Improved minimax estimation of a normal precision matrix

Let S_{p × p} have a Wishart distribution with parameter matrix Σ and n degrees of freedom. We consider here the problem of estimating the precision matrix Σ^?1 under the loss functions L₁(σ) tr (σ) - log |σ| and L₂(σ) = tr (σ). James-Stein-type estimators have been derived for an arbitrary p. We also obtain an orthogonal invariant and a diagonal invariant minimax estimator under both loss functions. A Monte-Carlo simulation study indicates that the risk improvement of the orthogonal invariant estimators over the James-Stein type estimators, the Haff (1979) estimator, and the “testimator” given by Sinha and Ghosh (1987) is substantial.     

Turning Three Sides into a Delta at General Motors: Enhancing Partnership Integration on Corporate Ventures

General Motors had enjoyed global operations that were based on exports, acquisitions, joint ventures and strategic alliances throughout the 20th century. Its global-programme strategy appeared assured. However, the carmaker came unstuck when it came to creating a large-scale collaborative effort involving its own internal units. The Delta Small Car Program, involving three fully internal GM units, was terminated after its goals failed to materialise. The carmaker’s long-standing cultural tradition of autonomy for its units was what made collaboration between them difficult. We examine this GM global programme, placing it in its historical context. Our primary database consists of the perceptions and experiences offered to us by a cross-section of programme participants. By examining partner integration at the working level, we identify insights and offer recommendations pertaining to venture structure and dynamics and their role in venture success.     

ASSESSING THE RELATIONSHIP BETWEEN SYMPTOMS OF ALLERGIC RHINOCONJUNCTIVITIS AND POLLEN COUNTS

The relationship between daily pollen counts during the peak pollen season and hay fever symptoms in known sufferers of pollen allergy was investigated in a Sydney hospital‐based study. This paper develops statistical models for both the short term (day to day) associations and the longer term relationships between these time series. Possible effects of asthma status are investigated. The analyses illustrate how different relationships between time series may be explored in a simple way by working on different time scales with suitably transformed variables.     

Challenging the Rhetoric of Choice in Maternal Labour‐Force Participation: Preferred Versus Contracted Work Hours

The notion of choice in maternal labour‐force participation (LFP) is a contentious one, with assertions that LFP is a direct result of either personal inclinations, such as employment commitment or external factors, such as historically available opportunities. This article suggests an alternative framework for understanding and testing choice in LFP using preferred versus contracted work hours. It explores these constructs quantitatively in a group of working mothers (N = 275) with dependent children and investigates qualitatively the underlying reasons for discrepant preferred versus contracted work hours in a sub‐sample of these women with under‐school‐aged children (N = 20). The results show that nearly two‐thirds of women working full time would prefer to work part time and the major reasons for not acting on their preferences is because of the nature of the job and the lack of career opportunities available for part‐time employees.     

Understanding Social Change in Conducting Research on Adolescence

In the present essay, we focus on G. Stanley Hall's contributions to the study of the role of social change for adolescent development. After introducing Hall's main ideas, we discuss recent demands adolescents face because of social change and how Hall's work could inform research on adolescent development in times of social change.     

Temporal Aggregation of Stationary and Non-stationary Continuous-Time Processes

Abstract.  We study the autocorrelation structure of aggregates from a continuous-time process. The underlying continuous-time process or some of its higher derivative is assumed to be a stationary continuous-time auto-regressive fractionally integrated moving-average (CARFIMA) process with Hurst parameter H . We derive closed-form expressions for the limiting autocorrelation function and the normalized spectral density of the aggregates, as the extent of aggregation increases to infinity. The limiting model of the aggregates, after appropriate number of differencing, is shown to be some functional of the standard fractional Brownian motion with the same Hurst parameter of the continuous-time process from which the aggregates are measured. These results are then used to assess the loss of forecasting efficiency due to aggregation.     

第6章新官上任

《龟航》是一部职场小说, 同时也是一部代表了特定人群的小说,在这个大时代下,境遇变迁,冲击着每一个人。海归的遭遇更能折射出现实的意义。本刊现独家连载这部原创作品。您从作者清新幽默的文笔下读到的将是新世纪《围城》的深刻和感同身受。