Publications with Fulltext
Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
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Publication Open Access Production control of a pull system with production and demand uncertainty(Institute of Electrical and Electronics Engineers (IEEE), 2002) Department of Business Administration; Tan, Barış; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 28600We consider a continuous material-flow manufacturing system with an unreliable production system and a variable demand source which switches randomly between zero and a maximum level. The failure and repair times of the production system and the switching times of the demand source are assumed to be exponentially distributed random variables. The optimal production flow control policy that minimizes the expected average inventory carrying and backlog costs is characterized as a double-hedging policy. The optimal hedging levels are determined analytically by minimizing the closed-form expression of the cost function. We investigate two approximate single hedging policies. It is empirically shown that an approximate policy that uses a single hedging level which is the sum of a production uncertainty term and a demand uncertainty term gives accurate results for the expected average cost.Publication Open Access Capacity planning for effective cohorting of hemodialysis patients during the coronavirus pandemic: a case study(Elsevier, 2023) Bozkır, C.D.C.; Özmemiş, C.; Kurbanzade, A.K.; Balçık, B.; Tuğlular, S.; Department of Business Administration; Güneş, Evrim Didem; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 51391Planning treatments of different types of patients have become challenging in hemodialysis clinics during the COVID-19 pandemic due to increased demands and uncertainties. In this study, we address capacity planning decisions of a hemodialysis clinic, located within a major public hospital in Istanbul, which serves both infected and uninfected patients during the COVID-19 pandemic with limited resources (i.e., dialysis machines). The clinic currently applies a 3-unit cohorting strategy to treat different types of patients (i.e., uninfected, infected, suspected) in separate units and at different times to mitigate the risk of infection spread risk. Accordingly, at the beginning of each week, the clinic needs to allocate the available dialysis machines to each unit that serves different patient cohorts. However, given the uncertainties in the number of different types of patients that will need dialysis each day, it is a challenge to determine which capacity configuration would minimize the overlapping treatment sessions of different cohorts over a week. We represent the uncertainties in the number of patients by a set of scenarios and present a stochastic programming approach to support capacity allocation decisions of the clinic. We present a case study based on the real-world patient data obtained from the hemodialysis clinic to illustrate the effectiveness of the proposed model. We also compare the performance of different cohorting strategies with three and two patient cohorts.Publication Open Access Modeling and analysis of an auction-based logistics market(Elsevier, 2008) Ağralı, Semra; Department of Business Administration; Department of Industrial Engineering; Tan, Barış; Karaesmen, Fikri; Faculty Member; Faculty Member; Department of Business Administration; Department of Industrial Engineering; College of Administrative Sciences and Economics; College of Engineering; 28600; 3579We consider a logistics spot market where the transportation orders from a number of firms are matched with two types of carriers through a reverse auction. In the spot market, local carriers compete with in-transit carriers that have lower costs. In order to analyze the effects of implementing a logistics spot market on these three parties: firms, local carriers, and in-transit carriers and also the effects of various system parameters, we develop a two-stage stochastic model. We first model the auction in a static setting and determine the expected auction price based on the number of carriers engaging in the auction and their cost distributions. We then develop a continuous-time Markov chain model to evaluate the performance of the system in a dynamic setting with random arrivals and possible abandonment of orders and carriers. By combining these two models, we evaluate the performance measures such as the expected auction price, price paid to the carriers, distribution of orders between local and in-transit carriers, and expected number of carriers and orders waiting at the logistics center in the long run. We present analytical and computational results related to the performance of the system and discuss operation of such a logistics spot market in Turkey.Publication Open Access Front-office multitasking between service encounters and back-office tasks(Elsevier, 2020) Legros, Benjamin; Jouini, Oualid; Koole, Ger; Department of Business Administration; Karaesmen, Zeynep Akşin; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 4534We model the work of a front-line service worker as a queueing system. The server interacts with customers in a multi-stage process with random durations. Some stages require an interaction between server and customer, while other stages are performed by the customer as a self-service task or with the help of another resource. Random arrivals by customers at the beginning and during an encounter create random lengths of idle time in the work of the server (breaks and interludes respectively). The server considers treatment of an infinite amount of back-office tasks, or tasks that do not require interaction with the customer, during these idle times. We consider an optimal control problem for the server's work. The main question we explore is whether to use the interludes in service encounters for treating back-office, when the latter incur switching times. Under certain operating environments, working on back-office during interludes is shown to be valuable. Switching times play a critical role in the optimal control of the server's work, at times leading the server to prefer remaining idle during breaks and interludes, instead of working on back-office, and at others to continue back-office in the presence of waiting customers. The optimal policy for use of the interludes is one with multiple thresholds depending on both the customers queueing for service, and the ones who are in-service. We illustrate that in settings with multiple interludes in an encounter, if at all, the back-office work should be concentrated on fewer, longer and later interludes.Publication Open Access Building social cohesion in ethnically mixed schools: an intervention on perspective taking(Oxford University Press (OUP), 2021) Alan, Şule; Baysan, Ceren; Kubilay, Elif; Department of Economics; Gümren, Mert; Researcher; Department of Economics; Graduate School of Social Sciences and HumanitiesWe evaluate the effect of an educational program that aims to build social cohesion in ethnically mixed schools by developing perspective-taking ability in children. The program is implemented in Turkish elementary schools affected by a large influx of Syrian refugee children. We measure a comprehensive set of outcomes that characterize a cohesive school environment, including peer violence incidents, the prevalence of interethnic social ties, and prosocial behavior. Using randomized variation in program implementation, we find that the program significantly lowers peer violence and victimization on school grounds. The program also reduces the likelihood of social exclusion and increases interethnic social ties in the classroom. We find that the program significantly improves prosocial behavior, measured by incentivized tasks: treated students exhibit significantly higher trust, reciprocity, and altruism toward each other as well as toward anonymous out-school peers. We show that this enhanced prosociality is welfare improving from the ex post payoff perspective. We investigate multiple channels that could explain the results, including ethnic bias, impulsivity, empathetic concern, emotional intelligence, behavioral norms, and perspective taking. Children's increased effort to take others' perspectives emerges as the most robust mechanism to explain our results.Publication Open Access Subcontracting with availability guarantees: production control and capacity decisions(Taylor _ Francis, 2004) Department of Economics; Department of Business Administration; Tan, Barış; Faculty Member; Department of Economics; Department of Business Administration; College of Administrative Sciences and Economics; 28600We present a simplified model of a system with a producer, a subcontractor and a random demand. The demand level alternates between a high level and a low level with exponential switching times. The producer does not have enough capacity to meet the high demand. Therefore, it either produces to stock in advance or uses a subcontractor to receive additional capacity when it needs. The subcontractor serves a number of manufacturers and guarantees a long-term availability that is defined as the long-term probability that the subcontractor will be available when it is requested, to each manufacturer. Therefore, a manufacturer may not receive the requested capacity from the subcontractor immediately and waits until the subcontractor becomes available. The times that the subcontractor is available and not available are also exponential random variables. The producer uses a threshold-type policy that depends on the state of the inventory/backlogto decide how much to produce and how much to request from the subcontractor. This system is modeled analytically based on a stochastic flow rate control problem with continuous flow and discrete states in a Markovian setting. A numerical analysis of the model is used to analyze the effects of guaranteed availability on the manufacturer’s and subcontractor’s performances. Extensions to the producer’s and subcontractor’s capacity decisions and the subcontractor’s pricing decisions are also discussed.Publication Open Access Can the desired service level be achieved when the demand and lost sales are unobserved?(Institute of Electrical and Electronics Engineers (IEEE), 2002) Department of Business Administration; Tan, Barış; Karabatı, Selçuk; Faculty Member; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 28600; 38819In this paper we consider an inventory management problem in the retail industry with unobserved lost sales. The retailer does not know the demand for a particular product but she has access to Point-Of-Sale (POS) data. The retailer uses a fixed review period, order-up-to level system to control the inventory. The objective of the retailer is to achieve a pre-specified service level. We define the service level as the fraction of demand satisfied from inventory. However, due to the unobserved lost sales nature of the problem, the retailer cannot exactly measure the current service level. We propose a POS data-based mechanism for periodic updating of the order-up-to level. We show that the periodic updating approach yields the desired service level without using any inventory information. Once the periodic updating scheme converges, it also gives the actual demand distribution.Publication Open Access Markov chain test for time dependence and homogeneity: an analytical and empirical evaluation(Elsevier, 2002) Department of Business Administration; Department of Economics; Tan, Barış; Yılmaz, Kamil; Faculty Member; Faculty Member; Department of Business Administration; Department of Economics; College of Administrative Sciences and Economics; 28600; 6111This paper evaluates the small and large sample properties of Markov chain time-dependence and time-homogeneity tests. First, we present the Markov chain methodology to investigate various statistical properties of time series. Considering an auto-regressive time series and its associated Markov chain representation, we derive analytical measures of the statistical power of the Markov chain time-dependence and time-homogeneity tests. We later use Monte Carlo simulations to examine the small-sample properties of these tests. It is found that although Markov chain time-dependence test has desirable size and power properties, time-homogeneity test does not perform well in statistical size and power calculations.Publication Open Access Formulation and a two-phase matheuristic for the roaming salesman problem: application to election logistics(Elsevier, 2020) Shahmanzari, Masoud; Salhi, Said; Department of Business Administration; Aksen, Deniz; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 40308In this paper we investigate a novel logistical problem. The goal is to determine daily tours for a traveling salesperson who collects rewards from activities in cities during a fixed campaign period. We refer to this problem as the Roaming Salesman Problem (RSP) motivated by real-world applications including election logistics, touristic trip planning and marketing campaigns. RSP can be characterized as a combination of the traditional Periodic TSP and the Prize-Collecting TSP with static arc costs and time-dependent node rewards. Commercial solvers are capable of solving small-size instances of the RSP to near optimality in a reasonable time. To tackle large-size instances we propose a two-phase matheuristic where the first phase deals with city selection while the second phase focuses on route generation. The latter capitalizes on an integer program to construct an optimal route among selected cities on a given day. The proposed matheuristic decomposes the RSP into as many subproblems as the number of campaign days. Computational results show that our approach provides near-optimal solutions in significantly shorter times compared to commercial solvers.Publication Open Access Assortment-based cooperation between two make-to-stock firms(Taylor _ Francis, 2014) Department of Business Administration; Tan, Barış; Akçay, Yalçın; Faculty Member; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 28600; 51400Cooperation can potentially improve competitiveness and profitability of firms with limited resources and production capacities. We present a continuous-time Markov chain model to study an assortment-based cooperation between two independent firms with limited capacity. An assortment-based cooperation is an agreement for combining the product assortments of the two firms and offering the combined assortment to each firm’s customers. We study both centralized and decentralized cooperations. In a centralized cooperation, firms jointly make replenishment decisions, whereas in the decentralized case, firms operate under independent base stock policies and manage product exchanges through a discount-based contract where each firm supplies its own product to the other firm at a discounted price and at an agreed fill rate. Under this scheme, assortment-based cooperation also mandates each firm to effectively ration their inventories since they have to deal with two different demand streams. The discount-based contract yields the results of the centralized operation by using specific values of the contract parameters. We also prove that assortment based cooperation is always beneficial for two symmetrical firms in both a centralized and a decentralized cooperation. Our numerical experiments reveal that assortment-based cooperation is not always beneficial if the firms are not symmetrical.