On a tobit–Birnbaum–Saunders model with an application to medical data

The tobit model allows a censored response variable to be described by covariates. Its applications cover different areas such as economics, engineering, environment and medicine. A strong assumption of the standard tobit model is that its errors follow a normal distribution. However, not all applications are well modeled by this distribution. Some efforts have relaxed the normality assumption by considering more flexible distributions. Nevertheless, the presence of asymmetry could not be well described by these flexible distributions. A real-world data application of measles vaccine in Haiti is explored, which confirms this asymmetry. We propose a tobit model with errors following a Birnbaum–Saunders (BS) distribution, which is asymmetrical and has shown to be a good alternative for describing medical data. Inference based on the maximum likelihood method and a type of residual are derived for the tobit–BS model. We perform global and local influence diagnostics to assess the sensitivity of the maximum likelihood estimators to atypical cases. A Monte Carlo simulation study is carried out to empirically evaluate the performance of these estimators. We conduct a data analysis for the mentioned application of measles vaccine based on the proposed model with the help of the R software. The results show the good performance of the tobit–BS model.     

Bootstrap tests for time varying cointegration

This article proposes wild and the independent and identically distibuted (i.i.d.) parametric bootstrap implementations of the time-varying cointegration test of Bierens and Martins (2010 Bierens, H. J., Martins, L. F. (2010). Time varying cointegration. Econometric Theory 26:1453–1490.[Crossref], [Web of Science ®] , [Google Scholar]). The bootstrap statistics and the original likelihood ratio test share the same first-order asymptotic null distribution. Monte Carlo results suggest that the bootstrap approximation to the finite-sample distribution is very accurate, in particular for the wild bootstrap case. The tests are applied to study the purchasing power parity hypothesis for twelve Organisation for Economic Cooperation and Development (OECD) countries and we only find evidence of a constant long-term equilibrium for the U.S.–U.K. relationship.     

Two-phase outcome-dependent studies for failure times and testing for effects of expensive covariates

Two- or multi-phase study designs are often used in settings involving failure times. In most studies, whether or not certain covariates are measured on an individual depends on their failure time and status. For example, when failures are rare, case–cohort or case–control designs are used to increase the number of failures relative to a random sample of the same size. Another scenario is where certain covariates are expensive to measure, so they are obtained only for selected individuals in a cohort. This paper considers such situations and focuses on cases where we wish to test hypotheses of no association between failure time and expensive covariates. Efficient score tests based on maximum likelihood are developed and shown to have a simple form for a wide class of models and sampling designs. Some numerical comparisons of study designs are presented.     

‘Drink a 12 box before you go’: pre-loading among young people in Aotearoa New Zealand

The practice of pre-loading—drinking large amounts of alcohol rapidly in private spaces prior to socialising in the night-time economy—has come to notice recently in the study of alcohol-related harm, but no studies have explored these phenomena in Aotearoa New Zealand. We used a theoretical framework developed with public health alcohol studies for understanding drinking cultures that conceptualises patterns of behaviours as arising within a dynamic interaction between forces of hedonism, function and control. We report findings from 34 focus groups conducted with 18–25 year olds as part of a project supported by the Marsden Fund, between 2011 and 2012, to investigate drinking cultures among young people. Our thematic analyses of participants’ accounts of pre-loading show that the term is in common use, applying to a range of practices motivated by price of alcohol but influenced by the pleasures of intoxication, the importance of peer processes and certain aspects of the regulatory system. We conclude with a discussion of the usefulness of the framework and the implications of the findings for public health policy that aims to reduce alcohol consumption and the harm that arises from it.     

A Hybrid Geometric Phase II/III Clinical Trial Design based on Treatment Failure Time and Toxicity

The problem of comparing several experimental treatments to a standard arises frequently in medical research. Various multi-stage randomized phase II/III designs have been proposed that select one or more promising experimental treatments and compare them to the standard while controlling overall Type I and Type II error rates. This paper addresses phase II/III settings where the joint goals are to increase the average time to treatment failure and control the probability of toxicity while accounting for patient heterogeneity. We are motivated by the desire to construct a feasible design for a trial of four chemotherapy combinations for treating a family of rare pediatric brain tumors. We present a hybrid two-stage design based on two-dimensional treatment effect parameters. A targeted parameter set is constructed from elicited parameter pairs considered to be equally desirable. Bayesian regression models for failure time and the probability of toxicity as functions of treatment and prognostic covariates are used to define two-dimensional covariate-adjusted treatment effect parameter sets. Decisions at each stage of the trial are based on the ratio of posterior probabilities of the alternative and null covariate-adjusted parameter sets. Design parameters are chosen to minimize expected sample size subject to frequentist error constraints. The design is illustrated by application to the brain tumor trial.     

Convolution power kernels for density estimation

We propose a new type of non-parametric density estimators fitted to random variables with lower or upper-bounded support. To illustrate the method, we focus on nonnegative random variables. The estimators are constructed using kernels which are densities of empirical means of m i.i.d. nonnegative random variables with expectation 1. The exponent m   plays the role of the bandwidth. We study the pointwise mean square error and propose a pointwise adaptive estimator. The risk of the adaptive estimator satisfies an almost oracle inequality. A noteworthy result is that the adaptive rate is in correspondence with the smoothness properties of the unknown density as a function on (0,+∞)$(0,+\infty )$. The adaptive estimators are illustrated on simulated data. We compare our approach with the classical kernel estimators.     

Competitive Strategies and Firm Performance: a Comparative Analysis of Pure,Hybrid and ‘Stuck‐in‐the‐middle’ Strategies in Spanish Firms

The purpose of this study was to examine the viability of hybrid competitive strategies, which combine differentiation and cost elements, and their impact on organizational performance in comparison to pure strategies and ‘stuck‐in‐the‐middle’ combinations. Previous studies have focused above all on US data. The analysis carried out in this paper has centred on a multisectorial sample of 164 Spanish firms. The findings show that a large number of the organizations use different types of hybrid strategies and also that such strategies tend to be associated with higher levels of firm performance, particularly those strategies which place emphasis on a greater number of strategic dimensions, and specifically on innovation differentiation.     

Half integer extreme points in the linear relaxation of the 2-edge-connected subgraph polyhedron

This paper studies the graphs for which the linear relaxation of the 2-connected spanning subgraph polyhedron has integer or half-integer extreme points. These graphs are called quasi-integer. For these graphs, the linear relaxation of the k-edge connected spanning subgraph polyhedron is integer for all k=4r, r≥1. The class of quasi-integer graphs is closed under minors and contains for instance the class of series-parallel graphs. We discuss some structural properties of graphs which are minimally non quasi-integer graphs, then we examine some basic operations which preserve the quasi-integer property. Using this, we show that the subdivisions of wheels are quasi-integer.     

Three Korean perspectives on U.S. internet public diplomacy

This study identifies perspectives of relationships publics have about countries other than their own and examines whether publics engaged through social media-based public diplomacy programs demonstrate different relationship perspectives. Q methodology and survey research were used to investigate these issues. Data come from South Korean adult internet users, including members of Café USA, an online community run by the U.S. Embassy in Seoul. Three relationship perspectives were identified: outcome-based, sincerity-based, and access-based. Compared with other groups, Café USA members put more emphasis on sincerity in their relationships with the United States. The results of this study indicate that individuals’ subjectivity should be considered as far more contextualized and nuanced than has been the case in previous research on national image or country reputation.     

Screening for Alcohol Problems Among 4-Year Colleges and Universities

Abstract

Objective: To assess the use of alcohol screening tools across US colleges. Participants: Directors of health services at 333 four-year colleges. Methods: An online survey was conducted regarding the use of alcohol screening tools. Schools reporting use of formal tools were further described in terms of 4 tools (AUDIT, CUGE, CAPS, and RAPS) that the authors judged to be the most favorable based on prior empirical comparative studies. Results: Forty-four percent of colleges reported use of at least 1 formal alcohol screening tool and nearly all of these used a tool appropriate for college students. However, less than half of the 44% of colleges that used a screening tool used 1 of the 4 most favorable tools. Conclusions: Continued efforts are needed to encourage colleges to use the most effective available screening tools to identify alcohol-related problems that require intervention among students.