Properties of preliminary test estimators and shrinkage estimators for evaluating multiple exposures - Application to questionnaire data from the SONIC study

Epidemiology studies increasingly examine multiple exposures in relation to disease by selecting the exposures of interest in a thematic manner. For example, sun exposure, sunburn, and sun protection behavior could be themes for an investigation of sun-related exposures. Several studies now use pre-defined linear combinations of the exposures pertaining to the themes to estimate the effects of the individual exposures. Such analyses may improve the precision of the exposure effects, but they can lead to inflated bias and type I errors when the linear combinations are inaccurate. We investigate preliminary test estimators and empirical Bayes type shrinkage estimators as alternative approaches when it is desirable to exploit the thematic choice of exposures, but the accuracy of the pre-defined linear combinations is unknown. We show that the two types of estimator are intimately related under certain assumptions. The shrinkage estimator derived under the assumption of an exchangeable prior distribution gives precise estimates and is robust to misspecifications of the user-defined linear combinations. The precision gains and robustness of the shrinkage estimation approach are illustrated using data from the SONIC study, where the exposures are the individual questionnaire items and the outcome is (log) total back nevus count.     

Advances in the statistical theory of comparison of lifetimes of machines under the generalized weibull distribution

Some new censoring schemes for comparing lifetimes of machines were introduced in Srivastava (1987), wherein it was shown that in general, these improve on the accuracy and also reduce the total expected time under experiment as well as the expected length of the experiment. Although the above studies were initiated under the generalized WeibuU distribution, the detailed development of the theory for the expected time or length were made under the Weibull only. In this paper, for a certain important special case, this development is extended to the generalized Weibull. Also, an error in the above paper is pointed out, and corrected, showing that the new schemes are actually superior to what they appeared to be there. Furthermore, under a realistic cost function, the problem of planning such experiments is discussed. An example for machines with a series connection is provided.     

Minimal 2-coverings of a finite affine space based on GF(2)

Let EG(m, 2) denote the m-dimensional finite Euclidean space (or geometry) based on GF(2), the finite field with elements 0 and 1. Let T be a set of points in this space, then T is said to form a q-covering (where q is an integer satisfying 1?q?m) of EG(m, 2) if and only if T has a nonempty intersection with every (m-q)-flat of EG(m, 2). This problem first arose in the statistical context of factorial search designs where it is known to have very important and wide ranging applications. Evidently, it is also useful to study this from the purely combinatorial point of view. In this paper, certain fundamental studies have been made for the case when q=2. Let N denote the size of the set T. Given N, we study the maximal value of m.     

INTERNATIONAL TECHNOLOGY AND OPERATIONS MANAGEMENT: RESOURCE TOOLKIT FOR RESEARCH AND TEACHING

Increasing globalization has affected the way that firms are managed today. While its impact on competitive strategy, marketing, and finance has been well accepted and well studied, its effect on the firm's technology and operations has not. This paper provides resources for research and teaching in international technology and operations management (itom). Included are an extensive bibliography of papers and an overview of large-scale survey research initiatives in the area. By bringing together and categorizing this body of work, we hope to facilitate further work in the area and to help define ITOM'S scope.     

On Exact Statistical Properties of Multidimensional Indices Based on Principal Components,Factor Analysis,MIMIC and Structural Equation Models

Recent empirical literature has seen many multidimensional indices emerge as well-being or poverty measures, in particular indices derived from principal components and various latent variable models. Though such indices are being increasingly and widely employed, few studies motivate their use or report the standard errors or confidence intervals associated with these estimators. This paper reviews the different underlying models, reaffirms their appropriateness in this context, examines the statistical properties of resulting indices, gives analytical expressions of their variances and establishes certain exact relationships among them.      

Optimal Design of a Two-Level Transportation Network with a Different Unit Cost for Each Secondary Link*

The objective of this paper is to further develop Singhal's (1990) framework for designing a two-level hierarchical transportation network consisting of a trunk or primary link and several feeder or secondary links. Secondary links are perpendicular lines from each of the given points and the primary link is a straight line or curve connecting the feet of two extreme perpendicular lines. The problem and the associated strategic and operational considerations such as cost, time, feasibility, and preferred regions for the primary link in the context of rural highway planning are discussed. Two-level networks are also common in electricity transmission, pipelines, and telecommunication design. The core of the framework is a model for finding the path of a primary link such that a weighted sum of the lengths of the perpendicular lines from each point to a linear primary link and the distance between the feet of the two extreme perpendicular lines is minimized. The analysis shows that for almost every problem there exists a wide range of solutions for which the total cost is only slightly higher than that of the optimal solution. This offers considerable flexibility to the decision maker. These solutions can be evaluated in view of the broader objectives and constraints that are not included in the model. The use of computer graphics and the option of a nonlinear or piecewise linear primary link are also discussed.     

Economic Manufacturing Quantity and Its Integrating Implications

Several contradictions are noted among the Economic Order Quantity (EOQ), Just‐In‐Time (JIT), and Optimized Production Technology (OPT) approaches and the economic framework for profit maximization. A fundamental model referred to as the Economic Manufacturing Quantity (EMO) is developed and examined for its integrating implications for the three approaches. An implication for the classic EOQ approach is that the balance between setup and inventory carrying costs is valid when a production facility is operating at or below a certain critical level but not when operating above that level. An implication for the JIT approach is that one must reduce setup cost at non‐bottlenecks and setup time at bottlenecks to reduce inventory. An implication for the OPT approach is that trade‐offs between setup and inventory carrying costs may indeed be ignored while determining process batch sizes, provided each facility in a production system is operating at or above Its critical level. Economic theoretic analysis of the EMO model provides a basis for unification of JIT which advocates stability in operating level as a key to improved productivity and quality, and OPT that advocates maximizing operating level with resultant emphasis on bottlenecks as a key to increased profits. This unifying basis states that a profit‐maximizing production facility or system will operate at the full and stable level as long as market demand remains relatively sensitive to price and operating at the full (maximum) level provides positive unit contribution.