【正文】 銷售員易過度受到最近經(jīng)驗(yàn)的影響。 因此，這系統(tǒng)中， 任何階段或?qū)蛹壍碾A層存貨(echelon inventory)等於此階層中現(xiàn)有的存貨，再加上所有的下游存貨(downstream inventory)。 二項(xiàng)重要且合理的假設(shè)： 1. 存貨決策由單一決策者制訂 ，且這決策者的目標(biāo)是使整體系統(tǒng)成本減到最小 。在許多實(shí)際情況下，運(yùn)送至倉庫的前置時(shí)間，必須假設(shè)為常態(tài)機(jī)率分配，平均前置時(shí)間以 AVGL表示及標(biāo)準(zhǔn)差以 STDL表示 。 前置時(shí)間 ，從下訂單到收到貨品間的時(shí)間，為 0。 Stock plunges. Dell acknowledged that the pany was sharply off in its forecast of demand, resulting in inventory write downs” (WSJ, August 1993) Inventory Management 8 Key factors affecting Inventory policy Customer demand Characteristics Replenishment lead Time Number of Products Service level requirements Cost Structure Objectives – Service level – Minimize costs Inventory Management 9 Cost Structure Order costs – Fixed – Variable Holding Costs – Insurance – Maintenance and Handling – Taxes – Opportunity Costs – Obsolescence Inventory Management 10 Example Book Store Mug Sales – Demand is constant, at 20 units a week – Fixed order cost of $, no lead time – Holding cost of 25% of inventory value annually – Mugs cost $, sell for $ Question – How many, when to order? Inventory Management 11 Economic Lot Size Model (EOQ) (Ford W. Harris, 1915) Time Inventory Order Size Note: ? No Stockouts ? Order when no inventory ? Order Size determines policy Avg. Inven Cycle Time =T Inventory Management 12 Assumptions of EOQ model 每天 的需求為 D， 且 保持穩(wěn)定。 每次訂購量為固定，以 Q表示 (Q的數(shù)量保持固定 )，亦即每次倉庫訂一次貨為 Q個(gè)數(shù)量。 期初存貨 為 0。在此情況下，再訂購點(diǎn) R的計(jì)算如下： 其中 AVG AVGL表示平均前置時(shí)間內(nèi)的平均需求，而 為平均前置時(shí)間內(nèi)的平均需求標(biāo)準(zhǔn)差。 2. 決策者有 獲取各零售商和倉庫存貨資訊 的管道。 Inventory Management 87 Managing inventory in the supply chain3 Echelon inventory position – The echelon inventory at the echelon, plus those items ordered by the echelon that have not yet arrived minus all items that are backordered. Inventory Management 88 The warehouse echelon inventory1 Supplier Warehouse Retailers Warehouse echelon lead time Inventory Management 89 The warehouse echelon inventory2 The reorder point of warehouse is ees L A V G z S T D L? ? ? ?Where Le=echelon lead time, defined as the lead time between the retailers and the warehouse plus the lead time between the warehouse and its supplier AVG=average demand across all retailers STD=standard deviation of demand across all retailers Inventory Management 90 Inventory Management: Best Practice Periodic inventory reviews Tight management of usage rates, lead times and safety stock (the smaller variation) ABC approach Reduced safety stock levels Shift more inventory, or inventory ownership, to suppliers Quantitative approaches Inventory Management 91 Inventory Turnover Ratio Inventory turnover ratio = annual sales/avg. inventory level Increase in inventory turnover leads to – Decrease in average inventory levels – Higher level of liquidity – Smaller risk of obsolescence – Reduced investment in inventory – Increases the risk of lost sales Inventory Management 92 Indus tr y Upper Quart ile Medi an Low er Quart ile Dair y Product s Electronic Com ponent Electronic Com puters Books: publis hing Household audio amp。 Inventory Management 96 Judgment Methods2 Panels of experts – internal, external, both Delphi method – Each member surveyed – Opinions are piled – Each member is given the opportunity to change his opinion Inventory Management 97 Market Research Methods Particularly valuable for developing forecasts of newly introduced products Market testing – Focus groups assembled. – Responses tested. – Extrapolations to rest of market made. Market surveys – Data gathered from potential customers – Interviews, phonesurveys, written surveys, etc. Inventory Management 98 Time Series Methods Past data is used to estimate future data Examples include – Moving averages – average of some prev。 –缺點(diǎn) 銷售員無法區(qū)分顧客想做與實(shí)際會做的事。 在配銷系統(tǒng)中，每一階段或?qū)蛹?(亦即倉庫或零售商 )被視為一階層 (echelon)。公司的 目的 在 管理其存貨以降低整體系統(tǒng)成本 ；因此 考慮不同設(shè)施間的互動及這些互動對每個(gè)設(shè)施應(yīng)採取存貨政策的影響是很重要的。 (無窮的 ) Inventory Management 13 EOQ: Calculating Total Cost Purchase Cost Constant Holding Cost (h): (Avg. Inven) * (Holding Cost) Ordering (Setup Cost) (k): Number of Orders * Order Cost Goal: Find the Order Quantity that Minimizes These Costs: 2??K D hQT O CQInventory Management 14 EOQ:Total Cost 0204060801001201401600 500 1000 1500O r d e r Q u a n t i t yCostTotal Cost Order Cost Holding Cost Inventory Management 15 EOQ: Optimal Order Quantity So for our problem (Mug sales), the optimal quantity is 316 2 KDQh? ?* 2 2 0 5 2 1 2Q 3 1 60 . 2 5 1? ? ????Inventory Management 16 EOQ: Important Observations Tradeoff between setup costs and holding costs when determining order quantity. In fact, we order so that these costs are equal per unit time Total Cost is not particularly sensitive to the optimal order quantity Order Qua ntit y 50 % 80 % 90 % 100 % 110 % 120 % 150 % 200 % Co st Increa se 125 % 103 % 101 % 100 % 101 % 102 % 108 % 125 % Inventory Management 17 The Effect of Demand Uncertainty Most panies treat the world as if it were predictable: – Production and inventory planning are based on forecasts of demand made far in advance of the selling season – Companies are aware of demand uncertainty when they create a forecast, but they design their planning process as if the forecast truly represents reality Recent technological advances have increased the level of demand uncertainty: – Short product life cycles – Increasing product variety Inventory Management 18 Demand Forecast The three principles of all forecasting techniques: – Forecasting is always wrong – The longer the forecast horizo