Estimation with Interval Censored Data and Covariates

In biostatistical applications interest often focuses on the estimation of the distribution of time T between two consecutive events. If the initial event time is observed and the subsequent event time is only known to be larger or smaller than an observed monitoring time C, then the data conforms to the well understood singly-censored current status model, also known as interval censored data, case I. Additional covariates can be used to allow for dependent censoring and to improve estimation of the marginal distribution of T. Assuming a wrong model for the conditional distribution of T, given the covariates, will lead to an inconsistent estimator of the marginal distribution. On the other hand, the nonparametric maximum likelihood estimator of FT requires splitting up the sample in several subsamples corresponding with a particular value of the covariates, computing the NPMLE for every subsample and then taking an average. With a few continuous covariates the performance of the resulting estimator is typically miserable. In van der Laan, Robins (1996) a locally efficient one-step estimator is proposed for smooth functionals of the distribution of T, assuming nothing about the conditional distribution of T, given the covariates, but assuming a model for censoring, given the covariates. The estimators are asymptotically linear if the censoring mechanism is estimated correctly. The estimator also uses an estimator of the conditional distribution of T, given the covariates. If this estimate is consistent, then the estimator is efficient and if it is inconsistent, then the estimator is still consistent and asymptotically normal. In this paper we show that the estimators can also be used to estimate the distribution function in a locally optimal way. Moreover, we show that the proposed estimator can be used to estimate the distribution based on interval censored data (T is now known to lie between two observed points) in the presence of covariates. The resulting estimator also has a known influence curve so that asymptotic confidence intervals are directly available. In particular, one can apply our proposal to the interval censored data without covariates. In Geskus (1992) the information bound for interval censored data with two uniformly distributed monitoring times at the uniform distribution (for T has been computed. We show that the relative efficiency of our proposal w.r.t. this optimal bound equals 0.994, which is also reflected in finite sample simulations. Finally, the good practical performance of the estimator is shown in a simulation study. This revised version was published online in July 2006 with corrections to the Cover Date.     

The EM Algorithm—an Old Folk-song Sung to a Fast New Tune

Celebrating the 20th anniversary of the presentation of the paper by Dempster, Laird and Rubin which popularized the EM algorithm, we investigate, after a brief historical account, strategies that aim to make the EM algorithm converge faster while maintaining its simplicity and stability (e.g. automatic monotone convergence in likelihood). First we introduce the idea of a 'working parameter' to facilitate the search for efficient data augmentation schemes and thus fast EM implementations. Second, summarizing various recent extensions of the EM algorithm, we formulate a general alternating expectation–conditional maximization algorithm AECM that couples flexible data augmentation schemes with model reduction schemes to achieve efficient computations. We illustrate these methods using multivariate t -models with known or unknown degrees of freedom and Poisson models for image reconstruction. We show, through both empirical and theoretical evidence, the potential for a dramatic reduction in computational time with little increase in human effort. We also discuss the intrinsic connection between EM-type algorithms and the Gibbs sampler, and the possibility of using the techniques presented here to speed up the latter. The main conclusion of the paper is that, with the help of statistical considerations, it is possible to construct algorithms that are simple, stable and fast.     

Patches of grief and rage: Visitor responses to the NAMES project AIDS Memorial Quilt

This paper presents an analysis of comments written on signature square panels of the NAMES Project AIDS Memorial Quilt. In our paper we explore how collective memory regarding AIDS deaths emerges from conflict among members of vernacular, mainstream and official culture. The cultural contest waged over how individuals who have died from AIDS-related illnesses will be remembered in society fits within the realm of the sociology of collective memory. Visitor responses to the Quilt illustrate that as a form of commemoration the Quilt is interpreted as a therapeutic device and a political tool. Like other memorials, especially those for which there is social disdain or conflict (e.g., the Vietnam Veteran's Memorial), signature square comments provide an alternative conception of public memory. The fact that the Quilt is used to symbolically commemorate the dead, providing an outlet for collective grieving and inspiring social action, is what makes it an important sociological phenomenon. In our paper we put forward a conception of the politics of collective memory that extends previous work in this area. In doing so this paper contributes to an emerging understanding of public commemoration and to the debate over how different segments of society are to be remembered.     

Recurrent events analysis in the presence of time-dependent covariates and dependent censoring

Summary.  Recurrent events models have had considerable attention recently. The majority of approaches show the consistency of parameter estimates under the assumption that censoring is independent of the recurrent events process of interest conditional on the covariates that are included in the model. We provide an overview of available recurrent events analysis methods and present an inverse probability of censoring weighted estimator for the regression parameters in the Andersen–Gill model that is commonly used for recurrent event analysis. This estimator remains consistent under informative censoring if the censoring mechanism is estimated consistently, and it generally improves on the naïve estimator for the Andersen–Gill model in the case of independent censoring. We illustrate the bias of ad hoc estimators in the presence of informative censoring with a simulation study and provide a data analysis of recurrent lung exacerbations in cystic fibrosis patients when some patients are lost to follow-up.     

争取食品权的国际人权

在人权中具体讨论人类有关食品的权利有利于促进了人权理论在学术界的发展(Craven,1995年,第110-114页)。对此,本篇论文首先介绍了有关人权法学理论的一般发展状况,并在此基础上重点强调了食品权利的发展以及两者之间的相互作用。此外,人权发展不仅体现出国际特色、国家特色,也因为欧盟这些区域组织的作用而颇具区域特色,为此本文也论述了欧盟对于人权和食品权的落实和保障情况。     

A versatile bivariate distribution on a bounded domain: another look at the product moment correlation

The Farlie-Gumbel-Morgenstern (FGM) family has been investigated in detail for various continuous marginals such as Cauchy, normal, exponential, gamma, Weibull, lognormal and others. It has been a popular model for the bivariate distribution with mild dependence. However, bivariate FGMs with continuous marginals on a bounded support discussed in the literature are only those with uniform or power marginals. In this paper we study the bivariate FGM family with marginals given by the recently proposed two-sided power (TSP) distribution. Since this family of bounded continuous distributions is very flexible, the properties of the FGM family with TSP marginals could serve as an indication of the structure of the FGM distribution with arbitrary marginals defined on a compact set. A remarkable stability of the correlation between the marginals has been observed.     

Sexual inactivity among men who have sex with men living with hiv in europe

Objectives: Most people living with HIV stay sexually active, but some remain inactive. This study investigated prevalence and correlates of sexual inactivity among European HIV-positive men who have sex with men (MSM). Methods: An anonymous sexual health questionnaire was distributed in 17 HIV outpatient clinics throughout Europe. Ninety-seven (11.6%) of 838 respondents reported absence of sexual activity. Results: Multivariable regression analysis identified older age, relationship status, and less satisfaction with general health and sexual desire as being associated with sexual inactivity. Conclusions: Prevalence of sexual inactivity has declined since the introduction of antiretroviral therapy to levels similar to those of HIV-negative MSM, suggesting a tendency toward normalization of HIV-positive MSM's sex lives.     

Economic evaluation of brief sex counseling for sexually dysfunctional adolescents

Objectives: In a randomized controlled trial brief sex counselling (BSC), intensive sexual healthcare (ISH) and no treatment (NT) for adolescents with a sexual dysfunction were compared. The aim of this study was to assess the cost-effectiveness and cost-utility of BSC versus ISH and NT from a societal perspective. Methods: Costs, sexual functioning and quality of life were measured during 6 months. Primary outcome measures were measured with the Female Sexual Functional Index, the International Index of Erectile Function and the utilities reflecting Quality of Life based on the SF-36. Uncertainty was dealt with by using bootstrap replications and sensitivity analyses. Results: Results show that the societal costs were the highest for ISH followed by NT and BSC. The difference in costs between ISH compared to NT and BSC was significant. Furthermore, there were no significant group differences in sexual functioning or quality of life. With respect to the cost-effectiveness and cost-utility, BSC can be considered to be a suitable treatment for adolescents with a sexual dysfunction. Conclusions: Due to the lack of a significant difference in clinical effect, it can be concluded that BSC is the cheapest treatment option when implementing the intention-to-treat, besides a no treatment option, for adolescents with a sexual dysfunction.     

Increasing parental involvement of English Language Learner families: What the research says

The nearly 10 million English Language Learners (ELLs) represent the fastest-growing segment of the US's public school student population. While research continually finds that ELL parents, generally speaking, place a high value on their children's education, many immigrant, refugee, and ELL parents experience their relationships with their children's schools very differently from mainstream English-speaking families. Schools often struggle to meet the unique instructional and linguistic needs of these students, and communities with large ELL populations face the additional challenge of communicating with parents, who may have limited fluency in English and comparatively low levels of literacy in their native languages [Arias, M. B., and M. Morillo-Campbell. 2008. Promoting ELL Parental Involvement: Challenges in Contested Times. Education Policy Research Unit. http://epsl.asu.edu/epru/documents/EPSL-0801-250-EPRU.pdf.]. Additionally, immigrant and ELL parents may have had negative experiences with educational institutions or less exposure to formal schooling. Thus, for schools to increase parental involvement most effectively, both traditional and nontraditional approaches to family engagement must be implemented within practices that are culturally and linguistically appropriate. This article provides an overview of the barriers that limit ELL parental involvement, recommends strategies that promote family engagement, and concludes with a case study of one public school district's successful outreach efforts.