Controlling inventories in a supply chain: A case study

This article presents a case study on the joint replenishment problem. Particularly we analyze the effect of minimum order quantities and a complex transportation cost structure on inventories. Several types of inventory policies are tested in a simulation model with real data.     

Replenish-up-to inventory control policy with random replenishment intervals

The multi period inventory problems have been studied under two main assumptions. Continuous review assumption where an order can be made at any time depending on inventory position and periodic review assumption where an order can be initiated only at discrete time epochs. In this study, we analyze a multi period inventory problem that falls under neither of these two assumptions. In the case we consider, there are periodic replenishments but the replenishment intervals are taken to be i.i.d. random variables. This setting represents the real life cases where a supplier visits a retailer with random inter arrival times and the retailer replenishes his inventories based on a replenish-up-to-level inventory control policy. We also assume that only a certain fraction of unmet demand is backordered and the rest of it is lost.In this setting under general distribution between replenishment epochs, we show the concavity of the expected profit function and give the condition that must hold for the optimal replenish-up-to-level. We also present the specific solutions and analysis under two different distributions, namely, uniform and exponential distributions, together with some numerical examples.     

Inventory management: Is there a knock-on effect?

Over the last several years a number of independent empirical studies have shown that organizational performance is related to a portfolio of management techniques, clearly demonstrating that there is no single “silver bullet.” In fact, these studies indicate that performance is positively correlated with the number of techniques employed and the depth of their implementation. Operational outcomes in areas like product quality, on-time delivery and productivity, as well as market measures like margins and export sales are both positively related to the implementation of a variety of techniques. This paper explores the relationship between the use of effective inventory management practices (as reflected in inventory turnover) and the implementation of other manufacturing practices. The hypothesis is that effective inventory management practices have a positive knock-on effect on the implementation of other practices. Since organizational performance is related to the implementation of multiple practices, the knock-on effect should have a positive effect on performance as well. The results show that inventory turnover is significantly related to the implementation of other techniques and weakly related to an index of overall company performance. This suggests a positive knock-on effect, but that it takes more than inventory management to achieve high levels of performance. Having established the knock-on relationship adds more evidence to the notion that management excellence in one area begets management excellence in others.     

A simple approach for assessing the cost of system nervousness

A well-known problem in coordinating supply chain inventories is that replenishment decisions are revised due to stochastic demands. This issue is often referred to as system nervousness. The literature distinguishes between two types of nervousness: setup-oriented and quantity-oriented. It is widely accepted that cost of nervousness is difficult to measure. We argue that this cost can be evaluated by means of three well-established inventory control strategies: static uncertainty, dynamic uncertainty, and static-dynamic uncertainty. These strategies reflect extreme cases with regard to the setup- and the quantity-oriented nervousness, and provide a simple yet an objective measure to assess the cost of system nervousness. Our results are of practical importance. We highlight that the setup-oriented nervousness, which is considered to be the most critical in practice, can be eliminated with a minor cost penalty. This is, however, not the case for the quantity-oriented nervousness.     

The complex relationship between inventory control and organisational setting: Theory and practice

Within companies it is widely recognised nowadays that the performance of inventory systems is not only determined by the way the inventory system is planned and controlled but also by its organisational architecture. Notwithstanding its importance, the field of production and operations management still lacks however, a comprehensive body of knowledge integrating both control and organisational aspects of inventory systems. One of the reasons for this shortcoming seems to be a lack of understanding of how inventory planning interacts with its organisational embedding in practice. In this article, this interdependence is further explored. Firstly, a conceptual framework is presented. The framework has been the starting point for five case studies performed during the last years. In this article data gathered over a period of more than fifteen years concerning one company is further explored. One of the main conclusions derived from this longitudinal case study is that organisations often try to neutralise shortcomings in the inventory planning and control system by applying organisational measures. In addition to this positive congruence, negative forms of congruence were also found. Shortcomings in the inventory control system are then negatively re-enforced by its organisational setting. The findings of the case study also suggest that companies often do not apply a clear and well-defined policy regarding the organisational setting of advanced inventory planning and control systems. We end this article by arguing that objectified notions on (re)designing inventory planning and control and its organisational design often under-emphasise irrational behaviour of the parties involved. A further elaboration of the framework presented in this article integrating operations management concepts and organisational theory therefore seems to be worthwhile.     

构建高效率组织工作机制 深化物资供应质量管理

从现行的物资供应组织工作机制和质量管理现状入手，阐述了物资供应的职责和已建立的质量保证体系模式，通过对工作中存在问题的调查分析，发现有章不循，责任心差是影响质量控制关键，为此推行了以明确岗位职责，建立目标体系为内容的岗位责任推进机制；以动态管理和三岗制为内容的工作创新机制，以理顺业务流程，以制度管人为内容的监督约束机制；以提高自身素质为内容的教育机制。     

A novel differential evolution algorithm for joint replenishment problem under interdependence and its application

In this paper, we propose a new differential evolution (DE) algorithm for joint replenishment of inventory using both direct grouping and indirect grouping which allows for the interdependence of minor ordering costs. Since solutions to the joint replenishment problem (JRP) can be represented by integer decision variables, this makes the JRP a good candidate for the DE algorithm. The results of testing randomly generated problems in contrastive numerical examples and two extended experiments show that the DE algorithm provides close to optimal results for some problems than the evolutionary algorithm (EA), which has been proved to be an efficient algorithm. Moreover, the DE algorithm is faster than the EA for most problems. We also conducted a case study and application results suggest that the proposed model is successful in decreasing total costs of maintenance materials inventories significantly in two power companies.     

Managers’ view of a new inventory paradigm

The significance of inventories in business operations have never been denied. The actual role of inventories, however, is changing over time, as required by the business environment. This paper provides empirical background to the thesis, which says that the role of inventories in the “Golden Era” of inventory research, which was in the 1950s, was significantly different from that of today because of fundamental changes in business. This development requires new approaches in research as well.After a summary of the antecedents, the results of a survey are analysed, and they support the above thesis. The lack of difference between the inventory performance measured by the turnover rate of those companies, whose managers accept and those who deny the birth of the new paradigm calls attention to the need for the elaboration of a more complex inventory performance measurement.     

Stochastic optimization for transshipment problems with positive replenishment lead times

Transshipments, monitored movements of material at the same echelon of a supply chain, represent an effective pooling mechanism. Earlier papers dealing with transshipments either do not incorporate replenishment lead times into their analysis, or only provide a heuristic algorithm where optimality cannot be guaranteed beyond settings with two locations. This paper uses infinitesimal perturbation analysis by combining with a stochastic approximation method to examine the multi-location transshipment problem with positive replenishment lead times. It demonstrates the computation of optimal base stock quantities through sample path optimization. From a methodological perspective, this paper deploys a duality-based gradient computation method to improve computational efficiency. From an application perspective, it solves transshipment problems with non-negligible replenishment lead times. A numerical study illustrates the performance of the proposed approach.     

Knowledge inventories and managerial myopia

Managing knowledge inventories is the central issue posed by the knowledge‐based view of the firm. Knowledge inventory management involves acquiring, retaining, deploying, idling, and abandoning technologies. Because of future opportunities to switch technologies over time, managing knowledge inventories requires valuing flexibility. Real option theory presents normative conceptual frameworks and pricing formulas for valuing flexibility. These formulas assume managers consider the full time horizon of technologies as well as all available substituting and complementary technologies. This study considers the implications of violations of these assumptions (i.e., temporal and spatial myopia) for managers' technology investment decisions. Specifying decision criteria under alternative forms of myopia reveals possible sources and patterns of technology management decision errors. This study highlights the cognitive sources of path dependencies and organizational rigidities. Copyright © 2002 John Wiley & Sons, Ltd.     

Inventory management in a multi-echelon spare parts supply chain

In many industrial sectors, firms are dealing with a demand which is more and more uncertain often due to the supply chain structure. One of the most critical effects of demand uncertainty is the simultaneous increase of inventories and decrease of customer service. This work describes an integrated system for managing inventories in a multi-echelon spare parts supply chain, in which customers of different size lay at the same level of the supply chain. The differences in size generate demand peaks and thus a very variable and lumpy demand pattern. The analysis presented in the paper stems from a case study in the field of durable goods spare parts. The paper contributes in three ways: on the one hand, it shows that consistency between managerial solutions and supply chain structure enables to enhance operative performances. On the other hand, it provides a new solution to a problem that characterises several different industrial contexts. Eventually, it highlights that the exploitation of a larger and more reliable set of information dramatically improves performance.     

Perspectives on roadmaps: how organizations talk about the future

Roadmaps, in the traditional sense, are concerned mostly with space and position. In the way that industrial and public research institutions use the term, though, roadmaps reveal the time dimension of technological progress. The many and increasing applications of roadmaps have generated confusion about what they are for and what they accomplish amid the many tools and techniques of managing technology. Roadmapping is itself a trend, while it seeks to exploit the trends underlying technology. The practice has side effects, biases, and behavioral consequences that are often not visible to practitioners, and until now, were unexamined by researchers. This study provides an organizational perspective on roadmapping as currently practiced, presents the experience of several organizations that have implemented it, and evaluates the results. Using a case‐based, exploratory method the author addresses several practical questions, such as: What are the effects of roadmapping? How are they measured? Is roadmapping always appropriate? How would an organization know if it was roadmapping well? What are the various kinds of roadmaps and how do they relate to each other? In addition, some more general lessons about organizational behavior emerge from the case data. Cases were selected from several large industrial firms participating in a research consortium exploring the modern challenges and tools of technology management. These firms granted extensive access to the research team and actively participated in the analysis, demonstrating a unique and productive model of research collaboration between academic researchers and business practitioners. Central to an understanding of roadmapping behavior is noticing the tension between its dual nature. Roadmaps are both forecasts of what is possible or likely to happen, as well as plans that articulate a course of action. They are, in a sense, personal to their authors. Roadmaps can be used to align organizations in times of predictable change, but have limited insight into disruptive change. The most influential roadmaps originate as responses to perceived threats, and link the technical storyline to organizational and personal concerns. For those who would implement the technique, the article suggests practical ways to use these and other findings and offers basic definitions and vital questions for future research.     

A multiple objective particle swarm optimization approach for inventory classification

This paper presents a particle swarm optimization approach for inventory classification problems where inventory items are classified based on a specific objective or multiple objectives, such as minimizing costs, maximizing inventory turnover ratios, and maximizing inventory correlation. In addition, this approach determines the best number of inventory classes and how items should be categorized for the desired objectives at the same time. Experiments are employed to determine the best combination of algorithm parameter values. Extensive numerical studies are conducted and results are compared to other known classification methods. The performance of the algorithm on a practical case is also presented.     

Environmentally responsible inventory models: Non-classical models for a non-classical era

Mathematical models of inventory typically include the three inventory associated costs of surplus, shortage and ordering. These classic inventory models are then analysed so as to choose inventory parameters that usually minimise the total cost of operating the inventory system being investigated.Unfortunately, classic inventory models do not provide a meaningful basis for analysing many real and increasingly important practical inventory problems and situations. It is therefore not surprising that over recent years, several authors have discussed these issues in broad terms and suggested that a new paradigm needs to be developed.This paper develops some specific aspects of this discussion. In particular, the paper identifies a range of inventory problems that are not covered appropriately by traditional inventory analysis. One of these is to design responsible inventory systems, i.e. systems that reflect the needs of the environment. The paper then examines the importance of inventory planning to the environment in greater detail. For example, packaging is important, not only because of its costs and the protection that it provides to the inventory items, but also because of its eventual effects on the environment in terms of the use of resources and potential landfill. For similar reasons, waste, which can result from poor inventory management, is highly important. The location of stores is important because location affects transport costs. Thus the influence of the secondary aspects of most inventory models; packaging, waste and location are important but, even more important are the inter-relations with the total system. In particular, the location of the manufacturing plants and the effect that inventory planning has on the logistics chain, potentially have considerable environmental implications. Inventory is part of a wider system.However, until the cost charged for an activity reflects the true environmental cost of that activity, it is likely that decisions will be made on the basis of erroneous data. In that situation, we are faced with either determining the environmental cost of specific actions or to use environmental costs that are somewhat contrived; in which case it may be more sensible to use very different performance measures and models. The paper discusses these ideas and ways in which inventory policies may reassure us with our environmental concerns.     

Management control and the production environment: A review

A modern production environment reasonably requires a management control system that is tailored to fit the specific circumstances of this kind of organization. A modern environment relates to many elements: flexible machines, low inventories, production to order, etc. This paper reviews and discusses the literature that addresses the relationship between elements of the production environment and aspects of management control systems. This review of the relevant conceptual and empirical research in the field reveals limited and inconclusive evidence on the extent to which organizations have aligned management control systems with the production environment. The review indicates that new forms of production and inventory control (e.g., just in time) are not always easily captured by the conventional classifications of technology that are commonly used in the empirical literature. The paper also addresses problems and issues related to the existing literature, and makes suggestions for avenues of future research.     

Business cycle stylized facts and inventory behaviour: New evidence for the Euro area

The purpose of this paper is for the first time to use Business Tendency Survey data, first, to identify new facts that are useful for the interpretation of the decline in the volatility of real activity in the Euro area, and, second, to test the inventory management hypothesis as an explanation for the Great Moderation in Europe. We present stylized facts from the Business Tendency data on series for inventories, current production, current orders, and expected production for the Euro area, emphasizing the decline in the volatility of the series. Further, we investigate whether the decline in inventory volatility can be attributed to an endogenous change in the persistence of shocks to the accumulation dynamics of inventories or to an exogenous change in the shocks hitting the inventory optimisation process. Our results at Euro level generally indicate that there is no evidence of a break in the inventory accumulation process. On the contrary, the impact of exogenous shocks on inventory volatility appears to be steadily declining over time, beginning from the mid-1980s.     

An alternative estimate of real inventory change for national economic accounts

In 1996, the Bureau of Economic Analysis adopted the Fisher index (a particular superlative index) for the national income and product accounts of the United States (NIPAs). This change was, on balance, a major improvement in the US national accounts, for it greatly improved the measurement of rates of growth of real economic aggregates. However, the change posed a special problem for inventories: the Fisher index is not suitable for time series that, like inventory change, exhibit changes in sign. The Fisher quantity relative is the geometric mean of Laspeyres and Paasche quantity relatives. When some detailed components of inventory change are negative, the Laspeyres and Paasche can easily differ in sign, in which case the Fisher index is undefined. In the NIPAs, this problem is avoided by measuring real-inventory change as the period-to-period change in inventory stocks. However, attempts to approximate real GDP by two-stage aggregation, combining this measure of inventory change and other major GDP subaggregates, show that the NIPA measures of real inventory change and of real GDP are inconsistent. This paper attempts to resolve the problem by investigating an alternative measure of inventory change.The alternative measure of inventory change considered is the difference between a Fisher index of inventory acquisitions and a Fisher index of inventory disposals. The consistency of this measure with real GDP is first examined analytically. It is shown that, for inventory changes likely to be observed, the approximation error in the aggregation of GDP components due to measuring inventory change as the difference between Fisher indexes of inventory acquisitions and disposals should be small. Second, the consistency of the alternative measure with real GDP is tested over the period 1977–98. The tests compare the actual approximation errors in the two-stage aggregation of GDP using inventory change based on the two methods: the present NIPA methodology and the alternative, Fisher difference method. These tests provide further support for the alternative method of estimating real inventory change.     

Distribution contracts to support optimal inventory holdings under demand uncertainty

When suppliers produce products for which demand is uncertain, they face a problem of inducing downstream distributors to stock inventory levels that the suppliers prefer. This paper considers a wide array of alternative supply contracts, each of which consists of a mixture of constant per-unit wholesale prices, buy-back arrangements, and post sale payments contingent on sales made, such as revenue sharing or buybacks. We show that linear supply contracts specifying any combination of two of these three instruments can implement the vertical integrated outcome for a monopoly, thereby generating the supplier's preferred inventory configuration and price distribution. We extend our results to differentiated product oligopoly, demonstrating that each supplier obtains its preferred inventory configuration and price distribution, given the choices of its rival. Distributors choose optimal inventories from the suppliers' standpoint, even if suppliers do not know the distribution of demand uncertainty, and, given the perfect competition among distributors, all profits in the supply chain are captured by suppliers. Thus, suppliers are able to deal with demand uncertainty with remarkably little information about demand, and without the need to control dealer actions in detail. In particular, suppliers need not specify either dealer inventories or resale prices, but instead encourage distributors to order based on information in their possession and to set prices that generate desirable resale price dispersion.     

The management of cross-disciplinary research

This paper is based on a critical review of the proceedings of the Second International Conference on Interdisciplinary Research held in 1981 in Manchester, England. Two sets of issues important for the management of collaborative research are identified. One is the need to remove ambiguities from the meanings of commonly used terminology, for which more precise definitions are proposed. Cross-disciplinary is used to describe only the nature of the task; multi- and inter-disciplinary are seen as describing alternative organizational ways of executing it. Which way should be used depends objectively on the circumstances.
The other set of issues is concerned with the effect institutional setting could have on the possibility of carrying out effective cross-disciplinary activity. Six different settings are identified and broad concepts of group dynamics are used to identify theoretical and practical problems and their possible solutions. The paper concludes with a survey of the practical implications for those managing cross-disciplinary research.     

Generalized single-vendor multi-buyer integrated inventory supply chain models with a better synchronization

Researchers have synchronized both the single-vendor single-buyer and the single-vendor multi-buyer integrated inventory supply chains by transferring the lot either only with equal-sized or only with unequal-sized sub-lot (batches). However, synchronization by transferring the lot with unequal and/or equal-sized batches (combination of unequal and equal sized batches along with the previous two cases), though available in the former case, the latter one lacks it. Accumulating the inventory in each of the vendor and the buyers, here we develop a generalized single-vendor multi-buyer supply chain model individually by extending the idea of such synchronization, and present logical development of their minimal cost solution techniques. Thereafter, we highlight theoretically the uniqueness of the present techniques by showing the best available methods as their special cases. Special cases are validated with the solutions of some numerical problems. Then their comparative studies with recently developed techniques on two numerical problems are carried out to show significant cost reductions.