IDEAS home Printed from https://ideas.repec.org/a/eee/jomega/v45y2014icp92-118.html
   My bibliography  Save this article

Competitive supply chain network design: An overview of classifications, models, solution techniques and applications

Author

Listed:
  • Farahani, Reza Zanjirani
  • Rezapour, Shabnam
  • Drezner, Tammy
  • Fallah, Samira

Abstract

Supply chain network design (SCND) determines the structure of a chain and affects its costs and performance. SCND deals with a variety of decisions such as determining number, size and location of facilities in a supply chain (SC) and may include tactical decisions (such as distribution, transportation and inventory management policies) as well as operational decisions (such as fulfilling customers demand). SCND has a voluminous literature. Most of the literature deals with a single SC and ignores the existing competitor SCs and future emerging ones. However, SCs compete together to capture more market shares. Even if there is not any competitor at the moment, SCs should be prepared for possible future competitive situation at the SCND stage. On the other hand, many competitive models assume that the supply chain network (SCN) and its structure already exist. Few research papers consider both aspects of design and competition. In this paper, we provide a review of SCND literature and highlight the effects of competitive environment on SCND. We review, classify, and introduce the major features of the proposed models in both SCND and competition literature. After investigating proposed competitive SCND models we develop a general framework for modeling the competitive SCND problems considering managerial insight and propose potential areas for future research.

Suggested Citation

  • Farahani, Reza Zanjirani & Rezapour, Shabnam & Drezner, Tammy & Fallah, Samira, 2014. "Competitive supply chain network design: An overview of classifications, models, solution techniques and applications," Omega, Elsevier, vol. 45(C), pages 92-118.
    • Handle: RePEc:eee:jomega:v:45:y:2014:i:c:p:92-118
    DOI: 10.1016/j.omega.2013.08.006
    as

    Download full text from publisher

    File URL: http://www.sciencedirect.com/science/article/pii/S030504831300090X
    Download Restriction: Full text for ScienceDirect subscribers only
    File URL: https://libkey.io/10.1016/j.omega.2013.08.006?utm_source=ideas
    LibKey link: if access is restricted and if your library uses this service, LibKey will redirect you to where you can use your library subscription to access this item
    ---><---

    As the access to this document is restricted, you may want to search for a different version of it.

    References listed on IDEAS

    as
    1. David L. Huff, 1966. "A Programmed Solution for Approximating an Optimum Retail Location," Land Economics, University of Wisconsin Press, vol. 42(3), pages 293-303.
    2. Wu, Tai-Hsi & Lin, Jen-Nan, 2003. "Solving the competitive discretionary service facility location problem," European Journal of Operational Research, Elsevier, vol. 144(2), pages 366-378, January.
    3. World Commission on Environment and Development,, 1987. "Our Common Future," OUP Catalogue, Oxford University Press, number 9780192820808.
    4. Listes, Ovidiu & Dekker, Rommert, 2005. "A stochastic approach to a case study for product recovery network design," European Journal of Operational Research, Elsevier, vol. 160(1), pages 268-287, January.
    5. ReVelle, Charles & Murray, Alan T. & Serra, Daniel, 2007. "Location models for ceding market share and shrinking services," Omega, Elsevier, vol. 35(5), pages 533-540, October.
    6. Lin, Cheng-Chang & Wang, Tsai-Hsin, 2011. "Build-to-order supply chain network design under supply and demand uncertainties," Transportation Research Part B: Methodological, Elsevier, vol. 45(8), pages 1162-1176, September.
    7. Vidal, Carlos J. & Goetschalckx, Marc, 1997. "Strategic production-distribution models: A critical review with emphasis on global supply chain models," European Journal of Operational Research, Elsevier, vol. 98(1), pages 1-18, April.
    8. Georgiadis, Michael C. & Tsiakis, Panagiotis & Longinidis, Pantelis & Sofioglou, Maria K., 2011. "Optimal design of supply chain networks under uncertain transient demand variations," Omega, Elsevier, vol. 39(3), pages 254-272, June.
    9. Mark Daskin & Collette Coullard & Zuo-Jun Shen, 2002. "An Inventory-Location Model: Formulation, Solution Algorithm and Computational Results," Annals of Operations Research, Springer, vol. 110(1), pages 83-106, February.
    10. Drezner, Tammy & Drezner, Zvi & Salhi, Said, 2002. "Solving the multiple competitive facilities location problem," European Journal of Operational Research, Elsevier, vol. 142(1), pages 138-151, October.
    11. Meng, Qiang & Huang, Yi Kai & Cheu, Ruey Long, 2007. "A note on supply chain network equilibrium models," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 43(1), pages 60-71, January.
    12. Fernando Bernstein & Awi Federgruen, 2005. "Decentralized Supply Chains with Competing Retailers Under Demand Uncertainty," Management Science, INFORMS, vol. 51(1), pages 18-29, January.
    13. Nagurney, Anna & Matsypura, Dmytro, 2005. "Global supply chain network dynamics with multicriteria decision-making under risk and uncertainty," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 41(6), pages 585-612, November.
    14. Eskigun, Erdem & Uzsoy, Reha & Preckel, Paul V. & Beaujon, George & Krishnan, Subramanian & Tew, Jeffrey D., 2005. "Outbound supply chain network design with mode selection, lead times and capacitated vehicle distribution centers," European Journal of Operational Research, Elsevier, vol. 165(1), pages 182-206, August.
    15. Sabri, Ehap H. & Beamon, Benita M., 2000. "A multi-objective approach to simultaneous strategic and operational planning in supply chain design," Omega, Elsevier, vol. 28(5), pages 581-598, October.
    16. Nagurney, Anna, 2006. "On the relationship between supply chain and transportation network equilibria: A supernetwork equivalence with computations," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 42(4), pages 293-316, July.
    17. Farahani, Reza Zanjirani & Miandoabchi, Elnaz & Szeto, W.Y. & Rashidi, Hannaneh, 2013. "A review of urban transportation network design problems," European Journal of Operational Research, Elsevier, vol. 229(2), pages 281-302.
    18. Fernando Bernstein & Awi Federgruen, 2004. "A General Equilibrium Model for Industries with Price and Service Competition," Operations Research, INFORMS, vol. 52(6), pages 868-886, December.
    19. Salema, Maria Isabel Gomes & Barbosa-Povoa, Ana Paula & Novais, Augusto Q., 2007. "An optimization model for the design of a capacitated multi-product reverse logistics network with uncertainty," European Journal of Operational Research, Elsevier, vol. 179(3), pages 1063-1077, June.
    20. Hammond, David & Beullens, Patrick, 2007. "Closed-loop supply chain network equilibrium under legislation," European Journal of Operational Research, Elsevier, vol. 183(2), pages 895-908, December.
    21. Kazemi Zanjani, Masoumeh & Ait-Kadi, Daoud & Nourelfath, Mustapha, 2010. "Robust production planning in a manufacturing environment with random yield: A case in sawmill production planning," European Journal of Operational Research, Elsevier, vol. 201(3), pages 882-891, March.
    22. Plastria, Frank, 2001. "Static competitive facility location: An overview of optimisation approaches," European Journal of Operational Research, Elsevier, vol. 129(3), pages 461-470, March.
    23. Drezner, Zvi, 1982. "Competitive location strategies for two facilities," Regional Science and Urban Economics, Elsevier, vol. 12(4), pages 485-493, November.
    24. Aboolian, Robert & Berman, Oded & Krass, Dmitry, 2007. "Competitive facility location model with concave demand," European Journal of Operational Research, Elsevier, vol. 181(2), pages 598-619, September.
    25. Rhim, Hosun & Ho, Teck H. & Karmarkar, Uday S., 2003. "Competitive location, production, and market selection," European Journal of Operational Research, Elsevier, vol. 149(1), pages 211-228, August.
    26. Xiao, Tiaojun & Yang, Danqin, 2008. "Price and service competition of supply chains with risk-averse retailers under demand uncertainty," International Journal of Production Economics, Elsevier, vol. 114(1), pages 187-200, July.
    27. Nagurney, Anna & Dong, June & Zhang, Ding, 2002. "A supply chain network equilibrium model," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 38(5), pages 281-303, September.
    28. Albert Y. Ha & Lode Li & Shu-Ming Ng, 2003. "Price and Delivery Logistics Competition in a Supply Chain," Management Science, INFORMS, vol. 49(9), pages 1139-1153, September.
    29. Pokharel, Shaligram, 2008. "A two objective model for decision making in a supply chain," International Journal of Production Economics, Elsevier, vol. 111(2), pages 378-388, February.
    30. Aboolian, Robert & Berman, Oded & Krass, Dmitry, 2007. "Competitive facility location and design problem," European Journal of Operational Research, Elsevier, vol. 182(1), pages 40-62, October.
    31. Longinidis, Pantelis & Georgiadis, Michael C., 2011. "Integration of financial statement analysis in the optimal design of supply chain networks under demand uncertainty," International Journal of Production Economics, Elsevier, vol. 129(2), pages 262-276, February.
    32. Rajiv D. Banker & Inder Khosla & Kingshuk K. Sinha, 1998. "Quality and Competition," Management Science, INFORMS, vol. 44(9), pages 1179-1192, September.
    33. Franca, Rodrigo B. & Jones, Erick C. & Richards, Casey N. & Carlson, Jonathan P., 2010. "Multi-objective stochastic supply chain modeling to evaluate tradeoffs between profit and quality," International Journal of Production Economics, Elsevier, vol. 127(2), pages 292-299, October.
    34. Birendra K. Mishra & Srinivasan Raghunathan, 2004. "Retailer- vs. Vendor-Managed Inventory and Brand Competition," Management Science, INFORMS, vol. 50(4), pages 445-457, April.
    35. Melo, M.T. & Nickel, S. & Saldanha-da-Gama, F., 2009. "Facility location and supply chain management - A review," European Journal of Operational Research, Elsevier, vol. 196(2), pages 401-412, July.
    36. Kurata, Hisashi & Yao, Dong-Qing & Liu, John J., 2007. "Pricing policies under direct vs. indirect channel competition and national vs. store brand competition," European Journal of Operational Research, Elsevier, vol. 180(1), pages 262-281, July.
    37. Fernandez, Pascual & Pelegrin, Blas & Garcia Perez, Maria Dolores & Peeters, Peter H., 2007. "A discrete long-term location-price problem under the assumption of discriminatory pricing: Formulations and parametric analysis," European Journal of Operational Research, Elsevier, vol. 179(3), pages 1050-1062, June.
    38. Jia Shu & Chung-Piaw Teo & Zuo-Jun Max Shen, 2005. "Stochastic Transportation-Inventory Network Design Problem," Operations Research, INFORMS, vol. 53(1), pages 48-60, February.
    39. Goetschalckx, Marc & Vidal, Carlos J. & Dogan, Koray, 2002. "Modeling and design of global logistics systems: A review of integrated strategic and tactical models and design algorithms," European Journal of Operational Research, Elsevier, vol. 143(1), pages 1-18, November.
    40. Daniel, J. Sudhir Ryan & Rajendran, Chandrasekharan, 2006. "Heuristic approaches to determine base-stock levels in a serial supply chain with a single objective and with multiple objectives," European Journal of Operational Research, Elsevier, vol. 175(1), pages 566-592, November.
    41. Schütz, Peter & Tomasgard, Asgeir & Ahmed, Shabbir, 2009. "Supply chain design under uncertainty using sample average approximation and dual decomposition," European Journal of Operational Research, Elsevier, vol. 199(2), pages 409-419, December.
    42. Leonardi, Giorgio & Tadei, Roberto, 1984. "Random utility demand models and service location," Regional Science and Urban Economics, Elsevier, vol. 14(3), pages 399-431, August.
    43. Zuo-Jun Max Shen & Mark S. Daskin, 2005. "Trade-offs Between Customer Service and Cost in Integrated Supply Chain Design," Manufacturing & Service Operations Management, INFORMS, vol. 7(3), pages 188-207, September.
    44. Azaron, A. & Brown, K.N. & Tarim, S.A. & Modarres, M., 2008. "A multi-objective stochastic programming approach for supply chain design considering risk," International Journal of Production Economics, Elsevier, vol. 116(1), pages 129-138, November.
    45. Van de Voorde, Eddy E.M., 2005. "What Future the Maritime Sector: Some Considerations on Globalisation, Co-Operation and Market Power," Research in Transportation Economics, Elsevier, vol. 13(1), pages 253-277, January.
    46. Andy A. Tsay & Narendra Agrawal, 2000. "Channel Dynamics Under Price and Service Competition," Manufacturing & Service Operations Management, INFORMS, vol. 2(4), pages 372-391, August.
    47. Osman, Hany & Demirli, Kudret, 2010. "A bilinear goal programming model and a modified Benders decomposition algorithm for supply chain reconfiguration and supplier selection," International Journal of Production Economics, Elsevier, vol. 124(1), pages 97-105, March.
    48. Gunasekaran, Angappa & Lai, Kee-hung & Edwin Cheng, T.C., 2008. "Responsive supply chain: A competitive strategy in a networked economy," Omega, Elsevier, vol. 36(4), pages 549-564, August.
    49. Dong, June & Zhang, Ding & Nagurney, Anna, 2004. "A supply chain network equilibrium model with random demands," European Journal of Operational Research, Elsevier, vol. 156(1), pages 194-212, July.
    50. Sawik, Tadeusz, 2011. "Selection of supply portfolio under disruption risks," Omega, Elsevier, vol. 39(2), pages 194-208, April.
    51. Elias Olivares-Benitez & José González-Velarde & Roger Ríos-Mercado, 2012. "A supply chain design problem with facility location and bi-objective transportation choices," TOP: An Official Journal of the Spanish Society of Statistics and Operations Research, Springer;Sociedad de Estadística e Investigación Operativa, vol. 20(3), pages 729-753, October.
    52. Goh, Mark & Lim, Joseph Y.S. & Meng, Fanwen, 2007. "A stochastic model for risk management in global supply chain networks," European Journal of Operational Research, Elsevier, vol. 182(1), pages 164-173, October.
    53. Shiode, Shogo & Drezner, Zvi, 2003. "A competitive facility location problem on a tree network with stochastic weights," European Journal of Operational Research, Elsevier, vol. 149(1), pages 47-52, August.
    54. Max Shen, Zuo-Jun & Qi, Lian, 2007. "Incorporating inventory and routing costs in strategic location models," European Journal of Operational Research, Elsevier, vol. 179(2), pages 372-389, June.
    55. Fernando Bernstein & Awi Federgruen, 2007. "Coordination Mechanisms for Supply Chains Under Price and Service Competition," Manufacturing & Service Operations Management, INFORMS, vol. 9(3), pages 242-262, January.
    56. Pranab Majumder & Ashok Srinivasan, 2008. "Leadership and Competition in Network Supply Chains," Management Science, INFORMS, vol. 54(6), pages 1189-1204, June.
    57. Wang, H.S., 2009. "A two-phase ant colony algorithm for multi-echelon defective supply chain network design," European Journal of Operational Research, Elsevier, vol. 192(1), pages 243-252, January.
    58. Zuo-Jun Max Shen & Collette Coullard & Mark S. Daskin, 2003. "A Joint Location-Inventory Model," Transportation Science, INFORMS, vol. 37(1), pages 40-55, February.
    59. Meng, Qiang & Huang, Yikai & Cheu, Ruey Long, 2009. "Competitive facility location on decentralized supply chains," European Journal of Operational Research, Elsevier, vol. 196(2), pages 487-499, July.
    60. Fernandez, Jose & Pelegri'n, Blas & Plastria, Frank & Toth, Boglarka, 2007. "Solving a Huff-like competitive location and design model for profit maximization in the plane," European Journal of Operational Research, Elsevier, vol. 179(3), pages 1274-1287, June.
    61. Romeijn, H. Edwin & Shu, Jia & Teo, Chung-Piaw, 2007. "Designing two-echelon supply networks," European Journal of Operational Research, Elsevier, vol. 178(2), pages 449-462, April.
    62. Ahmadi Javid, Amir & Azad, Nader, 2010. "Incorporating location, routing and inventory decisions in supply chain network design," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 46(5), pages 582-597, September.
    63. Asgari, Nasrin & Farahani, Reza Zanjirani & Goh, Mark, 2013. "Network design approach for hub ports-shipping companies competition and cooperation," Transportation Research Part A: Policy and Practice, Elsevier, vol. 48(C), pages 1-18.
    64. Meixell, Mary J. & Gargeya, Vidyaranya B., 2005. "Global supply chain design: A literature review and critique," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 41(6), pages 531-550, November.
    65. Jayaraman, Vaidyanathan & Patterson, Raymond A. & Rolland, Erik, 2003. "The design of reverse distribution networks: Models and solution procedures," European Journal of Operational Research, Elsevier, vol. 150(1), pages 128-149, October.
    66. Leung, Stephen C.H. & Tsang, Sally O.S. & Ng, W.L. & Wu, Yue, 2007. "A robust optimization model for multi-site production planning problem in an uncertain environment," European Journal of Operational Research, Elsevier, vol. 181(1), pages 224-238, August.
    67. Barnes, Paul & Oloruntoba, Richard, 2005. "Assurance of security in maritime supply chains: Conceptual issues of vulnerability and crisis management," Journal of International Management, Elsevier, vol. 11(4), pages 519-540, December.
    68. Santoso, Tjendera & Ahmed, Shabbir & Goetschalckx, Marc & Shapiro, Alexander, 2005. "A stochastic programming approach for supply chain network design under uncertainty," European Journal of Operational Research, Elsevier, vol. 167(1), pages 96-115, November.
    69. Zhang, Ding, 2006. "A network economic model for supply chain versus supply chain competition," Omega, Elsevier, vol. 34(3), pages 283-295, June.
    70. Charles J. Corbett & Uday S. Karmarkar, 2001. "Competition and Structure in Serial Supply Chains with Deterministic Demand," Management Science, INFORMS, vol. 47(7), pages 966-978, July.
    Full references (including those not matched with items on IDEAS)

    Most related items

    These are the items that most often cite the same works as this one and are cited by the same works as this one.
    1. Baghalian, Atefeh & Rezapour, Shabnam & Farahani, Reza Zanjirani, 2013. "Robust supply chain network design with service level against disruptions and demand uncertainties: A real-life case," European Journal of Operational Research, Elsevier, vol. 227(1), pages 199-215.
    2. Rezapour, Shabnam & Farahani, Reza Zanjirani & Dullaert, Wout & De Borger, Bruno, 2014. "Designing a new supply chain for competition against an existing supply chain," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 67(C), pages 124-140.
    3. Rezapour, Shabnam & Hassani, Ashkan & Farahani, Reza Zanjirani, 2015. "Concurrent design of product family and supply chain network considering quality and price," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 81(C), pages 18-35.
    4. Melo, M.T. & Nickel, S. & Saldanha-da-Gama, F., 2009. "Facility location and supply chain management - A review," European Journal of Operational Research, Elsevier, vol. 196(2), pages 401-412, July.
    5. S Rezapour & R Zanjirani Farahani & T Drezner, 2011. "Strategic design of competing supply chain networks for inelastic demand," Journal of the Operational Research Society, Palgrave Macmillan;The OR Society, vol. 62(10), pages 1784-1795, October.
    6. Asgari, Nasrin & Farahani, Reza Zanjirani & Goh, Mark, 2013. "Network design approach for hub ports-shipping companies competition and cooperation," Transportation Research Part A: Policy and Practice, Elsevier, vol. 48(C), pages 1-18.
    7. Rezapour, Shabnam & Allen, Janet K. & Mistree, Farrokh, 2015. "Uncertainty propagation in a supply chain or supply network," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 73(C), pages 185-206.
    8. Mohammaddust, Faeghe & Rezapour, Shabnam & Farahani, Reza Zanjirani & Mofidfar, Mohammad & Hill, Alex, 2017. "Developing lean and responsive supply chains: A robust model for alternative risk mitigation strategies in supply chain designs," International Journal of Production Economics, Elsevier, vol. 183(PC), pages 632-653.
    9. Jahani, Hamed & Abbasi, Babak & Sheu, Jiuh-Biing & Klibi, Walid, 2024. "Supply chain network design with financial considerations: A comprehensive review," European Journal of Operational Research, Elsevier, vol. 312(3), pages 799-839.
    10. Fattahi, Mohammad & Govindan, Kannan & Keyvanshokooh, Esmaeil, 2017. "Responsive and resilient supply chain network design under operational and disruption risks with delivery lead-time sensitive customers," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 101(C), pages 176-200.
    11. Jahani, Hamed & Abbasi, Babak & Alavifard, Farzad & Talluri, Srinivas, 2018. "Supply chain network redesign with demand and price uncertainty," International Journal of Production Economics, Elsevier, vol. 205(C), pages 287-312.
    12. Rezapour, Shabnam & Srinivasan, Ramakrishnan & Tew, Jeffrey & Allen, Janet K. & Mistree, Farrokh, 2018. "Correlation between strategic and operational risk mitigation strategies in supply networks," International Journal of Production Economics, Elsevier, vol. 201(C), pages 225-248.
    13. Longinidis, Pantelis & Georgiadis, Michael C., 2014. "Integration of sale and leaseback in the optimal design of supply chain networks," Omega, Elsevier, vol. 47(C), pages 73-89.
    14. Liu, Songsong & Papageorgiou, Lazaros G., 2013. "Multiobjective optimisation of production, distribution and capacity planning of global supply chains in the process industry," Omega, Elsevier, vol. 41(2), pages 369-382.
    15. Gohram Baloch & Fatma Gzara, 2020. "Strategic Network Design for Parcel Delivery with Drones Under Competition," Transportation Science, INFORMS, vol. 54(1), pages 204-228, January.
    16. Yamada, Tadashi & Imai, Koji & Nakamura, Takamasa & Taniguchi, Eiichi, 2011. "A supply chain-transport supernetwork equilibrium model with the behaviour of freight carriers," Transportation Research Part E: Logistics and Transportation Review, Elsevier, vol. 47(6), pages 887-907.
    17. Rezapour, Shabnam & Farahani, Reza Zanjirani & Pourakbar, Morteza, 2017. "Resilient supply chain network design under competition: A case study," European Journal of Operational Research, Elsevier, vol. 259(3), pages 1017-1035.
    18. Govindan, Kannan & Fattahi, Mohammad, 2017. "Investigating risk and robustness measures for supply chain network design under demand uncertainty: A case study of glass supply chain," International Journal of Production Economics, Elsevier, vol. 183(PC), pages 680-699.
    19. Yuxiang Yang & Zuqing Huang & Qiang Patrick Qiang & Gengui Zhou, 2017. "A Mathematical Programming Model with Equilibrium Constraints for Competitive Closed-Loop Supply Chain Network Design," Asia-Pacific Journal of Operational Research (APJOR), World Scientific Publishing Co. Pte. Ltd., vol. 34(05), pages 1-31, October.
    20. Gebennini, Elisa & Gamberini, Rita & Manzini, Riccardo, 2009. "An integrated production-distribution model for the dynamic location and allocation problem with safety stock optimization," International Journal of Production Economics, Elsevier, vol. 122(1), pages 286-304, November.

    Corrections

    All material on this site has been provided by the respective publishers and authors. You can help correct errors and omissions. When requesting a correction, please mention this item's handle: RePEc:eee:jomega:v:45:y:2014:i:c:p:92-118. See general information about how to correct material in RePEc.

    If you have authored this item and are not yet registered with RePEc, we encourage you to do it here. This allows to link your profile to this item. It also allows you to accept potential citations to this item that we are uncertain about.

    If CitEc recognized a bibliographic reference but did not link an item in RePEc to it, you can help with this form .

    If you know of missing items citing this one, you can help us creating those links by adding the relevant references in the same way as above, for each refering item. If you are a registered author of this item, you may also want to check the "citations" tab in your RePEc Author Service profile, as there may be some citations waiting for confirmation.

    For technical questions regarding this item, or to correct its authors, title, abstract, bibliographic or download information, contact: Catherine Liu (email available below). General contact details of provider: http://www.elsevier.com/wps/find/journaldescription.cws_home/375/description#description .

    Please note that corrections may take a couple of weeks to filter through the various RePEc services.

    IDEAS is a RePEc service. RePEc uses bibliographic data supplied by the respective publishers.