Manufacturing lead time accuracy

This article addresses the question of accuracy of planned lead times (PLTs) that are used with a material requirements planning system. Lead time error is defined as the difference between an item's PLT and the actual lead time (flow time) of an order to replenish the item. Three related topics are discussed: the relationship between system performance and average lead time error, the transient effect on work-in-process (WIP) inventory of increasing PLTs, and the relative accuracy of three methods of determining PLTs. A distinction is made between available and WIP inventory. The former includes any purchased item, fabricated part, assembly, or finished good that is in storage and available for use or delivery. WIP denotes materials associated with open orders on the shop floor.It was concluded that average lead time error has a considerable affect on system performance. PLTs that are on average too long or too short increase available inventory; and the further the average error is from zero, the more pronounced the increase. Contrary to conventional wisdom, increasing PLTs will increase the service level (decrease backorders), unless PLTs are already severely inflated and MPS uncertainty (forecast error) is small. If PLTs are inflated, decreasing them will decrease the number of setups per unit time in the case of considerable demand uncertainty. Contrary to conventional wisdom, increasing PLTs causes only a transient rise WIP inventory.The fact that the average lead time error has a significant effect on the three areas of system effectiveness mentioned above does not imply that a given order's lead time should be managed in a way that forces its actual lead time to match the PLT. Stated another way, the material planner may use the latest information to manage a given order's lead time; however, if the average discrepancy between the actual and planned lead times is large, system performance can be improved by changing the PLTs to approximate the average flow times.Three methods that have been proposed for determining PLTs are compared. They are historical averages of the actual flow times, calculated lead times based on standard times and historical averages of the queuing time at the appropriate work centers, and the QUOAT lead time proposed by Hoyt. The third was found to perform poorly unless the work content of all operations is identical. With one exception, no differences were found between the first two methods. The simpler historical average method was superior to the calculated lead time in the case where the work content of each operation varies and when considerable demand uncertainty exists.The results are based on simulation experiments employing a generalized MRP/Job-Shop stochastic simulation model. The program launches orders based on standard MRP logic, reschedules open orders by moving the due date in or out to coincide with revised need dates, moves manufacturing orders through a job shop, schedules the delivery of purchase orders, and updates inventory levels. The product structure tree contained eight distinct items, with four levels and one end item. There is no reason to believe that the conclusions would be any different had a larger system been studied.     

因子分析模型L的优良性和应用

传统的因子分析模型,公因子解不能降维或失去一些变量解释,故因子分析需要找出一个优良模型。而优良模型的确定需要更好的量化优良性准则,为此本文用因子分析的基本思想和目的,提出因子分析降维性、解释性、拟合性更好的量化优良性准则,并证明有目标函数最大化的因子分析模型L是因子分析的一个优良模型和方法。实证表明,因子分析模型L具有优良性,传统因子分析模型的最大似然法、主因子法误差较大。     

Cobweb dynamics and price dispersion in illicit drug markets

Extreme price dispersion is a hallmark of illegal drug markets, and this apparent contradiction to the law of one price has long puzzled drug market economists. We propose a novel explanation for this dispersion: the coupling of dealers’ unwillingness to hold inventory with dealers’ imperfect foresight concerning future prices and/or random lead times when “ordering” drugs from higher-level suppliers. Unwillingness to hold inventory means drug markets might operate consistent with a cobweb model. The classic cobweb model was inspired by the observation of cyclic (typically annual) fluctuations in commodity prices. However, with minor changes that make the model more realistic the resulting price trajectories can be highly variable or even chaotic, not just periodic. Cobweb dynamics can also amplify the variability created by supply chain disruptions.     

THE RESURGENCE OF INVENTORY RESEARCH: WHAT HAVE WE LEARNED?

Abstract. Recent empirical and theoretical research on business inventories is surveyed and critically evaluated. While most inventory research has had macroeconomic motivations, we focus on its microtheoretic basis and on potential conflicts between theory and evidence. The paper asks two principal questions. First, how can inventories, which are allegedly used by firms to stabilize production, nonetheless be a destabilizing factor at the macroeconomic level? Second, why, if firms are following the production-smoothing model, is production more variable than sales in many industries? We suggest that the so-called (S,s) model may help answer both questions.     

Development and validation of an Integrated Organizational Safety Climate Questionnaire with multilevel confirmatory factor analysis

Meta-analytic and traditional reviews on safety climate reveal theoretical and methodological safety climate issues still open. The main aim of this study is to propose a questionnaire which combines recent and different approaches to safety climate, trying to give a contribute about these issues. The present research led to the development of a new questionnaire to measure safety climate, suitable for blue-collar workers, and to the evaluation of its psychometric properties, and usefulness to measure safety climate in the industrial sector. Multilevel confirmatory factor analysis (MCFA) was used to properly evaluate the factor structure underlying the safety climate questionnaire composed of three scales: organizational safety climate scale, supervisor’s safety climate scale and co-workers’ safety climate scale. The clear distinction, made with the use of three different scales, among safety agents (organization, supervisor, co-workers), allows the assessment of workers’ perceptions focused on each level, and allows to deeply explore, for instance, lateral relationships of supervisor’s safety climate and co-workers’ safety climate, analysing the interactions between the roles of these two safety agents. A two-level design was used, considering the individual level and the work-group level. Data collection involved 1,617 blue-collars from eight Italian manufacturing companies. The MCFA results demonstrated the importance to use proper analysis to study the factor structure of a multilevel construct as safety climate, and confirmed the theoretical structure of safety climate purposed from Griffin and colleagues, using not only psychological climate (i.e., the individual level), but also the group level safety climate.     

Double symmetry model and its decompositions in square contingency tables having ordered categories

In this paper, we have employed the non-standard log-linear models to fit the double symmetry models and some of its decompositions to square contingency tables having ordered categories. SAS PROC GENMOD was employed to fit these models although we could similarly have used GENLOG in SPSS or GLM in STATA. A SAS macro generates the factor or scalar variables required to fit these models. Two sets of \(4 \times 4\) unaided distance vision data that have been previously analyzed in (Tahata and Tomizawa, Journal of the Japan Statistical Society 36:91–106, 2006) were employed for verification of results. We also extend the approach to the Danish \(5 \times 5\) Mobility data as well as to the \(3 \times 3\) Danish longitudinal study data of subjective health, firstly reported in (Andersen, The Statistical Analysis of Categorical Data, Springer:Berlin, 1994) and analyzed in (Tahata and Tomizawa, Statistical Methods and Applications 19:307–318, 2010). Results obtained agree with those published in previous literature on the subject. The approaches suggest here eliminate any programming that might be required in order to apply these class of models to square contingency tables.     

What inventories tell us about aggregate fluctuations—A tractable approach to (S,s) policies

We estimate a DSGE model with (S,s) inventory policies. We find that (i) taking inventories into account can significantly improve the empirical fit of DSGE models in matching the standard business-cycle moments (in addition to explaining inventory fluctuations); (ii) (S,s) inventory policies can significantly amplify aggregate output fluctuations, in contrast to the findings of the recent general-equilibrium inventory literature; and (iii) aggregate demand shocks become more important than technology shocks in explaining the business cycle once inventories are incorporated into the model. An independent contribution of our paper is that we develop a solution method for analytically solving (S,s) inventory policies in general equilibrium models with heterogeneous firms and a large aggregate state space, and we illustrate how standard log-linearization methods can be used to solve various versions of our inventory model, generate impulse response functions, and estimate the model׳s deep structural parameters.     

Humorous coping scales and their fit to a stress and coping framework

The focus of this study is on the fit between the item content of scales measuring humorous coping and basic concepts of stress and coping theory. To investigate this fit 81 items from seven currently available humorous coping scales have been subjected to a facet analysis, using the tool of a mapping sentence. Three facets derived from stress and coping theory were part of this mapping sentence: external demands, humorous responses and coping aims. Because of the claim that humorous coping may be related to physical health dimensions, special attention has been paid to two health-related coping aims: cognitive reappraisal and response-focused coping responses. Five raters categorized the facets and their respective categories. Some humorous coping scales showed an underrepresentation of “external demands” and “humorous responses” and only a few scales covered the “aims” facet adequately. Reliability and agreement parameters varied considerably among scales, both on facet level and on category level. The Waterloo University Humor Inventory (WUHI) was a positive exception to this pattern. Findings are discussed in the light of specific characteristics of the scales included. Possible improvements of humorous coping measurement in health-related research are proposed, as well as adaptations to the rating procedures.     

汽车配件供应链库存分析及需求预测实例

通过对汽车售后配件供应链中的安全库存进行分析,运用正态分布原理,采集足够的库存数据,对汽车保养件、易损件、季节波动零件等三种不同类型的配件需求建立起科学合理的预测模型,在实际的配件物流工作中起到一定的指导作用。     

周期性订货策略中的牛鞭效应分析与控制

利用随机变量数学期望理论,在由一个制造商和一个零售商所组成的周期订货供应链系统中,建立了延迟需求信息和即时需求信息下的制造商日平均库存模型,并分析影响两者库存量差异变动的因素,提出库存策略中牛鞭效应的产生主要是由制造商的周期服务水平(CSL)以及零售商和制造商的订货时间差所导致。     

基于单元化分装对企业备品备件的储存空间优化研究

文章主要针对日立环球存储科技公司的技术革新、产品升级、机器更换升级后造成备品备件的库存大量增加,空间利用率下降这一问题．通过现场库存和物料特性分析,运用单元化分装管理方法,达到库存优化管理和空间的有效利用,降低物流管理成本．规范公司备品备件的管理。     

敏捷供应链下库存管理的财务影响

敏捷供应链强调供应链的快速响应能力、快速重构能力，库存控制是其研究热点。论文就敏捷供应链下库存管理的财务影响展开论述．在分析库存优化现状的基础上。提出库存持有成本是衡量库存管理对财务影响的重要因素，得出企业设立库存周转目标时，必须准确计算库存持有成本，充分证明各种选择策略将如何影响其他物流成本。并将其与节约的库存持有成本相比较．     

A formal set of algorithms for project scheduling with critical path scheduling/material requirements planning

A hybrid model combining the critical path method (CPM) with material requirements planning (MRP) has been suggested (Aquilano) as a more robust method for scheduling projects and resources. The primary advantage of this technique is that resource acquisition lead times as well as inventory records are integrated into the process of computing the project schedule. This paper presents a set of formal CPM/MRP algorithms that may be used to compute the early and late start schedules as well as the critical sequence. A number of modifications have been incorporated into the CPM/MRP technique to improve the viability of CPM/MRP as a tool for application to actual project scheduling problems. A simple example project is used to demonstrate the CPM/MRP model.The CPM/MRP technique is designed to overcome a basic shortcoming of previously suggested project scheduling methodologies. CPM was initially designed to schedule projects subject to technological constraints only. Later, additional techniques were introduced to consider constraints upon various aspects of resource availability (Davis). None of the suggested techniques attempted to integrate resource acquisition lead time with the generation of requirements for resources. Obviously such a technique would require the integration of inventory records into the scheduling technique.The combination of CPM and MRP provides a possible vehicle for overcoming this drawback in CPM. Both CPM and MRP are linear models that generate schedules based upon precedence relationships. An integrated approach is useful since activities could be scheduled subject to information about the inventory position. An activity may be scheduled as soon as all resources are on hand. It is only delayed by those resources which must be acquired and activities which proceed it in the project network.CPM/MRP also shows promise as an aid to constrained resource scheduling since computations regarding resource availability are an integrated part of the technique. The effect of resource allocation decisions is immediately evident in the MRP-type time phased records.Results of the tests run on short projects of up to 300 activities and resources have shown that the program does work satisfactorily. Execution time for a 300 item network tested was approximately ten seconds on a CYBER 175.     

INNOVATIVE ATTITUDES, VALUES, AND INTENTIONS IN ORGANIZATIONS [1]

In this paper we explore the relationships between some of the various concepts and scales that have been used to characterize innovative attitudes and behaviours. A sample (N = 123) of undergraduate and graduate business students with full-time jobs or the equivalent provided questionnaire data to test two initial hypotheses. It was found that four attitude-value scales which have been used to measure change values, innovation orientation, readiness for change, and innovativenesss, are intercorrelated significantly when no distinction is made between the respondents’intentions to be innovative and respondents’actual innovative behaviour. Innovative and change attitudes, as measured by these same four scales, do consistently predict multiple innovative intentions and behaviours but not, as expected, single behaviours or single intentions. The single best predictor in a multiple regression of the combined multiple innovative intention-behaviour measure was found to be a creative scale (R²= 0.43, p < 0.01). The innovative behaviour scale was tentatively called attitude toward being innovative. A factor analysis of this scale revealed dimensions related to innovative behaviour in organizations: the innovator, the preserver of the status quo, and the unchallenged, dissatisfied person. Two other hypotheses were also tested. The first was that the perceived organization risk-taking climate would moderate the relationship between these attitude measures. This hypothesis was not supported by the overall results. However, moderate support was found for the hypothesis that the more formal authority a person has in an organization, the greater the consistency between change attitudes and innovative behaviours as measured by self-report methods.     

The effects of workforce strategies on manufacturing operations

This paper tests the effects of three workforce strategies on a multistage, multiproduct manufacturing system under various operating conditions. These three strategies are those that are predominant in today's world economics. One type is a chase strategy, often used by firms that employ low skilled workers and faced with seasonal product demands, where workforce levels fluctuate according to increases and decreases in production requirements. A level-flexible strategy, commonly called the Toyota system, keeps the aggregate workforce at a constant size but by having flexibly trained workers it can allow transfers of workers between various departments and processes as production requirements dictate. The third major type is a level-inflexible strategy, such as that used on mass-assembly lines employed by American automobile manufacturers. Under this strategy, the number of workers remains constant in each department as well as at the aggregate level regardless of short term changes in the production requirements. The manufacturing system is envisioned as a sequential planning process with interrelated decisions made at the levels of aggregate planning, master production scheduling, and departmental planning. This process is modeled as a zero-one mixed integer program. The operating conditions under which the strategies are tested are the variability of demand, the level of service, and the degree of inventory investment. The strategies are statistically tested as to their effects on five criteria: average weekly workforce size, average quarterly inventory investment, average weekly overtime, total setups and the average weekly ratio of departmental load to capacity.Using four different products and an experimental manufacturing environment described within the paper, we tested for any statistical differences between the three strategies. We found that none of the strategies had significantly different workforce sizes. Also, the chase strategy had the smallest average quarterly inventory investment. This implies that the “Japanese” level-inflexible strategy does not have the smallest workforce or lowest inventory as claimed by some.We also tested the effects of the operating conditions on the workforce strategies. The level-flexible strategy was most insulated from the seasonality, inventory restrictions, and service level. Thus again the claims made for the level-inflexible strategy about being insulated from its environment were not substantiated. It was found that the seasonality of demand had the greatest impact on the three workforce strategies. Also, the level of service greatly affected the utilization of labor resources.The major overall conclusion is that the level-flexible strategy which is associated with some Japanese manufacturing firms does not achieve the claims that some of its adherents have made for it. It does not have the smallest workforce or the lowest inventory nor is it best insulated from its environment. Furthermore, the seasonality of demand and the level of service do effect this strategy.     

Development of a multidimensional measure of perceived product quality

A measure of perceived product quality (i.e., the Perceived Product Quality Measure of PPQM) was developed that had scales for measuring four general dimensions of quality, i.e., flawlessness, durability, appearance, and distinctiveness. Results of two studies (preliminary and main) showed support for the reliability and validity of the PPQM. Results of analyses of data from the main, experimental study showed that: (1) the PPQM scales had fairly high levels of internal consistency; (2) manipulations of several attributes of products led to expected changes in the responses to PPQM scales; (3) the PPQM scales predicted a willingness to buy criterion; and (4) the results of a confirmatory factor analysis showed support for the hypothesized multidimensionality of the PPQM. In view of these findings, we believe that the PPQM should prove useful in future research on perceived product quality. It should also prove helpful in efforts to manage the quality of products produced by individuals on work organizations.     

基于因子分析的安徽省区域物流发展对策研究

通过选取区域物流发展的部分评价指标,对安徽省近几年及其17个地级市06年数据进行因子分析,根据析出的4个主因子客观地对安徽省及其地级市物流水平进行综合评价,在此基础上提出安徽省区域物流的发展对策。     

Measuring cost savings in the operating room: a matter of FOCUS (follow-through, obsolescence, control, utilization, standardization)

In summary, changes came slowly at first in the OR. The biggest inventory reduction came about a year after the effort had begun. It would seem that tackling obsolescence, standardizing products, etc. would give an initial "big chunk" savings, with small amounts coming later as inventories are lowered. What we expected did not occur, due to the inherent nature of practices and what was customarily being used. For example, previously, some equipment purchases were paid for by purchasing product, which resulted in long-term commitments and high supply costs. The OR at times seemed more like a process of managing change, not just supplies. FOCUS is a daily process. How many times can you afford to overlook this process? How often are the mistakes of the past repeated? In review, we suggest this simple outline: Follow-through (review each case from start to finish), Obsolescence (schedule inventories every six months), Control (achieve inventory management by adjusting PAR levels to needs), Utilization (utilize product/equipment with your control), and Standardization (standardize products and measure results).     

关于R²的几点质疑

本文论证线性回归分析指标R2拟合意义的局限性,并质疑R2检验的正当性。通过经济计量学课程实验资料揭示R2的局限,实证分析经济计量学软件Eviews R2计算公式的误区。此外,本文提出并建议采用残差误差率指标作为回归分析拟合优度的可决系数较为适宜。     

Analysis of high dimensional multivariate stochastic volatility models

This paper is concerned with the Bayesian estimation and comparison of flexible, high dimensional multivariate time series models with time varying correlations. The model proposed and considered here combines features of the classical factor model with that of the heavy tailed univariate stochastic volatility model. A unified analysis of the model, and its special cases, is developed that encompasses estimation, filtering and model choice. The centerpieces of the estimation algorithm (which relies on MCMC methods) are: (1) a reduced blocking scheme for sampling the free elements of the loading matrix and the factors and (2) a special method for sampling the parameters of the univariate SV process. The resulting algorithm is scalable in terms of series and factors and simulation-efficient. Methods for estimating the log-likelihood function and the filtered values of the time-varying volatilities and correlations are also provided. The performance and effectiveness of the inferential methods are extensively tested using simulated data where models up to 50 dimensions and 688 parameters are fit and studied. The performance of our model, in relation to various multivariate GARCH models, is also evaluated using a real data set of weekly returns on a set of 10 international stock indices. We consider the performance along two dimensions: the ability to correctly estimate the conditional covariance matrix of future returns and the unconditional and conditional coverage of the 5% and 1% value-at-risk (VaR) measures of four pre-defined portfolios.