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.     

Statistical approaches for conducting network meta-analysis in drug development

We introduce health technology assessment and evidence synthesis briefly, and then concentrate on the statistical approaches used for conducting network meta-analysis (NMA) in the development and approval of new health technologies. NMA is an extension of standard meta-analysis where indirect as well as direct information is combined and can be seen as similar to the analysis of incomplete-block designs. We illustrate it with an example involving three treatments, using fixed-effects and random-effects models, and using frequentist and Bayesian approaches. As most statisticians in the pharmaceutical industry are familiar with SAS? software for analyzing clinical trials, we provide example code for each of the methods we illustrate. One issue that has been overlooked in the literature is the choice of constraints applied to random effects, and we show how this affects the estimates and standard errors and propose a symmetric set of constraints that is equivalent to most current practice. Finally, we discuss the role of statisticians in planning and carrying out NMAs and the strategy for dealing with important issues such as heterogeneity.     

Semiparametric Stochastic Modeling of the Rate Function in Longitudinal Studies

In longitudinal biomedical studies, there is often interest in the rate functions, which describe the functional rates of change of biomarker profiles. This paper proposes a semiparametric approach to model these functions as the realizations of stochastic processes defined by stochastic differential equations. These processes are dependent on the covariates of interest and vary around a specified parametric function. An efficient Markov chain Monte Carlo algorithm is developed for inference. The proposed method is compared with several existing methods in terms of goodness-of-fit and more importantly the ability to forecast future functional data in a simulation study. The proposed methodology is applied to prostate-specific antigen profiles for illustration. Supplementary materials for this paper are available online.     

HOW WELL DO SELECTION MODELS PERFORM? ASSESSING THE ACURACY OF ART AUCTION PRE-SALE ESTIMATES

Art auction catalogs provide a pre-sale prediction interval for the price each item is expected to fetch. When the owner consigns art work to the auction house, a reserve price is agreed upon, which is not announced to the bidders. If the highest bid does not reach it, the item is brought in. Since only the prices of the sold items are published, analysts only have a biased sample to examine due to the selective sale process. Relying on the published data leads to underestimating the forecast error of the pre-sale estimates. However, we were able to obtain several art auction catalogs with the highest bids for the unsold items as well as those of the sold items. With these data we were able to evaluate the accuracy of the predictions of the sale prices or highest bids for all item obtained from the original Heckman selection model that assumed normal error distributions as well as those derived from an alternative model using the t(2) distribution, which yielded a noticeably better fit to several sets of auction data. The measures of prediction accuracy are of more than academic interest as they are used by auction participants to guide their bidding or selling strategy, and similar appraisals are accepted by the US Internal Revenue Services to justify the deductions for charitable contributions donors make on their tax returns.     

Estimating the prevalence of sensitive behaviour and cheating with a dual design for direct questioning and randomized response

Randomized response is a misclassification design to estimate the prevalence of sensitive behaviour. Respondents who do not follow the instructions of the design are considered to be cheating. A mixture model is proposed to estimate the prevalence of sensitive behaviour and cheating in the case of a dual sampling scheme with direct questioning and randomized response. The mixing weight is the probability of cheating, where cheating is modelled separately for direct questioning and randomized response. For Bayesian inference, Markov chain Monte Carlo sampling is applied to sample parameter values from the posterior. The model makes it possible to analyse dual sample scheme data in a unified way and to assess cheating for direct questions as well as for randomized response questions. The research is illustrated with randomized response data concerning violations of regulations for social benefit.     

交通模型网络分配技术

拥挤的城市道路网络的建模工作一直是世界各国研究的课题．各种方法的准确程度也不相同,一个基本的方面就是在追求战略规划目的的同时取得成效,和在准确地表示地域范围内交通运转情况需要之间寻求一种平衡．在许多城市中,需要能够同时评价这两者的灵活的模型．OveArup＆Partners最近已经开发并成功地运用了一个EMME／2模型对奥克兰市、新西兰市做了交通项目估计,同时具体表达了交汇处的延误．这部分的前期工作是由ROHill所承担的,此模型的研究和开发强调了模型中出现的许多重要问题．本文讨论了与拥挤分配模型开发有关的问题,阐述了建立城市范围内的模型的方法．内容包括：a．分配方法;b．稳定性和收敛性;c．城市范围模型的具体水平;d．联系和转弯惩罚函数;e．在没有信号灯的交叉口的延误函数．     

马克思《资本论》的方法与逻辑问题

列宁在其黑格尔《逻辑学》一书摘要中经常把这一本书同《资本论》加以比较。这种比较对研究《资本论》的逻辑问题,乃至对研究马克思列宁主义的逻辑学,都具有重大意义。将《精神现象学》同《资本论》加以比较,也是这样。 在《资本论》中,也和在《精神现象学》中一样,从具体上升到抽象和从抽象上升到具体这两种上升方法,不是彼此对立的,而是一个统一的认识方法。     

LET''''S MAKE ENGLISH MORE EXCITING

"Teacher, what are we going to do next class?" This is an exciting question which I often hear from my students. As an English conversation teacher, I can make the learning of English either a dull and boring duty, or an exciting adventure. It is very important that I make my students hate missing my class, and keep them guessing just what will happen the next time! Let us consider some important ways that any teacher can help his other students want to do their     

培养未来教师的语言表达能力

作者在文中对哈萨克语表达方面的正确性、词汇性、准确性、逻辑性等进行了研究,并总结出了形成语言表达能力的科学结论。     

政治决策实施过程中的价值冲突

在政治决策过程中会遇到不可回避的原则冲突。不同原则由不同的道德理论支撑,诸如实用主义或专制主义。政治决策的实施力度与当代社会发展同步增加。探讨在公共社会生活中道德理论实施的前景和可能,是十分重要的。