Durk-Jouke van der Zee

Simulation modelling for food supply chain redesign; integrated decision making on product quality, sustainability and logistics

Food supply chains are confronted with increased consumer demands on food quality and sustainability. When redesigning these chains the analysis of food quality change and environmental load of new scenarios is as important as the analysis of efficiency and responsiveness requirements. Simulation tools are often used for supporting decision-making on supply chain (re)design when logistic uncertainties are in place, building on their inherent modelling flexibility. Mostly, the underlying assumption is that product quality is not influenced by or does not influence chain design. Clearly, this is not true for food supply chains, as quality change is intrinsic to the industry. We propose a new integrated approach towards logistics, sustainability and food quality analysis, and implement the approach by introducing a new simulation environment, ALADIN™. It embeds food quality change models and sustainability indicators in discrete event simulation models. A case example illustrates the benefits of its use relating to speed and quality of integrated decision making, but also to creativity in terms of alternative solutions.

Proportion of Patients Treated With Thrombolysis in a Centralized Versus a Decentralized Acute Stroke Care Setting

Background and Purpose— Today, treatment of acute stroke consists of tissue-type plasminogen activator (tPA), admission to a stroke unit, and aspirin. Although tPA treatment is the most effective, there is substantial undertreatment. Centralized care may affect rate, timing, and outcome of thrombolysis compared to decentralized treatment in community hospitals. The present study aimed to assess the impact of organizational models on the proportion of patients undergoing tPA treatment. Methods— A prospective, multicenter, observational study among 13 hospitals in the North of the Netherlands was conducted. In the centralized model, tPA treatment for 4 hospitals was administered in 1 stroke center. The decentralized model comprised 9 community hospitals. Primary outcome was the proportion of patients treated with tPA. Secondary outcome measures were proportion of patients arriving within 4.5 hours, safety, 90-day functional outcome, and onset-to-door, door-to-needle, and onset-to-needle times. Potential confounders were adjusted using logistic regression analysis. Results— Two hundred eighty-three and 801 ischemic stroke patients were enrolled in the centralized and decentralized settings. Numbers of patients treated with tPA were 62 (21.9%) and 113 (14.1%) (OR, 1.72; 95% CI, 1.22–2.43). Adjusting for potential confounders did not alter results (OR, 2.03; 95% CI, 1.39–2.96). In the centralized setting, significantly more patients arrived at the hospital within the 4.5-hour time window (P<0.01), and shorter door-to-needle times were reached (35 versus 47 minutes). Other secondary outcome measures did not differ across setting. Conclusions— In a centralized setting, the results demonstrate a 50% increased likelihood of treatment. Prehospital factors seem to contribute to this result.

Conceptual Modeling for Discrete-Event Simulation

Bringing together an international group of researchers involved in military, business, and health modeling and simulation, Conceptual Modeling for Discrete-Event Simulation presents a comprehensive view of the current state of the art in the field. The book addresses a host of issues, including: What is a conceptual model? How is conceptual modeling performed in general and in specific modeling domains? What is the role of established approaches in conceptual modeling? Each of the books six parts focuses on a different aspect of conceptual modeling for simulation. The first section discusses the purpose and requirements of a conceptual model. The next set of chapters provides frameworks and tools for conceptual modeling. The book then describes the use of soft systems methodology for model structuring as well as the application of software engineering methods and tools for model specification. After illustrating how conceptual modeling is adopted in the military and semiconductor manufacturing, the book concludes with a discussion on future research directions. This volume offers a broad, multifaceted account of the field by presenting diverse perspectives on what conceptual modeling entails. It also provides a basis upon which these perspectives can be compared.

Safety stock or safety lead time: coping with unreliability in demand and supply

Safety stock and safety lead time are common measures used to cope with uncertainties in demand and supply. Typically, these uncertainties are studied in isolated instances, ignoring settings with uncertainties both in demand and in supply. The current literature largely neglects case study based contexts and, often, single product situations are investigated in which machine set-ups are not considered. Based on the problems and findings in a case study, we investigate the effects of safety stock and safety lead time on delivery performance in a multi-product setting. The outcomes of the extensive simulation study indicate that utilising a safety lead time results in a higher delivery performance where there is a variable supply, whereas having a safety stock results in a higher delivery performance where there is unreliable demand information. In contrast to earlier findings in the single product situation, this study shows that managers facing the combination of unreliability in demand information and supply variability in a multiple product situation should opt for a safety lead time as the most effective way of improving their delivery performance.Safety stock and safety lead time are common measures used to cope with uncertainties in demand and supply. Typically, these uncertainties are studied in isolated instances, ignoring settings with uncertainties both in demand and in supply. The current literature largely neglects case study based contexts and, often, single product situations are investigated in which machine set-ups are not considered. Based on the problems and findings in a case study, we investigate the effects of safety stock and safety lead time on delivery performance in a multi-product setting. The outcomes of the extensive simulation study indicate that utilising a safety lead time results in a higher delivery performance where there is a variable supply, whereas having a safety stock results in a higher delivery performance where there is unreliable demand information. In contrast to earlier findings in the single product situation, this study shows that managers facing the combination of unreliability in demand information and supply variability in a multiple product situation should opt for a safety lead time as the most effective way of improving their delivery performance.

Thrombolysis in acute ischemic stroke: a simulation study to improve pre- and in-hospital delays in community hospitals.

Background Various studies demonstrate better patient outcome and higher thrombolysis rates achieved by centralized stroke care compared to decentralized care, i.e. community hospitals. It remains largely unclear how to improve thrombolysis rate in decentralized care. The aim of this simulation study was to assess the impact of previously identified success factors in a central model on thrombolysis rates and patient outcome when implemented for a decentral model. Methods Based on a prospectively collected dataset of 1084 ischemic stroke patients, simulation was used to replicate current practice and estimate the effect of re-organizing decentralized stroke care to resemble a centralized model. Factors simulated included symptom onset call to help, emergency medical services transportation, and in-hospital diagnostic workup delays. Primary outcome was proportion of patients treated with thrombolysis; secondary endpoints were good functional outcome at 90 days, Onset-Treatment-Time (OTT), and OTT intervals, respectively. Results Combining all factors might increase thrombolysis rate by 7.9%, of which 6.6% ascribed to pre-hospital and 1.3% to in-hospital factors. Good functional outcome increased by 11.4%, 8.7% ascribed to pre-hospital and 2.7% to in-hospital factors. The OTT decreased 17 minutes, 7 minutes ascribed to pre-hospital and 10 minutes to in-hospital factors. An increase was observed in the proportion thrombolyzed within 1.5 hours; increasing by 14.1%, of which 5.6% ascribed to pre-hospital and 8.5% to in-hospital factors. Conclusions Simulation technique may target opportunities for improving thrombolysis rates in acute stroke. Pre-hospital factors proved to be the most promising for improving thrombolysis rates in an implementation study.

Conceptual modeling for simulation-based serious gaming

In recent years several simulation-based serious games have been developed for mastering new business concepts in operations management. This indicates the high potential of simulation use for pedagogical purposes. Unfortunately, this potential is hardly reflected in simulation methodology. We consider this issue by identifying alternative demands game use of simulation sets for model building and application. Moreover, we propose a framework for conceptual modelling for simulation-based serious gaming, which addresses relevant issues in a systematic, step-wise manner. Use of the framework is illustrated by two case examples, highlighting simulation use for training and education respectively.

Dynamic one-way traffic control in automated transportation systems

In a project on underground freight transportation using Automated Guided Vehicles, single lanes for traffic in two directions are constructed to reduce infrastructure investment. Intelligent control rules are required to manage vehicle flows such, that collision is avoided and waiting times are minimised. In contrast to standard traffic control at intersections, these control rules should take into account significant driving times along the single lane (in our application up to 8 min). Whereas periodic control rules are often applied in traffic theory, we focus on adaptive rules such as look-ahead heuristics and dynamic programming algorithms. Numerical experiments show that our control rules reduce waiting times by 10–25% compared to a straightforward periodic rule. Dynamic programming yields the best results in terms of mean waiting times.

Simulation and gaming as a support tool for lean manufacturing systems: a case example from industry

In this article we illustrate how simulation and gaming can be used to support lean manufacturing systems. More in particular we study a case example from industry - a manual assembly line for mail-inserting systems - for which we have developed a simulation game. This paper focuses on the development steps of the simulation game. The objective of the game is to support the introduction of lean principles in an existing assembly line. The simulation game can be used to demonstrate applicability of a lean control concept at the assembly line and to train workers to make appropriate control decisions within this concept. In this paper, we indicate a definite need for the development of this game. The systematic way in which it is developed, the use of a general simulation language in the design phase, and its usefulness may stimulate the introduction of simulation games in more industrial settings

Building insightful simulation models using Petri Nets - A structured approach

Petri Nets have essential strengths in capturing a systems static structure and dynamics, its mathematical underpinning, and providing a graphical representation. However, visual simulation models of realistic systems based on Petri Nets are often perceived as too large and too complex to be easily understood. This constrains stakeholders in participating in such modeling and solution finding, and limits acceptance. We address this issue by considering a structured approach for guiding the analyst in creating more insightful models. Key elements are a domain-related reference architecture that supports conceptual modeling coupled with uniform rules for mapping high-level concepts onto low-level Petri Net components. The proposed approach is implemented and illustrated in the manufacturing domain.

Family-based dispatching: anticipating future jobs (Submitted for Phd Prize)

Group Technology exploits similarities in product and process design to meet the diversity of customer demand in an economic way. In this paper we consider one of the implementations of this concept – family-based dispatching. Intrinsic to family-based dispatching is the grouping of similar types of products for joint processing. In this way the number of set-ups may be reduced. Consequently, lead-time performance of the shop can be improved. We extend existing rules for family-based dispatching by including data on upcoming job arrivals. Typically, this type of data resides in the minds of the operators, or is stored in a shop-floor control system. Its availability allows for (1) better estimates of the composition of a process batch for a family, (2) the consideration of families for which no products are available at the decision moment, and (3) the possibility to start set-ups in anticipation of future job arrivals. The potential of including forecast data in decision-making is demonstrated by an extensive simulation study of a single-machine shop. Results indicate the possibility of significant improvements of flow time performance.