http://repub.eur.nl/res/aut/10388/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/32038/ 2012-04-01T00:00:00Z Most inventory management systems at hospital departments are characterised by lost sales, periodic reviews with short lead times, and limited storage capacity. We develop two types of exact models that deal with all these characteristics. In a capacity model, the service level is maximised subject to a capacity restriction, and in a service model the required capacity is minimised subject to a service level restriction. We also formulate approximation models applicable for any lost-sales inventory system (cost objective, no lead time restrictions etc). For the capacity model, we develop a simple inventory rule to set the reorder levels and order quantities. Numerical results for this inventory rule show an average deviation of 1% from the optimal service levels. We also embed the single-item models in a multi-item system. Furthermore, we compare the performance of fixed order size replenishment policies and (R, s, S) policies. http://repub.eur.nl/res/pub/32040/ 2012-03-06T00:00:00Z Competitive retail environments are characterized by service levels and lost sales in case of excess demand. We contribute to research on lost-sales models with a service level criterion in multiple ways. First, we study the optimal replenishment policy for this type of inventory system as well as base-stock policies and (R, s, S) policies. Furthermore, we derive lower and upper bounds on the order-up-to level, and we propose efficient approximation procedures to determine the order-up-to level. The procedures find values of the inventory control variables that are close to the best (R, s, S) policy and comply to the service level restriction for most of the instances, with an average cost increase of 2.3% and 1.2% for the case without and with fixed order costs, respectively. http://repub.eur.nl/res/pub/25161/ 2011-03-01T00:00:00Z In 2008, Dutch anti-Islam Member of Parliament Geert Wilders produced a short video called Fitna to visualize his argument that Islam is a dangerous religion. Thousands of men and women across the globe uploaded their own videos to YouTube to criticize or support the film. In this article, we look at these alternative videos from a feminist perspective, contrasting the gender portrayal and narratives in Fitna with those in the alternative videos. We contend that Fitna expressed an extremist Orientalist discourse, in which women are presented as the current and future victims of the oppression of Muslim men and Islam. In contrast, the YouTube videos give voice to women themselves who come from across the globe, are relatively young and often active Muslims. Second, they express different view points in generically new ways, criticizing and ridiculing Wilders or producing serious and committed explanations of their own understanding of Islam. Third, although relatively few women appeared in the videos, those that did speak for themselves, not only take on Wilders, but also claim their right to speak within Islam. We propose to understand these videos as acts of citizenships through which women constitute themselves as global citizens, in some cases by engaging in deliberation as it is understood in feminist political theory, in other cases by taking a voice that can be responded to. http://repub.eur.nl/res/pub/32047/ 2010-07-01T00:00:00Z Field services are a particular type of after-sales service performed at the customer's location where technicians repair malfunctioning machines. The inventory decisions about which spare part types to take to the repair site and in what quantities is called the repair kit problem. This problem is characterized by an order-based performance measure since a customer is only satisfied when all required spare parts are available to fix the machine. As a result, the service level in the decision making process is defined as a job fill rate. In this paper we derive a closed-form expression for the expected service level and total costs for the repair kit problem in a general setting, where multiple units of each part type can be used in a multi-period problem. Such an all-or-nothing strategy is a new characteristic to investigate, but commonly used in practice. Namely, items are only taken from the inventory when all items to perform the repair are available in the right quantity. We develop a new algorithm to determine the contents of the repair kit both for a service and cost model while incorporating this new expression for the job fill rate. We show that the algorithm finds solutions which differ on average 0.2% from optimal costs. We perform a case study to test the performance of the algorithm in practice. Our approach results in service level improvements of more than 30% against similar holding costs. http://repub.eur.nl/res/pub/16463/ 2009-03-01T00:00:00Z We consider the problem of scheduling the storage and retrieval of containers in the storage area of a container terminal. Some arcs in the underlying directed network must be visited; other arcs may be-but need not be-visited. We can. therefore, consider this problem to be a special case of the directed rural postman problem. We show that this problem can be reformulated as an asymmetric Steiner traveling salesman problem. This reformulation can be efficiently solved to optimality by a combination of optimal assignments in bipartite networks for parts of the problem and dynamic programming for the connections between those parts. http://repub.eur.nl/res/pub/13641/ 2008-11-01T00:00:00Z Order picking is the warehousing process by which products are retrieved from their storage locations in response to customers' orders. Its efficiency can be influenced through the layout of the area and the operating policies. We present a model that minimizes travel distances in the picking area by identifying an appropriate layout structure consisting of one or more blocks of parallel aisles. The model has been developed for one commonly used routing policy, but it is shown to be fairly accurate for some other routing policies as well. http://repub.eur.nl/res/pub/13640/ 2008-02-05T00:00:00Z Automated Storage and Retrieval Systems (AS/RSs) are warehousing systems that are used for the storage and retrieval of products in both distribution and production environments. This paper provides an overview of literature from the past 30 years. A comprehensive explanation of the current state of the art in AS/RS design is provided for a range of issues such as system configuration, travel time estimation, storage assignment, dwell-point location, and request sequencing. The majority of the reviewed models and solution methods are applicable to static scheduling and design problems only. Requirements for AS/RSs are, however, increasingly of a more dynamic nature for which new models will need to be developed to overcome large computation times and finite planning horizons, and to improve system performance. Several other avenues for future research in the design and control of AS/RSs are also specified http://repub.eur.nl/res/pub/11873/ 2005-05-01T00:00:00Z Products can be transported in containers from one port to another. At a container terminal these containers are transshipped from one mode of transportation to another. Cranes remove containers from a ship and put them at a certain time (i.e., release time) into a buffer area with limited capacity. A vehicle lifts a container from the buffer area before the buffer area is full (i.e., in due time) and transports the container from the buffer area to the storage area. At the storage area the container is placed in another buffer area. The advantage of using these buffer areas is the resultant decoupling of the unloading and transportation processes. We study the case in which each container has a time window [release time, due time] in which the transportation should start. The objective is to minimize the vehicle fleet size such that the transportation of each container starts within its time window. No literature has been found studying this relevant problem. We have developed an integer linear programming model to solve the problem of determining vehicle requirements under time-window constraints. We use simulation to validate the estimates of the vehicle fleet size by the analytical model. We test the ability of the model under various conditions. From these numerical experiments we conclude that the results of the analytical model are close to the results of the simulation model. Furthermore, we conclude that the analytical model performs well in the context of a container terminal. http://repub.eur.nl/res/pub/11868/ 2003-05-16T00:00:00Z At container terminals, containers are transshipped from one mode of transportation to another. Within a terminal different types of material handling equipment are used to transship containers from ships to barges, trucks and trains and vice versa. Over the past decades, ships have strongly increased in size, up to 8000 TEU (Twenty feet equivalent unit container). In order to use these big ships efficiently, the docking time at the port must be as small as possible. This means that large amounts of containers have to be loaded, unloaded and transshipped in a short time span, with a minimum use of expensive equipment. This paper gives a classification of the decision problems that arise at container terminals. For various decision problems, an overview of relevant literature is presented. Quantitative models from this literature, which try to solve the problems are discussed. Finally, some general conclusions and subjects for further research are given. http://repub.eur.nl/res/pub/866/ 2002-05-17T00:00:00Z Iris Vis was born in 1974 in Leidschendam. May 2002 - Assistant professor at the School of Economics and Business Administration, Vrije Universiteit Amsterdam 1999, Visiting scholar at Georgia Institute of Technology, School of Industrial and Systems Engineering, october - december September 1997 - May 2002, Ph.D. candidate at the Rotterdam School of Management/Faculteit Bedrijfskunde, Erasmus University Rotterdam Research Areas logistics, material handling systems, cross-docking centres, number of vehicles, scheduling, storage and retrieval, container terminals http://repub.eur.nl/res/pub/11856/ 2001-01-01T00:00:00Z This paper describes the development of a minimum flow algorithm to determine the number of automated guided vehicles (AGVs) required at a semi-automated container terminal. At such a terminal the containers are transported by AGVs from the quay cranes to the automated stacking cranes and vice versa. A model and a strongly poly~lomial time algorithm are developed to solve the case in which containers are available for transport at known time instants.