On the effectiveness of top‐down strategy for forecasting autoregressive demands

We investigate the relative effectiveness of top‐down versus bottom‐up strategies for forecasting the demand of an item that belongs to a product family. The demand for each item in the family is assumed to follow a first‐order univariate autoregressive process. Under the top‐down strategy, the aggregate demand is forecasted by using the historical data of the family demand. The demand forecast for the items is then derived by proportional allocation of the aggregate forecast. Under the bottom‐up strategy, the demand forecast for each item is directly obtained by using the historical demand data of the particular item. In both strategies, the forecasting technique used is exponential smoothing. We analytically evaluate the condition under which one forecasting strategy is preferred over the other when the lag‐1 autocorrelation of the demand time series for all the items is identical. We show that when the lag‐1 autocorrelation is smaller than or equal to 1/3, the maximum difference in the performance of the two forecasting strategies is only 1%. However, if the lag‐1 autocorrelation of the demand for at least one of the items is greater than 1/3, then the bottom‐up strategy consistently outperforms the top‐down strategy, irrespective of the items' proportion in the family and the coefficient of correlation between the item demands. A simulation study reveals that the analytical findings hold even when the lag‐1 autocorrelation of the demand processes is not identical. © 2006 Wiley Periodicals, Inc. Naval Research Logistics, 2007.     

Bayesian estimation and optimal designs in partially accelerated life testing

A method of life testing is proposed which combines both ordinary and accelerated life-testing procedures. It is assumed that an item can be tested either in a standard environment or under stress. The amount of stress is fixed in advance and is the same for all items to be tested However, the time x at which an item on lest is taken out of the standard environment and put under stress can be chosen by the experimenter subject to a given cost structure. When an item is put under stress its lifetime is changed by the factor α. Let the random variable T denote the lifetime of an item in the standard environment, and let γ denote its lifetime under the partially accelerated test procedure just described. Then Y = T if T ≦ x, and Y = x + α (T > x) if T > x. It is assumed that T has an exponential distribution with parameter θ. The estimation of θ and α and the optimal design of a partially accelerated life test are studied in the framework of Bayesian decision theory.     

基于网络嗅探技术的试验数据实时录取

针对某系统在试验中出现的通信节点多、时序紧凑、数据量大等问题,提出了利用网络嗅探技术实现网络环境下的数据采集,采用GPS时钟硬中断和RTX环境下实时数据传输相结合的方法,实现试验数据的精确配时和实时处理,进而结合数据分类和处理,实现了试验数据的实时录取和分析。     

Insurance type item provisioning guidelines

Policy decisions for insurance type items, where zero or one unit is maintained at the depot, are more difficult and more critical than decisions for common supply items. This report presents results of developing initial provisioning guidelines for insurance type items. The guidelines are based on examination of lifetime costs and benefits. Costs of stocking an item as compared with not stocking are developed through a sinking fund annual payment formulation. Benefits of stocking are developed as stationary reduction in time weighted backorders experienced. A resource allocation formulation yields an optimal policy for allocating a fixed budget. The guideline is presented with refinements based on a sample of items. A figure of merit is calculated for each item, and if it is large the item is stocked while if small it is not stocked. Empirical definitions for large and small are developed based on sample data. Estimation techniques are discussed for deriving all of an item's parameters needed to compute its figure of merit. A Bayes procedure is suggested based on family group Experienced Demand Replacement Factors. This and other techniques are discussed.     

Cyclical schedules for the joint replenishment problem with dynamic demands

The Joint Replenishment Problem (JRP) involves production planning for a family of items. The items have a coordinated cost structure whereby a major setup cost is incurred whenever any item in the family is produced, and an item-specific minor setup cost is incurred whenever that item is produced. This paper investigates the performance of two types of cyclical production schedules for the JRP with dynamic demands over a finite planning horizon. The cyclical schedules considered are: (1) general cyclical schedules—schedules where the number of periods between successive production runs for any item is constant over the planning horizon—and (2) power-of-two schedules—a subset of cyclical schedules for which the number of periods between successive setups must be a power of 2. The paper evaluates the additional cost incurred by requiring schedules to be cyclical, and identifies problem characteristics that have a significant effect on this additional cost. © 1997 John Wiley & Sons, Inc. Naval Research Logistics 44: 577–589, 1997.     

恒定应力加速寿命试验的非参数统计方法

在恒定应力加速寿命试验 (简称恒加试验 )的统计分析中 ,非参数统计方法具有一定的实际价值 .在一定的假定下 ,文中给出了两种新估计 ,并证明了它们的优良性     

一种用于机载火控设备综合测试系统的数据库系统

介绍一种用于机载火控设备综合测试系统的检测数据库系统,着重论述了检测数据库系统的项目库和参数库的具体结构设计,并对测试软件操作检测数据库系统的过程以及一些相关问题进行了说明。     

A three-echelon,multi-item model for recoverable items

The main objective of this paper is to develop a mathematical model for a particular type of three-echelon inventory system. The proposed model is being used by the Air Force to evaluate inventory investment requirements for alternative logistic structures. The system we will model consists of a group of locations, called bases, and a central depot. The items of concern in our analysis are called recoverable items, that is, items that can be repaired when they fail. Furthermore, each item has a modular or hierarchical design. Briefly, the model is used to determine the stock levels at each location for each item so as to achieve optimum inventory-system performance for a given level of investment. An algorithm for the computation of stock levels for each item and location is developed and illustrated. Some of the ways the model can be used are illustrated with Air Force data.     

小子样条件下可靠性试验信息的融合方法

小子样条件下,不同状态的多源信息融合是可靠性工程领域的一个热门问题。综述了小子样条件下可靠性试验信息的融合方法。在一定的假设条件之下,利用分离可交换量模型对不同环境下的可靠性信息融合方法进行了研究,仿真算例说明这种融合方法充分利用了信息,评估结论更为合理。     

The inventory packing problem

We study the problem of finding the minimum number of identical storage areas required to hold n items for which demand is known and constant. The replenishments of the items within a single storage area may be time phased so as to minimize the maximum total storage capacity required at any time. This is the inventory-packing problem, which can be considered as a variant of the well-known bin-packing problem, where one constraint is nonlinear. We study the worst-case performance of six heuristics used for that earlier problem since the recognition version of the inventory-packing problem is shown to be NP complete. In addition, we describe several new heuristics developed specifically for the inventory-packing problem, and also study their worst-case performance. Any heuristic which only opens a bin when an item will not fit in any (respectively, the last) open bin needs, asymptotically, no more than 25/12 (resp., 9/4) times the optimal number of bins. Improved performance bounds are obtainable if the range from which item sizes are taken is known to be restricted. Extensive computational testing indicates that the solutions delivered by these heuristics are, for most problems, very close to optimal in value.     

The proper balancing of repair and preventive maintenance activities in the determination of coordinated shipboard allowance lists

The determination as to the cost-effective number of spares for given types of items or equipment to be carried on board various types of ships is studied. This spare pool, known as the ship's COSAL, must provide prespecified levels of protection against stockouts for all uses of that item on board the given ship. This article derives and illustrates the methodology for optimally trading off the reduction in the ship's COSAL that can be gained by improving repair/resupply capabilities or by lowering the failure rate of the equipment through more or different types of preventive maintenance. A flexible class of preventive maintenance/repair response functions to cost is studied which are nonlinear and exhibit realistic diminishing returns. Two different types of assumptions are possible regarding the interdependencies of the resupply times across different ship types. A tractable budget allocation method is presented which can be used in a multiitem, multiship, multiechelon repair environment where there is one budget to cover all spares, all repair/resupply, and preventive maintenance activities. The technique incorporates different criticalities of shortages by type of ship and item. It can be used either in a budget building mode or a budget execution mode.     

阅读理解测试中常用题型的对比研究

运用定量研究的方法,探讨阅读测试中多项选择、简答和正误判断三种题型与阅读理解能力之间的关系。研究设计中的受试者为390名武警学院的战士学员。统计程序使用SPSS对数据进行相关分析和回归分析。研究结果表明：多项选择、简答和正误判断三种题型都适合测试考生的阅读理解能力,但简答题是最有效的测试题型。     

结合全局与局部信息的主动轮廓分割模型

针对传统的基于区域的主动轮廓模型在分割灰度不均匀图像和噪声图像存在效果不佳的问题,提出结合全局项与局部项的主动轮廓分割模型。全局项由CV(Chan-Vese)模型的保真项构成,局部项的构建考虑局部区域信息的同时引入反映图像灰度特性的局部熵信息。依据图像灰度的特点,选择合理的全局项和局部项参数,并加入正则项保证曲线在演化过程中保持平滑,保障分割结果的可靠性。通过变分水平集方法最小化能量泛函,依据梯度下降流迭代更新水平集,完成曲线演化。采用模拟图像和实际图像进行实验分析,结果表明,所提出的结合全局项和局部项的主动轮廓模型可以高效地分割噪声严重以及灰度分布不均匀的图像。     

指控软件可用性工程生命周期模型

针对当前装甲指控软件的应用现状和存在的问题,运用可用性工程理论和方法,提出了指控软件的可用性工程生命周期模型,该模型包括可用性需求分析、模型构建、可用性设计、可用性测试与评估、安装部署与使用体验、用户反馈等过程。通过与一般软件测试的比较,强调可用性测试实验环境的作用。该模型突出用户的作用,重视用户反馈,可为指控软件的设计和开发提供理论指导。     

Customer delays in M/M/c repair systems with spares

A service center to which customers bring failed items for repair is considered. The items are exchangeable in the sense that a customer is ready to take in return for the failed item he brought to the center any good item of the same kind. This exchangeability feature makes it possible for the service center to possess spares. The focus of the article is on customer delay in the system—the time that elapses since the arrival of a customer with a failed item and his departure with a good one—when repaired items are given to waiting customers on a FIFO basis. An algorithm is developed for the computation of the delay distribution when the item repair system operates as an M/M/c queue.     

嵌入式装备测试系统设计研究

针对现代装备测试性需求,利用ARM微处理器构建了基于CAN总线网络的多路嵌入式数据采集系统,介绍了嵌入式数据采集系统硬件组成、硬件模块实现以及软件设计;检测数据经过CAN总线存入SD卡存储器,并进行后期数据的存取和处理．实验结果表明,该系统能够可靠、准确地对多路信号进行采集,实现对火炮装备的检测．     

Testing goodness of fit to the increasing failure rate family with censored data

One important thrust in the reliability literature is the development of statistical procedures under various “restricted family” model assumptions such as the increasing failure rate (IFR) and decreasing failure rate (DFR) distributions. However, relatively little work has been done on the problem of testing fit to such families as a null hypothesis. Barlow and Campo proposed graphical methods for assessing goodness of fit to the IFR model in single-sample problems. For the same problem with complete data, Tenga and Santner studied several analytic tests of the null hypothesis that the common underlying distribution is IFR versus the alternative that it is not IFR for complete data. This article considers the same problem for the case of four types of censored data: (i) Type I (time) censoring, (ii) Type I1 (order statistic) censoring, (iii) a hybrid of Type I and Type I1 censoring, and (iv) random censorship. The least favorable distributions of several intuitive test statistics are derived for each of the four types of censoring so that valid small-sample-size α tests can be constructed from them. Properties of these tests are investigated.     

嵌入式实时操作系统性能测试方法

为综合评价操作系统能否满足使用要求,研究了一种定量测试嵌入式实时操作系统功能和性能的方法,提出了具体的测试项目、测试环境和测试用例的要点,给出一种基于外部注入的功能测试、性能测试、应用测试方法及具体实现。测试结果证明,该方法对提高软件质量,缩短研制周期,增强可维护性等方面有显著效果。     

美军飞机易损性实弹测试现状及启示

飞机易损性分析与评估是指对飞机在战斗状态下遭受攻击后受损的难易程度的评价,是衡量飞机战斗性能的重要指标.研究飞机易损性对于飞机高生存力设计和飞机战伤预测评估具有重要意义.本文首先简要介绍了飞机易损性分析的一般方法与流程,收集并整理了美军飞机易损性实弹测试的历史沿革和条件建设现状,同时以F-35整机、F135发动机和C2...     

气象水文软件测评系统建设研究

介绍了气象水文软件测评系统建设和研究情况,重点分析了测评系统的网络环境、硬件环境、软件环境、测试技术能力、测试流程管理等方面的建设和研究情况.