Load Sharing Models

Consider a system of components whose lifetimes are governed by a probability distribution. Load sharing refers to a model of stochastic interdependency between components that operate within a system. If components are set up in a parallel system (see Parallel, Series, and Series–Parallel Systems) for example, the system survives as long as at least one component is operating. In a typical load-sharing system, once a component fails, the remaining components suffer an increase in failure rate due to the extra “load” they must encumber due to the failed component

Statistical Reliability with Applications

This chapter reviews fundamental ideas in reliability theory and inference. The first part of the chapter accounts for lifetime distributions that are used in engineering reliability analyis, including general properties of reliability distributions that pertain to lifetime for manufactured products. Certain distributions are formulated on the basis of simple physical properties, and other are more or less empirical. The first part of the chapter ends with a description of graphical and analytical methods to find appropriate lifetime distributions for a set of failure data. The second part of the chapter describes statistical methods for analyzing reliability data, including maximum likelihood estimation and likelihood ratio testing. Degradation data are more prevalent in experiments in which failure is rare and test time is limited. Special regression techniques for degradation data can be used to draw inference on the underlying lifetime distribution, even if failures are rarely observed. The last part of the chapter discusses reliability for systems. Along with the components that comprise the system, reliability analysis must take account of the system configuration and (stochastic) component dependencies. System reliability is illustrated with an analysis of logistics systems (e.g., moving goods in a system of product sources and retail outlets). Robust reliability design can be used to construct a supply chain that runs with maximum efficiency or minimum cost

Persistent Hypertension is Associated with Low Hdl in Young Adults: an Evidence from Taiwan Five Years Population Based Cohort Study

Background: Hypertension (HT) is one of major risk factor for cardiovascular disease (CVD) and its combination with other risk factors, i.e. dyslipidemia, could worsen the health. The burden of CVD is now shifting to younger age, and its development is even in the earlier life period. Young adults with rare health check and lack of awareness in CVD risk factor may result in suboptimal management. This study aimed to investigate the association of persistent HT and low HDL level in 5 years follow up among young adults aged 15-25 years. Subjects and Methods: This study was done using Tw3SHH, a Taiwan population based cohort study (2002 to 2007), among young adults 15-25 years of age. HT was defined as the SBP>120mmHg and/or DBP>80 mmHg and/or respondent was told of having HT by medical personnel. HT development was classified based on their HT status in 2002 and 2007: “Non-HT (no HT both in 2002 and 2007)”, “Improved HT (HT in 2002 and no HT in 2007)”, ”Worsened HT (no HT in 2002 and HT in 2007)”, and “Persistent HT (HT both in 2002 and 2007)”. HDL was grouped based on NCEP-ATP III, and the low HDL defined as HDL<40mg/dl. Exercise, fatty food consumption and smoking status were based in self-reported questions. The data were analyzed by a multiple logistic reg¬res¬sion model to estimate the association of persistent HT and low HDL level in 5-years. Covariates were sex, age, smoking habit, exercise habit, fatty food con¬sump¬tion, and HbA1C. Results: About 9.35% of 727 study subjects had persistent HT. Compared to non-HT group, persistent HT group had higher risk of having low HDL level (crude OR= 3.70; CI 95%= 1.90 to 7.2; p<0.001; adjusted OR= 2.44; CI 95%= 1.18 to 5.03; p= 0.015). This association persisted (crude OR= 4.90; CI 95% 2.29 to 10.49; p<0.001; adjusted OR= 2.89; CI 95%= 1.34 to 6.24; p= 0.007) in sub¬group analysis among study subjects without low baseline HDL. Conclusion: Persistent HT is associated with low HDL level in 5 years follow up among young adults 15-25 years of age. Although universal screening in lipid profile among children and youth is still controversial, awareness to this condition should be raised. Keywords: cardiovascular, hypertension, dyslipidemia, HDL, young adul

Reliability Modeling in Spatially Distributed Logistics System

This article proposes methods for modeling service reliability in a supply chain. The logistics system in a supply chain typically consists of thousands of retail stores along with multiple distribution centers (DC). Products are transported between DC & stores through multiple routes. The service reliability depends on DC location layouts, distances from DC to stores, time requirements for product replenishing at stores, DC\u27s capability for supporting store demands, and the connectivity of transportation routes. Contingent events such as labor disputes, bad weather, road conditions, traffic situations, and even terrorist threats can have great impacts on a system\u27s reliability. Given the large number of store locations & multiple combinations of routing schemes, this article applies an approximation technique for developing first-cut reliability analysis models. The approximation relies on multi-level spatial models to characterize patterns of store locations & demands. These models support several types of reliability evaluation of the logistics system under different probability scenarios & contingency situations. Examples with data taken from a large-scale logistics system of an automobile company illustrate the importance of studying supply-chain system reliability

Extending the Skill Test for Disease Diagnosis

For diagnostic tests, we present an extension to the skill plot introduced by Briggs and Zaretski (Biometrics 2008; 64:250–261). The method is motivated by diagnostic measures for osteopetrosis in a study summarized by Hans et al. (The Lancet 1996; 348:511–514). Diagnostic test accuracy is typically defined using the area (or partial area) under the receiver operator characteristic (ROC) curve. If partial area is used, the resulting statistic can be highly subjective because the focus region of the ROC curve corresponds to a set of low false‐positive rates that are chosen by the experimenter. This paper introduces a more objective diagnostic test for which the focus region depends on a skill score, which in turn depends on the loss functions associated with misdiagnosis. More specifically, the skill‐based diagnostic test serves as a more objective version of the nonparametric test introduced by Dodd and Pepe (Biometrics 2003; 59:614–623)

Adjusted Empirical Likelihood Models with Estimating Equations for Accelerated Life Tests

This article proposes an adjusted empirical likelihood estimation (AMELE) method to model and analyze accelerated life testing data. This approach flexibly and rigorously incorporates distribution assumptions and regression structures by estimating equations within a semiparametric estimation framework. An efficient method is provided to compute the empirical likelihood estimates, and asymptotic properties are studied. Real-life examples and numerical studies demonstrate the advantage of the proposed methodology

Matter-induced vertices for photon splitting in a weakly magnetized plasma

We evaluate the three-photon vertex functions at order $B$ and $B^{2}$ in a weak constant magnetic field at finite temperature and density with on shell external lines. Their application to the study of the photon splitting process leads to consider high energy photons whose dispersion relations are not changed significantly by the plasma effects. The absorption coefficient is computed and compared with the perturbative vacuum result. For the values of temperature and density of some astrophysical objects with a weak magnetic field, the matter effects are negligible.Comment: 14 pages, 1 figure. Accepted for publication in PR