A relaxation procedure for calculating (G–)minimax optimal designs

Summary: In nonlinear statistical models, standard optimality functions for experimental designs depend on the unknown parameters of the model. An appealing and robust concept for choosing a design is the minimax criterion. However, so far, minimax optimal designs have been calculated efficiently under various restrictive conditions only. We extend an iterative relaxation scheme originally proposed by Shimizu and Aiyoshi (1980) and prove its convergence under very general assumptions which cover a variety of situations considered in experimental design. Application to different specific design criteria is discussed and issues of practical implementation are addressed. First numerical results suggest that the method may be very efficient with respect to the number of iterations required.*Supported by a grant from the Deutsche Forschungsgemeinschaft. We are grateful to a referee for his constructive suggestions.     

“Getting pregnant might make me seem more normal to them”: Attitudes,experiences, and gendered nuances regarding pregnancy and parenting among youths experiencing homelessness

Homelessness among youths exacerbates an array of challenging life experiences, notable examples of which are pregnancy and parenting. Research is lacking on young men’s attitudes and experiences, and also the influences which homeless youths’ gender identities and sexualities may have on their pregnancy and parenting outcomes and trajectories. This study qualitatively explores gender differentials, including youths’ gender identities and sexual orientations, with regard to their attitudes and experiences of pregnancy and parenting while homeless. Qualitative data were obtained from interviews with a diverse sample of 30 female, male, and transgender homeless youths, ages 18 to 21. Findings revealed that topics of pregnancy and parenting represent a wide range of complexities, different standards, and threats for homeless youths. Although pregnancy is often viewed positively within this community, it is received and experienced differently for each gender and sexuality. Results demonstrate that youths’ unimpeded access to and knowledge of all available reproductive and sexual health care services is essential. These services must be designed and delivered for youths of all gender identities and sexual orientations. Interventions and resources on parenting and co-parenting, particularly models that more inclusively engage a greater diversity of young parents’ involvement, are also urgently needed.     

Study of the bivariate survival data using frailty models based on Lévy processes

Frailty models allow us to take into account the non-observable inhomogeneity of individual hazard functions. Although models with time-independent frailty have been intensively studied over the last decades and a wide range of applications in survival analysis have been found, the studies based on the models with time-dependent frailty are relatively rare. In this paper, we formulate and prove two propositions related to the identifiability of the bivariate survival models with frailty given by a nonnegative bivariate Lévy process. We discuss parametric and semiparametric procedures for estimating unknown parameters and baseline hazard functions. Numerical experiments with simulated and real data illustrate these procedures. The statements of the propositions can be easily extended to the multivariate case.

     

On the influence of market structure in modelling the US copper industry

Many of the contemporary models used to describe the behavior of the mineral industries assume a competitive market i.e. one in which market price is equal to marginal production cost. One such recent model of the worldwide copper industry is the MIDAS-II model developed for the Bureau of Mines [3, 4]. This model is used to project production and prices up through the year 2000. The purpose of this paper is to demonstrate the importance of the assumed market structure in the construction of these forecasts. If the market structure of the US copper industry is assumed to be comprised of a few large firms (an oligopoly), then forecasts based upon exactly the same data base differ significantly from the competitive market assumption.     

The revised safe at home instrument for assessing readiness to change intimate partner violence

This article describes the development and factor structure of the Revised Safe At Home instrument, a 35-item self-report measure designed to assess individuals' readiness to change their intimate partner violence behaviors. Seven new items have been added, representing content specific to the Maintenance stage, and other items have been revised to strengthen the assessment of earlier stages and address gender concerns. Confirmatory factor analysis using multisite data (two sites, a total of 281 men at intake) supported the conclusion that a four-factor model (Precontemplation, Contemplation, Preparation/Action, and Maintenance stages) was consistent with the observed covariances. A high degree of correlation between the Preparation/Action and Maintenance scales was observed, but subsequent testing indicated a need to treat the two as distinct factors in the model. It is recommended that scoring include only 31 items that perform well.     

Migrant earnings gaps in Gulf Cooperation Council countries: Employers' perceptions or opportunity costs?

This article discusses earnings differentials among skilled Western, Arab and Asian migrants, who constitute most of the private sector labour force in Gulf Cooperation Council (GCC) countries, during the period 2012–14. Using two standard decomposition methodologies, it investigates the view that Westerners are paid premium rates due to the unobserved perceptions of private employers. The results indicate that while one‐third to three‐quarters of real hourly earnings differentials are attributable to differing observed levels of productivity‐related characteristics, the remainder are due to the incorporation of higher opportunity costs for Westerners into the bargaining process. The potential effect of unobserved perceptions is found to be irrelevant to observed earnings differentials.     

Product value matrices help firms to focus their efforts

In management theory a gap seems to exist between consumer buying behaviour and the economic life cycles of products. This gap is the subject of this paper using product value matrices (PVMs). Brought together in this paper are some well-known theories dealing with consumer behaviour, buying behaviour and product life cycles (PLCs), in a manner that provides useful information to all firms. Too often it is found that these theories are elegant in isolation, but provide little help in focusing a company's efforts. Consumer behaviour has been researched for many decades, as has product life cycles, but the link between consumer buying behaviour and economic life cycles of products is mystical, with most chief executive officers (CEOs) believing it occurs like magic. The paper initially develops frameworks describing the economic life cycles of products and consumer buying behaviour. The paper then links the frameworks with the product value matrix and illustrates how they can be used in focusing a firm's efforts.