Impact of frequency of alignment of physical and information system inventories on out of stocks: A simulation study

Inaccuracy in the information system inventory as compared to the physical inventory may lead to out of stocks. Inaccuracy may occur for many reasons, a principal one being random losses such as theft. One way to reduce this inaccuracy is to adjust the inventory information in the systems at some regular frequency. Such alignments are quite expensive in practice. Thus how often to align the two inventories is the focus of this research. A simulation model is employed to investigate the effect of such loss defined by the stock loss parameter (λ) and the frequent alignment of physical and information system inventories on the stockout (S_out) and average inventory (I). A term to be called the effective value of stock loss parameter is introduced to signify the effect of frequency of alignment (f) on S_out. The results derived in this study provide a powerful tool in the hands of an inventory manager. It has been noted that, so far as stockout is concerned, by selecting a moderate value of alignment frequency (f), the effective value of stock loss parameter (λ_e) can be reduced to∼ λ/f. The accuracy of S_out and I values across a number of runs in the simulation studies, sensitivity of S_out and I on various parameters and the nature of stochastic demand distribution, and application of these results with or without deployment of RFID to reduce the loss due to stockout are also discussed. The results, verified under various scenarios, indicate that there is a significant reduction in stockout loss when the alignment is done monthly vs. annually, but it does not add much value beyond a monthly check.     

Design of a novel adaptive inventory control system based on the online identification of lead time

In this paper, an adaptation method for the online identification of lead time is incorporated in production–inventory control systems. Based on the lead time estimate, the tuning parameters are updated in real time to improve the efficiency of the system. Combination of the adaptive scheme with a proportional control law is able to eliminate the inventory drift that appears when the actual lead time is not known in advance or when it varies with time. A detailed analysis is provided for the proposed production–inventory system, including a stability analysis and the quantification of its bullwhip effect. Several examples and comparison with state-of-the-art alternative approaches illustrate the efficiency of the system.     

An investigation of setup instability in non-stationary stochastic inventory systems

In stochastic inventory systems unfolding uncertainties in demand lead to the revision of earlier replenishment plans which in turn results in an instability or so-called system nervousness. In this paper, we provide the grounds for measuring system nervousness in non-stationary demand environments, and gauge the stability and the cost performances of (R,S) and (s,S) inventory policies. Our results reveal that, both the stability and the cost performance of inventory policies are affected by the demand pattern as well as the cost parameters, and the (R,S) policy has the potential to replace the cost-optimal (s,S) policy for systems with limited flexibility.     

Just-in-time: A cross-sectional plant analysis

This paper uses a data base of quantitative and qualitative plant-level cross-sectional data to analyze the relative performance of Just-in-time (JIT) and non-JIT plants operating in two distinct manufacturing industries: electronic components and auto-parts. A number of conjectures made by the literature concerning the relationship between JIT manufacturing and plant inventory holdings, costs and profits are tested. Consistent with many of these conjectures, the results suggest that JIT manufacturing at the plant level is associated with greater productivity in inventory usage, lower total and variable costs, but not fixed costs, and higher profits. The success of JIT plants along these dimensions is found to be related to the length of experience with JIT manufacturing, and process quality and leanness but unrelated to product quality, quality control or the extent of plant unionization.     

The shaping of inventory systems in health services: A stakeholder analysis

Although many studies have addressed the diagnosing and redesign of inventory systems in an industrial setting, the field of operations management seems to lack a thorough understanding of the process of shaping inventory systems in a health care setting. In this article, a contribution is made to fill this gap by exploring the process of reshaping a hospital inventory system of medicines by means of an exploratory case study. In doing so, we concentrate on the question how the outcomes of this process are affected by the different stakeholders involved. Our case study indicates that decisions made during this reshaping process are heavily influenced by the dynamics of the relationships and interactions between the stakeholders involved in the project. Based on our case study there are also some strong indications especially in a health care setting, the existence of multiple stakeholders having a multi-goal focus regarding the inventory system can have a strong influence on the outcomes of inventory projects. For project managers it is important to be aware of these characteristics and circumstances in order to help health service organisations to develop and use inventory systems more effectively.     

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.     

A holistic network model for supply chain analysis

Supply chain researchers are experiencing a conceptual and analytical paradox. They are asked to move beyond dyadic analyses and investigate larger network effects with only a limited analytical toolkit. This research proposes the use of bilinear mixed-modeling to holistically analyze supply chain phenomena. Through this approach, researchers are able to account for multiple supply chain relationships, higher-order dependencies among member firms, and simultaneously evaluate covariates from buyer and seller perspectives. The model is validated through the lens of a pervasive supply chain problem commonly referred to as the bullwhip effect. A sample of firms from the US apparel industry in 2004 is analyzed and then the findings are confirmed using data from 2005. In addition to validating the model through the presence of the bullwhip effect, the bilinear model illuminates variables such as advertising, price deals, inventory turnover, and inventory backlogs that exacerbate or diminish inventory differences between firms in a supply chain. The results extend research on supply networks and supply efficiency to a more holistic level and show that higher-order dependencies are important drivers of supply chain phenomena.     

Reducing the mean supply delay of spare parts using lateral transshipments policies

Lateral transshipment has been studied lately as a promising policy for increasing the performances of multi-echelon spare parts inventory system. By lateral transshipment spare parts can be moved from one location with excess inventory to another location, at the same echelon, in shortage, with the aim of reducing supply delays of spare parts. This paper will examine the relative effectiveness of two lateral shipments approaches in reducing the mean supply delay (MSD) of a non-repairable item, with respect to a classical policy of no lateral shipments. A simulation model of a two echelon supply network has been implemented and an experiment has been performed by varying different parameters of the supply network, such as the number of warehouses (locations at the lower echelon), the supply lead time from the central depot, the spare parts demand uncertainty, and the size variability of the warehouses. Results show appreciable reductions of MSD when lateral shipments are allowed with respect to the classical policy, in almost every network configuration.     

Evaluating the robustness of lead time demand models

This paper examines the robustness of lead time demand models for the continuous review (r, Q) inventory policy. A number of classic distributions, (e.g. normal, lognormal, gamma, Poisson and negative binomial) as well as distribution selection rules are examined under a wide variety of demand conditions. First, the models are compared to each other by assuming a known demand process and evaluating the errors associated with using a different model. Then, the models are examined using a large sample of simulated demand conditions. Approximation results of inventory performance measures—ready rate, expected number of backorders and on-hand inventory levels are reported. Results indicate that distribution selection rules have great potential for modeling the lead time demand.     

The performance of two popular service measures on management effectiveness in inventory control

Service is one of the inventory managers' concerns and is frequently incorporated into the ordering decisions. Since there are multiple measures of service available for evaluating the efficiency of an inventory system, a comparative study is necessary and has not been addressed in the literature. This paper evaluates two popular service measures, which are the probability of no stockout during lead time and the fill rate, in the context of continuous inventory systems. The performance of the two measures is examined by evaluating the tradeoff among the cost, the level of service, and the inventory turnover ratio.     

Base stock policies with degraded service to larger orders

We study an inventory system controlled by a base stock policy assuming a compound renewal demand process. We extend the base stock policy by incorporating rules for degrading the service of larger orders. Two specific rules are considered, denoted as Postpone(q,t) and Split(q), respectively. The parameter q distinguishes between regular orders (of size less than or equal to q) and larger orders. We develop mathematical expressions for the performance measures: order fill rate of the regular orders and average on-hand inventory level. We make numerical experiments where the postpone parameter t and the base stock levels of each rule are such that all customers (of both order types) are indifferent between the two rules. When comparing the difference in the average on-hand inventory levels, we can then make an assessment of the threshold value of the cost of splitting an order (which may otherwise be hard to quantify) in the rule Split(q). Our numerical results indicate that this threshold value is increasing in the variance of the order sizes. Based on the numerical experiment our conclusion is therefore that when the variance of the order sizes is low, then Postpone(q,t) seems to be a good option, while when the variance is high, then Split(q) is more competitive.     

Comparison of benefits of radio frequency identification: Implications for business strategic performance in the U.S. and Korean retailers

This study explores a model to compare the benefits of RFID technology on supply chain management by focusing on retail industry. A path model was estimated to examine structural relationships among technological infrastructure, RFID benefits, and business strategic performance in the U.S. (n = 70) and Korean samples (n = 87). Result showed that technological infrastructure was required to more improve inventory management, store operation and demand management, leading to business strategic performance. Especially, data system automation was perceived as a prerequisite for improving inventory management for both countries. Hardware/software application was significantly related to the RFID benefit of inventory management for U.S. retailers, whereas, it was related to the benefits of efficient store operation and demand management for Korean retailers. Business strategic performance was significantly determined by RFID benefit factors (e.g., inventory management and demand management) for U.S. retailers and Korean retailers. The managerial implication for business to business strategic performance in the U.S. and Korean retail industries was discussed from a retailer's perspective.     

A perishable inventory model with Markovian renewal demands

In the inventory model, people usually assume that the inter-demand time is independently identical distributed which may not be true in reality. Here we study an (s,S) continuous review model for items with an exponential random lifetime and a general Markovian renewal demand process. By constructing Markovian renewal equations, we derive the mean and the variance of the reorder cycle time and lead to a simple expression for the total expected long run cost rate. The numerical results illustrate the system behavior and lead to managerial insights into controlling such inventory systems.     

MRP: A tool for product and sales management

One of the most important developments in the area of production and inventory control in recent years is the structuring and application of overall systems to control the flow of work and materials through the plant. The important aspects of these new systems are discussed below, as well as the benefits that such systems can provide. Attention is given to the opportunities that such systems afford the marketing function, in the context of product and sales management.     

An organizational perspective on inventory control: Theory and a case study

Inventory control is a well-covered area in literature. Nowadays, many concepts and techniques are available for effectively controlling inventories. Eminent examples are stochastic models to determine order quantities, techniques for forecasting demand and different kinds of ABC analysis. Remarkably enough, the theoretical backgrounds of these concepts and techniques show a great deal of similarity: they all stem from the field of operations management and operations research. Despite the value of the available concepts, such a one-sided approach has its limitations in practice. In this article it is argued that it is important to take a broader view when dealing with practical inventory control problems. First of all, an organizational perspective on inventory control is developed. The main idea behind this perspective is the importance of the organizational context of inventories. This means that in addition to the traditional points for attention, such as order quantities and replenishment strategies, other aspects need attention as well. They are, for example, the allocation of responsibilities and authorities regarding inventory management, the quality of inventory information and the relevant decision-making processes. These contextual factors as well as the traditional factors are mapped out and integrated into a framework to be used in solving practical inventory control problems. One of the main features of the framework is that it is useful in both analyzing and redesigning an inventory situation. Secondly, this article describes the results of a case study regarding the application of the organizational perspective to a practical inventory control problem. The central issue of the case study is the spare parts inventory of a missionary aviation organization in Africa. The case study shows that the organizational perspective on inventory control is very helpful in dealing with inventory control problems. It is illustrated that taking into account the organizational context of inventories, especially the responsibilities and authorities of the persons concerned, is a necessity to accurately understand inventory control problems. In addition, it is illustrated that the organizational perspective leads to a more appropriate redesign.     

Optimal two-stage lot sizing and inventory batching policies

This paper extends previous studies of two-stage lot sizing problems with finite production rates. We develop various lot sizing and inventory batching (i.e., operation–unit batching (OUB) and unit–unit batching (UUB)) models under different system characteristics and lot sizing and inventory policies. The analysis of the optimality of the lot size ratio between the two stages reveals (1) that both non-increasing and non-decreasing lot sizing policies can be optimal in both OUB and UUB, (2) that a non-integer lot size ratio can be optimal in OUB, and (3) that an integer lot size ratio is always optimal in UUB. We present a simple, easy-to-implement, optimal solution approach to the two-stage lot sizing and inventory batching problem, along with examples.     

Lead time reduction strategies in a single-vendor-single-buyer integrated inventory model with lot size-dependent lead times and stochastic demand

This paper studies alternative methods for reducing lead time and their impact on the safety stock and the expected total costs of a (Q,s) continuous review inventory control system. We focus on a single-vendor-single-buyer integrated inventory model with stochastic demand and variable, lot size-dependent lead time and assume that lead time consists of production and setup and transportation time. As a consequence, lead time may be reduced by crashing setup and transportation time, by increasing the production rate, or by reducing the lot size. We illustrate the benefits of reducing lead time in numerical examples and show that lead time reduction is especially beneficial in case of high demand uncertainty. Further, our studies indicate that a mixture of setup time and production time reduction is appropriate to lower expected total costs.     

Inventory model for an inventory system with time-varying demand rate

The standard inventory problems of the multi-period have been modeled under different situations. Specifically we have considered the demand subjects of a continuous distribution and a discrete distribution, and whether the demand of each period is unchanged or not. A method to get an economic order quantity in inventory systems with discrete and unchanged demand was presented in a previous paper, and this method has been generalized to an inventory model with varying continuous demand. However, it was not achieved due to there being many classified cases in the general situations. In this article the above method is discussed in the case discrete demand to determine whether it increases or decreases from period to period. A theoretical method is presented by using previous results and some examples are given which suggest how the concept can handle on inventory system. In order to make the decision, an algorithm is also presented under some conditions, and examples are shown by using the computer software program, Mathematica, which helps to explain the findings. In general cases, we view the optimal policy in the inventory problems in only a few periods.     

Order release planning with clearing functions: A queueing-theoretical analysis of the clearing function concept

We consider the problem of planning future order releases in hierarchical production planning and control systems. An established research direction is the clearing function concept: the planned material flow through a production unit is modelled by inventory balance equations for WIP and final products, and the consequences of the stochastic properties of the material flow are modelled by clearing functions, which is the functional relationship between the level of WIP and the maximum output of a work centre in a period.Using a transient M/M/1 model, our paper shows that the usual definition of a nonlinear clearing function suffers from substantial shortcomings concerning both the definition of the function and empirical estimation of its parameters. We propose an alternative transient clearing function and derive a procedure for its parameterization.     

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.