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The bullwhip effect is a supply chain phenomenon where orders to suppliers tend to have a larger variability than sales to buyers, which results in an amplified demand variability upstream. In part, this results in increasing swings in inventory in response to shifts in consumer demand as one moves further up the supply chain. The concept first appeared in Jay Forrester's Industrial Dynamics (1961) [1] and thus it is also known as the Forrester effect. It has been described as "the observed propensity for material orders to be more variable than demand signals and for this variability to increase the further upstream a company is in a supply chain". [2] Research at Stanford University helped incorporate the concept into supply chain vernacular using a story about Volvo. Suffering a glut in green cars, sales and marketing developed a program to sell the excess inventory. While successful in generating the desired market pull, manufacturing did not know about the promotional plans. Instead, they read the increase in sales as an indication of growing demand for green cars and ramped up production. [3]
Research indicates a fluctuation in point-of-sale demand of five percent will be interpreted by supply chain participants as a change in demand of up to forty percent. Much like cracking a whip, a small flick of the wrist - a shift in point of sale demand - can cause a large motion at the end of the whip - manufacturers' responses. [4]
Because customer demand is rarely perfectly stable, businesses must forecast demand to properly position inventory and other resources. Forecasts are based on statistics, and they are rarely perfectly accurate. Because forecast errors are given, companies often carry an inventory buffer called "safety stock".
Moving up the supply chain from end-consumer to raw materials supplier, each supply chain participant has greater observed variation in demand and thus greater need for safety stock. In periods of rising demand, down-stream participants increase orders. In periods of falling demand, orders fall or stop, thereby not reducing inventory. The effect is that variations are amplified as one moves upstream in the supply chain (further from the customer). This sequence of events is well simulated by the beer distribution game which was developed by MIT Sloan School of Management in the 1960s.
The causes can further be divided into behavioral and operational causes.
The first theories focusing onto the bullwhip effect were mainly focusing on the irrational behavior of the human in the supply chain, highlighting them as the main cause of the bullwhip effect. Since the 90's, the studies evolved, placing the supply chain's misfunctioning at the heart of their studies abandoning the human factors. [5] Previous control-theoretic models have identified as causes the tradeoff between stationary and dynamic performance [6] as well as the use of independent controllers. [7] In accordance with Dellaert et al. (2017), [8] one of the main behavioral causes that contribute to the bullwhip effect is the under-estimation of the pipeline. [9] In addition, the complementary bias, over-estimation of the pipeline, also has a negative effect under such conditions. Nevertheless, it has been shown that when the demand stream is stationary, the system is relatively robust to this bias. In such situations, it has been found that biased policies (both under-estimating and over-estimating the pipeline) perform just as well as unbiased policies.
Some others behavioral causes can be highlighted:
Human factors influencing the behavior in supply chains are largely unexplored. However, studies suggest that people with increased need for safety and security seem to perform worse than risk-takers in a simulated supply chain environment. People with high self-efficacy experience less trouble handling the bullwhip-effect in the supply chain. [10]
A seminal Lee et al. (1997) study found that the bullwhip effect did not solely result from irrational decision making: it found that under some circumstances it is rational for a firm to order with greater variability than variability of demand, i.e., distort demand and cause the bullwhip effect. They established a list of four major factors which cause the bullwhip effect: demand signal processing, rationing game, order batching, and price variations. [2] This list has become a standard and is used as a framework to identify bullwhip effect.[ citation needed ]
Other operational causes include:
In addition to greater safety stocks, the described effect can lead to either inefficient production or excessive inventory, as each producer needs to fulfill the demand of its customers in the supply chain. This also leads to a low utilization of the distribution channel.
In spite of having safety stocks there is still the hazard of stock-outs which result in poor customer service and lost sales. In addition to the (financially) hard measurable consequences of poor customer services and the damage to public image and loyalty, an organization has to cope with the ramifications of failed fulfillment which may include contractual penalties. Moreover, repeated hiring and dismissal of employees to manage the demand variability induces further costs due to training and possible lay-offs.
The impact of the bullwhip effect has been especially acute at the beginning stages of the COVID-19 pandemic, when sudden spikes in demand for everything from medical supplies such as masks or ventilators [13] to consumer items such as toilet paper or eggs created feedback loops of panic buying, hoarding, and rationing. [14]
Information sharing across the supply chain is an effective strategy to mitigate the bullwhip effect. For example, it has been successfully implemented in Wal-Mart's distribution system. Individual Wal-Mart stores transmit point-of-sale (POS) data from the cash register back to corporate headquarters several times a day. This demand information is used to queue shipments from the Wal-Mart distribution center to the store and from the supplier to the Wal-Mart distribution center. The result is near-perfect visibility of customer demand and inventory movement throughout the supply chain. Better information leads to better inventory positioning and lower costs throughout the supply chain.
Another recommended strategy to limit the bullwhip effect is order smoothing. [7] Previous research has demonstrated that order smoothing and the bullwhip effect are concurrent in industry. [15] It has been proved that order smoothing is beneficial for the system's performance when the demand is stationary. However, its impact is limited to the worst-case order amplification when the demand is unpredictable. Having said that, dynamic analysis reveals that order smoothing can degrade performance in the presence of demand shocks. The opposite bias (i.e., over-reaction to mismatches), on the other hand, degrades the stationary performance but can increase dynamic performance; controlled over-reaction can aid the system reach its new goals quickly. The system, nevertheless, is considerably sensitive to that behaviour; extreme over-reaction significantly reduces performance. Overall, unbiased policies offer in general good results under a large range of demand types. Although these policies do not result in the best performance under certain criteria. It is always possible to find a biased policy that outperforms an unbiased policy for any one performance metric.
Methods intended to reduce uncertainty, variability, and lead time:
Many studies demonstrate the bullwhip effect in a supply chain from different perspectives, including information sharing (Lee et al., 2000), [16] information distortion (Lee et al., 2004), [17] bankruptcy events (Lee et al., 2004, Mizgier et al., 2012 [18] ) and systematic risk (Osadchiy et al., 2015). [19] Most of them devote themselves to exploring the bullwhip effect from the perspectives of inventory flow risk and information flow risk rather than that of cash flow risk. For a firm's internal liquidity risk (Chen et al., 2011), [20] it is an appropriate proxy for a firm's financial risk.
Evolving from the notion of a stock derived bullwhip effect, there exists a similar, "financial bullwhip effect", explored in (Chen et al., 2013), [21] on bondholders' wealth along a supply chain by examining whether the internal liquidity risk effect on bond yield spreads becomes greater upwardly along the supply chain counterparties.
This is more generally modelled in (Proselkov et al., 2023), [22] which uses complex adaptive systems modelling to study cascade failures as a consequence of financial bullwhips. Specifically, they create an agent-based supply network simulation model capturing the behaviours of companies with asymmetric power dynamics with their partners. To remain operational, they maximise their liquidity by negotiating longer repayment terms and cheaper financing, thus distributing risk onto weaker companies and propagating financial stress. This results in network-wide breakdown.
Inventory or stock refers to the goods and materials that a business holds for the ultimate goal of resale, production or utilisation.
Kanban is a scheduling system for lean manufacturing. Taiichi Ohno, an industrial engineer at Toyota, developed kanban to improve manufacturing efficiency. The system takes its name from the cards that track production within a factory. Kanban is also known as the Toyota nameplate system in the automotive industry.
Vendor-managed inventory (VMI) is an inventory management practice in which a supplier of goods, usually the manufacturer, is responsible for optimizing the inventory held by a distributor.
The beer distribution game is an educational game that is used to experience typical coordination problems of a supply chain process. It reflects a role-play simulation where several participants play with each other. The game represents a supply chain with a non-coordinated process where problems arise due to lack of information sharing. This game outlines the importance of information sharing, supply chain management and collaboration throughout a supply chain process. Due to lack of information, suppliers, manufacturers, sales people and customers often have an incomplete understanding of what the real demand of an order is. The most interesting part of the game is that each group has no control over another part of the supply chain. Therefore, each group has only significant control over their own part of the supply chain. Each group can highly influence the entire supply chain by ordering too much or too little which can lead to a bullwhip effect. Therefore, the order taking of a group also highly depends on decisions of the other groups.
Safety stock is a term used by logisticians to describe a level of extra stock that is maintained to mitigate risk of stockouts caused by uncertainties in supply and demand. Adequate safety stock levels permit business operations to proceed according to their plans. Safety stock is held when uncertainty exists in demand, supply, or manufacturing yield, and serves as an insurance against stockouts.
Inventory control or stock control can be broadly defined as "the activity of checking a shop's stock". It is the process of ensuring that the right amount of supply is available within a business. However, a more focused definition takes into account the more science-based, methodical practice of not only verifying a business's inventory but also maximising the amount of profit from the least amount of inventory investment without affecting customer satisfaction. Other facets of inventory control include forecasting future demand, supply chain management, production control, financial flexibility, purchasing data, loss prevention and turnover, and customer satisfaction.
The business terms push and pull originated in logistics and supply chain management, but are also widely used in marketing and in the hotel distribution business.
The term demand chain has been used in a business and management context as contrasting terminology alongside, or in place of, "supply chain". Madhani suggests that the demand chain "comprises all the demand processes necessary to understand, create, and stimulate customer demand". Cranfield School of Management academic Martin Christopher has suggested that "ideally the supply chain should become a demand chain", explaining that ideally all product logistics and processing should occur "in response to a known customer requirement".
Demand-chain management (DCM) is the management of relationships between suppliers and customers to deliver the best value to the customer at the least cost to the demand chain as a whole. Demand-chain management is similar to supply-chain management but with special regard to the customers.
Behavioral operations management examines and takes into consideration human behaviours and emotions when facing complex decision problems. It relates to the behavioral aspects of the use of operations research and operations management. In particular, it focuses on understanding behavior in, with and beyond models. The general purpose is to make better use and improve the use of operations theories and practice, so that the benefits received from the potential improvements to operations approaches in practice, that arise from recent findings in behavioral sciences, are realized. Behavioral operations approaches have heavily influenced supply chain management research among others.
Production leveling, also known as production smoothing or – by its Japanese original term – heijunka (平準化), is a technique for reducing the mura (unevenness) which in turn reduces muda (waste). It was vital to the development of production efficiency in the Toyota Production System and lean manufacturing. The goal is to produce intermediate goods at a constant rate so that further processing may also be carried out at a constant and predictable rate.
Demand forecasting is the prediction of the quantity of goods and services that will be demanded by consumers at a future point in time. More specifically, the methods of demand forecasting entail using predictive analytics to estimate customer demand in consideration of key economic conditions. This is an important tool in optimizing business profitability through efficient supply chain management. Demand forecasting methods are divided into two major categories, qualitative and quantitative methods. Qualitative methods are based on expert opinion and information gathered from the field. This method is mostly used in situations when there is minimal data available for analysis such as when a business or product has recently been introduced to the market. Quantitative methods, however, use available data, and analytical tools in order to produce predictions. Demand forecasting may be used in resource allocation, inventory management, assessing future capacity requirements, or making decisions on whether to enter a new market.
Channel coordination aims at improving supply chain performance by aligning the plans and the objectives of individual enterprises. It usually focuses on inventory management and ordering decisions in distributed inter-company settings. Channel coordination models may involve multi-echelon inventory theory, multiple decision makers, asymmetric information, as well as recent paradigms of manufacturing, such as mass customization, short product life-cycles, outsourcing and delayed differentiation. The theoretical foundations of the coordination are based chiefly on the contract theory. The problem of channel coordination was first modeled and analyzed by Anantasubramania Kumar in 1992.
The term Lehman Wave refers to an economy-wide fluctuation in production and economic activity, with a wavelength of between 12 and 18 months, driven by a sudden major disruption of the economic system. The Lehman Wave is a damped, wave-like fluctuation around equilibrium. The amplitude of the Lehman Wave is larger for a business that is further away from its end market than for a business that is closer to its end market, which difference is caused by cumulative de-stocking of the intermediate supply chain. This term Lehman Wave has first been used by Dutch researchers in 2009 who gave that name to the economic wave that started in September 2008. They argue that the latter was caused by global de-stocking after the financial panic following the bankruptcy of Lehman Brothers on September 15, 2008. The Lehman Wave can have strong effects on the sales volume and therefore on the profitability of companies that are located upstream in the supply chain.
Active destocking in supply chain management is an active decision to reduce the inventory-to-sales ratio of a company. The inventory can include finished products, raw materials and goods in process. In general, active destocking is done following an autonomous, often financial decision by a company to improve its efficiency, free up cash and reduce its costs. Decisions for active destocking in general are made by financial executives or general managers.
Reactive destocking in supply chain management is a reduction of the inventory when expected demand goes down. When a company is only doing reactive destocking, the desired inventory to sales ratio, remains unchanged. Reactive destocking in general is done by operational managers of the logistical activities, without additional instructions. The inventory can include finished products, raw materials and/or goods in process.
Inventory optimization refers to the techniques used by businesses to improve their oversight, control and management of inventory size and location across their extended supply network. It has been observed within operations research that "every company has the challenge of matching its supply volume to customer demand. How well the company manages this challenge has a major impact on its profitability."
Maqbool Dada is a professor at Carey Business School, Johns Hopkins University, with expertise in the areas of operations management, healthcare, and marketing. He is also a core faculty member at the Johns Hopkins School of Medicine’s Armstrong Institute for Patient Safety and Quality.
Retail back-office software is used to manage business operations that are not related to direct sales efforts and interfaces that are not seen by consumers. Typically, the business processes managed with back-office software include some combination of inventory control, price book management, manufacturing, and supply chain management (SCM). Back-office software is distinct from front-office software, which typically refers to customer relationship management (CRM) software used for managing sales, marketing, and other customer-centric activities.
Guillermo Gallego is an American data scientist, academic and author. He is the Liu Family Emeritus professor at Columbia University, the Crown Worldwide Professor Emeritus at The Hong Kong University of Science and Technology and is the X.Q. Deng Presidential Chair Professorship at The Chinese University of Hong Kong, Shenzhen.