最后流落到了乌拉尔和俄国南部伏尔加河下游的草原地带。之后的一百年内,又有其它几支从卫拉特人中分离出来的卡尔梅克人与之融合。但到1771年1月,为了避免沙皇俄国的强权和同化,他们之中五分之四即17万人集体东移,返回了准噶尔。在1918—1920年俄国革命爆发的内战中,这批继续滞留在伏尔加河西岸的卡尔梅克人受了严重的人员、财物和牲畜的损失。他们中一部分曾反对布尔什维克的人由于俄国革命的胜利,不得不逃离俄国,取道土耳其,分散到法国、南斯拉夫、保加利亚和捷克斯洛伐克。但随着1944年秋到1945年春苏联军队向西欧东部和中欧的迅速推进,那些在保加利亚、南斯拉夫和捷克斯洛伐克的卡尔梅克逃亡者又急忙逃往奥地利、德国和法国。到第二次世界大战接近尾声时,在德国东部和北部的卡尔梅克人便已逃到了美军占领区。     

一种有效计算离散Gabor展开的方法(英文)

定义了一种有限和无限序列的实数形式离散Ｇａｂｏｒ展开方式,这种展开方式使用了类似于广泛应用的复数形式Ｇａｂｏｒ展开方式。但前者由于仅涉及实数运算,结果不仅节省大量的运算,而且更便于硬件和软件的实现。另外,实数形式离散Ｇａｂｏｒ展开方式与复数形式离散Ｇａｂｏｒ展开方式有着非常简单的关系,所以前者可以取代后者的运算。     

Semiparametric transformation models for multivariate panel count data with dependent observation process

This article discusses regression analysis of multivariate panel count data in which the observation process may contain relevant information about or be related to the underlying recurrent event processes of interest. Such data occur if a recurrent event study involves several related types of recurrent events and the observation scheme or process may be subject-specific. For the problem, a class of semiparametric transformation models is presented, which provides a great flexibility for modelling the effects of covariates on the recurrent event processes. For estimation of regression parameters, an estimating equation-based inference procedure is developed and the asymptotic properties of the resulting estimates are established. Also the proposed approach is evaluated by simulation studies and applied to the data arising from a skin cancer chemoprevention trial.     

Prediction based classification for longitudinal biomarkers

Assessment of circulating CD4 count change over time in HIV-infected subjects on antiretroviral therapy (ART) is a central component of disease monitoring. The increasing number of HIV-infected subjects starting therapy and the limited capacity to support CD4 count testing within resource-limited settings have fueled interest in identifying correlates of CD4 count change such as total lymphocyte count, among others. The application of modeling techniques will be essential to this endeavor due to the typically non-linear CD4 trajectory over time and the multiple input variables necessary for capturing CD4 variability. We propose a prediction based classification approach that involves first stage modeling and subsequent classification based on clinically meaningful thresholds. This approach draws on existing analytical methods described in the receiver operating characteristic curve literature while presenting an extension for handling a continuous outcome. Application of this method to an independent test sample results in greater than 98% positive predictive value for CD4 count change. The prediction algorithm is derived based on a cohort of n = 270 HIV-1 infected individuals from the Royal Free Hospital, London who were followed for up to three years from initiation of ART. A test sample comprised of n = 72 individuals from Philadelphia and followed for a similar length of time is used for validation. Results suggest that this approach may be a useful tool for prioritizing limited laboratory resources for CD4 testing after subjects start antiretroviral therapy.     

多效唑对亚热带低寒带桃“Flordaprince”物候循环的影响

Allan，p，George，A．P，Nissen，R．J，Rasmussen，T．S．和Morley－Bunker，M．J，1993。多效吐对亚热带澳大利亚的低寒带桃“Flordaprince”物候循环的影响。园艺科学，53：73-84。将低寒带桃“Flordaprince”种植在亚热带澳大利亚，用多效吐处理两次以矮化树体，或使用氮处理来强化树体。疏花之后，调整并统一单株果实数，使每棵树的果实具有充分空间。多效唑能显著地抑制竞争性的春梢生长，引起果实提早成熟，并使产量增加，果实增大，品质提高。多效吐减少根的春季生长，而导致秋季根的大量生长。在乔化树中，果实生长的第二阶段稍长，使得种子和胚的生长推迟，胚中干物质较多。淀粉储藏量以根中为最大，其次是根干、枝和叶。在硬核期和果实成熟阶段，淀粉储藏量降到最低水平，但在枝梢伸长生长结束之后，又会升高，到落叶时，再次达到峰值。叶分析显示，整个季节中N、P和K的浓度降低，而Ca和Mg的浓度升高。多效唑处理树中，K的水平显著低于乔化树，而Ca、Mg水平显著高于乔化树。     

英国史学研究的新方向

在过去一百多年中,英国的史学经历了两次革命,一次革命已经完成,另一次革命仍在进行中。在十九世纪八十年代之前,英国的历史编纂工作是隶属于非专业人员和那些创作文学名著的文人的学术研究领域。从那时以来,英国的史学研究日益成为专业学者的学术研究领域。在英国,这一专业化的过程是从十九世纪八十年代开始,到二