The Effect of Drop-Out on the Efficiency of Longitudinal Experiments

It is shown that drop-out often reduces the efficiency of longitudinal experiments considerably. In the framework of linear mixed models, a general, computationally simple method is provided, for designing longitudinal studies when drop-out is to be expected, such that there is little risk of large losses of efficiency due to the missing data. All the results are extensively illustrated using data from a randomized experiment with rats.     

An experiment in primary health care provision

Missing data and, more generally, imperfections in implementing a study design are an endemic problem in large scale studies involving human subjects. We present an analysis of an experiment in the interaction between general practitioners and their patients, in which the issue of missing data is addressed by a sensitivity analysis using multiple imputation. Instead of specifying a model for missingness we explore certain extreme ways of departing from the assumption of data missing at random and establish the largest extent of such departures which would still fail to supplant the evidence about the studied effect. An important advantage of the approach is that the algorithm intended for the complete data, to fit generalized linear models with random effects, is used without any alteration.     

从数据垄断走向数据开放：数据成为必需设施的竞争法分析

数字经济下,数据发挥着越来越重要的作用,而拥有大量数据的支配企业却往往拒绝其他经营者对数据的访问,意图垄断数据资源。竞争法有必要通过必需设施规则,对支配企业拒绝访问的行为予以规制,实现数据开放,促进数据资源的流通、利用。结合必需设施规则的发展历程,必需设施规则在数字经济背景下的适用需要有严格的条件限制。这些条件包括:在经营者具有市场支配地位的前提条件下,首先数据需要构成必需设施,即数据是经营者参与市场竞争所必不可少的;其次,支配企业拒绝数据访问的行为所产生的消极效果明显大于积极效果;最后,支配企业拒绝数据访问不存在合理的理由。满足了上述条件后,支配企业才需要承担允许数据访问的义务。在此基础上,竞争法需要针对数据访问的具体要求以及其可能与个人数据保护法、知识产权法之间的关系进行细化,以使数据访问在实践中更具有操作性。具体而言,竞争法对数据访问的要求主要有三点:其一,其他经营者对数据的访问应当是公平的;其二,其他经营者对数据的访问应当是现实可行的;其三,其他经营者对数据进行访问需要支付合理的费用。在竞争法与个人数据保护法之间的关系上,一方面,个人数据保护法在一定程度上阻碍了其他经营者对数据的访问;另一方面,个人数据保护法与竞争法在价值上存在着统一性。在竞争法与知识产权法之间的关系上,大多数情况下,支配企业拒绝其他经营者对受到知识产权保护的数据的访问的行为是合法的,但是支配企业也需注意在合理的范围内行使权利,不得滥用知识产权限制数据访问的实施。在数字经济时代,我国有必要在反垄断法规范体系中纳入必需设施规则,同时在相关的法律规范和指南中对数据访问的具体内容进行设计,加强与个人数据保护、知识产权等相关法律规范的协调,实现数据访问的本土化建构,促进市场竞争,挖掘数据的巨大价值。     

P-value adjustment for multiple binary endpoints

The p-value-based adjustment of individual endpoints and the global test for an overall inference are the two general approaches for the analysis of multiple endpoints. Statistical procedures developed for testing multivariate outcomes often assume that the multivariate endpoints are either independent or normally distributed. This paper presents a general approach for the analysis of multivariate binary data under the framework of generalized linear models. The generalized estimating equations (GEE) approach is applied to estimate the correlation matrix of the test statistics using the identity and exchangeable working correlation matrices with the model-based as well as robust estimators. The objectives of the approaches are the adjustment of p-values of individual endpoints to identify the affected endpoints as well as the global test of an overall effect. A Monte Carlo simulation was conducted to evaluate the overall family wise error (FWE) rates of the single-step down p-value adjustment approach from two adjustment methods to three global test statistics. The p-value adjustment approach seems to control the FWE better than the global approach Applications of the proposed methods are illustrated by analyzing a carcinogenicity experiment designed to study the dose response trend for 10 tumor sites, and a developmental toxicity experiment with three malformation types: external, visceral, and skeletal.     

高速SOI-LIGBT的数值分析

研制出Ｈｅ离子注入局域寿命控制ＳＯＩ横向绝缘栅双极晶体管（ＳＯＩ－ＬＩＧＢＴ），有效地提高了器件的关断速度，且与集成电路工艺相兼容。数值模拟表明，该器件的t_f～V_F折衷关系优于采用非局域寿命控制的器件，在相同关断速度下其正向压降可降低０．６～１．４ Ｖ，避免了阳极短路结构的正向快速返回现象。     

半线性抛物型方程解的熄灭时间

证明半线性抛物型方程柯西问题的解在一定条件之下在有限时刻熄灭 ,并在不同条件之下对解的熄灭时间分别进行估计     

A time series approach to life table construction

This paper deals with the construction of the life table. A discussion of basic facts about the life table is followed by the proposal of a nonstationary, autoregressive model for the life table. The moment structure of the nonstationary, autoregressive model is developed. Some estimation procedures are proposed followed by several examples.     

Smoothed estimates for nonlinear time series models with irregular data

Time series data observed at unequal time intervals (irregular data) occur quite often and this usually poses problems in its analysis. A recursive form of the exponentially smoothed estimated is here proposed for a nonlinear model with irregularly observed data and its asymptotic properties are discussed An alternative smoother to that of Wright (1985) is also derived. Numerical comparison is made between the resulting estimates and other smoothed estimates.     

Testing for lack of fit in linear multiresponse models based on exact or near replicates

Three procedures for testing the adequacy of a proposed linear multiresponse regression model against unspecified general alternatives are considered. The model has an error structure with a matrix normal distribution which allows the vector of responses for a particular run to have an unknown covariance matrix while the responses for different runs are uncorrelated. Furthermore, each response variable may be modeled by a separate design matrix. Multivariate statistics corresponding to the classical univariate lack of fit and pure error sums of squares are defined and used to determine the multivariate lack of fit tests. A simulation study was performed to compare the power functions of the test procedures in the case of replication. Generalizations of the tests for the case in which there are no independent replicates on all responses are also presented.     

Influence of human immunodeficiency virus infection on neurological impairment: an analysis of longitudinal binary data with informative drop-out

A study to investigate the human immunodeficiency virus (HIV) status on the course of neurological impairment, conducted by the HIV Center at Columbia University, followed a cohort of HIV positive and negative gay men for 5 years and assessed the presence or absence of neurological impairment every 6 months. Almost half of the subjects dropped out before the end of the study for reasons that might have been related to the missing neurological data. We propose likelihood-based methods for analysing such binary longitudinal data under informative and non-informative drop-out. A transition model is assumed for the binary response, and several models for the drop-out processes are considered which are functions of the response variable (neurological impairment). The likelihood ratio test is used to compare models with informative and non-informative drop-out mechanisms. Using simulations, we investigate the percentage bias and mean-squared error (MSE) of the parameter estimates in the transition model under various assumptions for the drop-out. We find evidence for informative drop-out in the study, and we illustrate that the bias and MSE for the parameters of the transition model are not directly related to the observed drop-out or missing data rates. The effect of HIV status on the neurological impairment is found to be statistically significant under each of the models considered for the drop-out, although the regression coefficient may be biased in certain cases. The presence and relative magnitude of the bias depend on factors such as the probability of drop-out conditional on the presence of neurological impairment and the prevalence of neurological impairment in the population under study.