Publications with Fulltext
Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
Browse
10 results
Search Results
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 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 The case against active pension funds: evidence from the Turkish Private Pension System(Elsevier, 2015) Yalçın, Atakan; Department of Business Administration; Gökçen, Umut; Department of Business Administration; College of EngineeringUsing data on private Turkish pension funds we show that most active managers are not able to provide performance beyond what could be achieved by passive indexing. The average fund beats its benchmark by only 26 basis points, before fees. We also observe herding behavior among managers' asset allocation decisions which can potentially explain their lack of overperformance. Our results strongly support the need for low-cost index funds in emerging market countries that are reforming their pension schemes. We further recommend regulatory oversight on the "activeness" of funds and introduction of default plans with more balanced asset allocations.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 Optimal sales and production rollover strategies under capacity constraints(Elsevier, 2021) Schwarz, Justus Arne; Department of Business Administration; Tan, Barış; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 28600Firms regularly replace their old product generation by a newer generation to sustain and increase their market share and profit. The product rollover problem of deciding on the number of old products to be pre-produced before the introduction of the new generation, and then deciding on the prices, sales volumes, and production volumes of the old and the new generation during the introduction under capacity constraint is considered. Production capacity limitations are common during the introduction period of a new product. We provide the first study that examines how a production capacity constraint affects the optimal decisions. The optimal decisions for a deterministic period-based model are provided in closed-form. A single sales/production rollover strategy implies that the sales/production of the old generation is discontinued before introducing the new generation. With a dual sales/production rollover strategy, the old and the new generation are sold/produced simultaneously. Depending on the capacity shortage, there are two types of mitigation actions: (i) increasing the prices, (ii) changing the sales and/or production rollover strategies with pre-production while adjusting the prices accordingly. If the capacity is unlimited, aligned sales and production rollover strategies are always optimal. We establish the conditions under which limited capacity leads to a combination of a single production rollover with a dual sales rollover strategy. We show that the selection of optimal rollover strategies is non-monotone in the available capacity. This implies that a change in the rollover strategy in response to limiting capacity has to be revoked for more severe capacity shortages.Publication Open Access Robust optimization of forecast combinations(Elsevier, 2019) Post, Thierry; Arvanitis, Stelios; Department of Business Administration; Karabatı, Selçuk; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 38819In this paper, we introduce RF energy harvesting paradigm into WNCS framework to study the optimal power control, energy harvesting and scheduling problem with the objective of providing maximum level of adaptivity under strict timing and reliability requirements employing the constant rate transmission model. We formulate the problem as a Mixed Integer Linear Programming Problem (MILP). We show the power allocation can be separated from the scheduling and harvesting at optimality. Then, we introduce a heuristic algorithm for the scheduling problem, periodic list scheduling (PLS), inspired from list scheduling of jobs with sequence dependent setup times on identical machines. We then demonstrate via extensive simulations the superiority of the proposed algorithm in terms of closeness to the optimal, adaptivity and runtime.Publication Open Access Cyclical dynamics of industrial production and employment: Markov chain-based estimates and tests(Elsevier, 2012) Gencer, Gözde; Department of Business Administration; Tan, Barış; Altuğ, Sumru; Faculty Member; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 28600; N/AThe purpose of this paper is to understand differences in cyclical phenomena across a broad range of developed and emerging countries based on the behavior of two key economic times series—industrial production and employment. The paper characterizes the series in question as a recurring Markov chain. Univariate processes are estimated for each series individually, and a composite indicator is constructed by using information on both series. Based on tests of equality of the estimated Markov chains across countries as well as the expected times to switch between different states, we find evidence that (i) the developed and emerging economies are “de-coupled” from each other in terms of their cyclical dynamics, and (ii) the behavior of industrial production and employment growth are “de-coupled” for the emerging economies. Our results suggest new directions for the analysis of emerging economy cyclical fluctuations.Publication Open Access Do physiological and spiritual factors affect economic decisions?(Wiley, 2021) Özbaş, Oğuzhan; Silva, Rui C.; Department of Business Administration; Demiroğlu, Cem; Ulu, Mehmet Fatih; Faculty Member; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 18073; N/AWe examine the effects of physiology and spiritual sentiment on economic decision-making in the context of Ramadan, an entire lunar month of daily fasting and increased spiritual reflection in the Muslim faith. Using an administrative data set of bank loans originated in Turkey during 2003 to 2013, we find that small business loans originated during Ramadan are 15% more likely to default within two years of origination. Loans originated in hot Ramadans, when adverse physiological effects of fasting are greatest, and those approved by the busiest bank branches perform worse. Despite their worse performance, Ramadan loans have lower credit spreads.Publication Open Access Effects of policy reforms on firm innovation(Springer Nature, 2022) Şeker, Murat; Department of Business Administration; Ulu, Mehmet Fatih; Faculty Member; Department of Business Administration; College of Administrative Sciences and Economics; 283267The regulatory environment in a country is an important factor that affects firm performance. This study investigates the impact of a particular regulation-license requirements for certain firm activities-on the innovation performance of Indian firms in the 1990s. Using a unique firm-level panel data set, it shows that the removal of license requirements led to an eight percentage points higher innovation rate within two years following the reform. We measure innovation as the introduction of new product varieties that had not been produced by the firm before. It takes a longer time for firms to innovate in industries in which they were not producing before. The findings of this study are also robust to the inclusion of controls for other policy reforms that occurred during the period of licensing reform. They also persist in tests with different subgroups of firms and with the use of alternative estimation methods.