Inventory Concepts库存概念Chapter 4

Hu Xiongying, LecturerThe Wuhan Institute of Technology

COST TRADEOFFS REQUIRED INMARKETING AND LOGISTICS

Marketing Objective:Allocate resources to the marketing mix in such a manner as to maximize the long-term profitability of the firm.

Logistics Objective:Minimize Total Costs given the customer service objective where total costs = Transportation Costs + Warehousing Costs + Order

Processing and Information Costs + Lot Quantity Costs + Inventory Carrying Costs

INVENTORY PIPELINE库存管道

WHY IS IT NECESSARY TO HAVE INVENTORIES?为什么持有库存• To achieve transportation economies• To achieve production economies• To take advantage of quantity purchase discounts• To maintain a source of supply• To support the firm’s customer service policies• To meet changing market conditions (e.g., Seasonality,

demand fluctuations, competition)• To overcome time and space differentials that exist

between producers and consumers• To accomplish least total logistics commensurate with a

desired level of customer service

Basic Reasons to Hold Inventory持有库存的基本原因

• Buffer against uncertainty in:– Demand– Supply– Delivery

Safety stock is the principal inventory tool to address uncertainty

Basic Reasons to Hold Inventory (cont.) 持有库存的基本原因（续）• Provide scale economies in:

– Procurement– Production– Transportation

Cycle stock is used to take advantage of scale economies.

Other Reasons for Holding Inventory持有库存的其他原因

• Supply and demand cannot be perfectly matched (uncertainty & production lots)

• Customer service through positioning and availability

• Anticipation--fertilizer and seasonal items• Decoupling of:

– production and supply– production and finished goods

Inventory Categories库存类型• Cycle (“working”) stock• Safety stock• Anticipation (“build”) stock• Intransit (“pipeline”) stock• Decoupling (“peace-keeper”) stock• Shadow (“phantom”) stock

Dimensions of Inventory Models库存模型的分类

• Deterministic versus Stochastic• Indefinite versus finite planning horizon• Independent versus dependent demand• Single versus multiple:

– item– location– echelon (interrelated locations)– indenture (interrelated items)

Echelon n.[ 军 ] 梯形 , 梯阵 , 梯次编队 vt. 排成梯队 vi. 排成梯队Indenture n. 契约

Relevant Inventory Costs相关的库存成本• Purchase cost of the item

– Volume discounts– Temporary discounts

• Ordering or purchasing costs– Administrative purchasing costs– Manufacturing setup costs

Costs Included in Inventory Models用于库存模型中的成本

• Holding or inventory carrying costs– capital costs– service costs– storage costs– risk costs

• Shortage costs– backorder costs– lost sales costs– lost customer costs

back order 留待将来交付的订货 , 未交清订货 , 未完成的订货 , 延迟交货，延期交货 ,再订货

INVENTORY MANAGEMENT ANSWERSSEVERAL IMPORTANT STRATEGIC QUESTIONS库存管理回答的几个重要战略问题

• What level of customer service?• Where should stock be (location)• What items to stock? (range)• How much of each item to stock? (depth)

INVENTORY MANAGEMENT ANSWERSFOUR IMPORTANT TACTICAL QUESTIONS库存管理回答的四个重要战术问题

• How to order?• When to order?• What to order?• How much to order?

THE EFFECT OF REORDER QUANTITY ON AVERAGE INVENTORY INVESTMENT WITH CONSTANT DEMAND AND

LEAD TIME需求和前置期稳定的前提下，再订货量对平均库存投资的影响

THE EFFECT OF REORDER QUANTITY ON AVERAGE INVENTORY INVESTMENT WITH CONSTANT DEMAND AND

LEAD TIME需求和前置期稳定的前提下，再订货量对平均库存投资的影响

THE EFFECT OF REORDER QUANTITY ON AVERAGE INVENTORY INVESTMENT WITH CONSTANT DEMAND AND

LEAD TIME需求和前置期稳定的前提下，再订货量对平均库存投资的影响

AVERAGE INVENTORY INVESTMENT UNDER CONDITIONS OF UNCERTAINTY不确定情况下的平均库存投资

AVERAGE INVENTORY INVESTMENT UNDER CONDITIONS OF UNCERTAINTY不确定情况下的平均库存投资

AVERAGE INVENTORY INVESTMENT UNDER CONDITIONS OF UNCERTAINTY不确定情况下的平均库存投资

Wilson’s* Economic Order Quantity (EOQ)经济订购批量

Wilson’s Economic Order Quantity (EOQ)经济订购批量

Wilson’s Economic Order Quantity (EOQ)经济订购批量

Relevant Costs in EOQEOQ 的相关成本

• Holding or Carrying cost (C)• Ordering cost (P)• Purchase cost of the item (V)

THE EOQ MODELEOQ 模型

COST TRADE-OFFS REQUIRED TO DETERMINETHE MOST ECONOMIC ORDER QUANTITY确定最经济的订货批量所需的成本权衡

AN EOQ MODEL EXAMPLE(The hard Way)

COST TRADE-OFFS REQUIRED TO DETERMINEAN ECONOMIC ORDER QUANTITY

By Trial & Error (a.k.a “Brute Force”)决定最经济订货批量所需的成本权衡

The Easy & Exact Way

ASSUMPTIONS OF THE SIMPLE EOQ MODEL简单 EOQ 模型的假设

• A continuous, constant, and known rate of demand.• A constant and known replenishment cycle or lead time.• A constant purchase price that is independent of the

order quantity or time.• A constant transportation cost that is independent of the

order quantity or time.• The satisfaction of all demand (no stock-outs are

permitted).• No inventory intransit.• Only one item in inventory, or at least no interaction

among items.• An infinite planning horizon.• No limit on capital availability.

RELATIONSHIP BETWEEN INVENTORYINVESTMENT AND CUSTOMER SERVICE LEVELS库存投资和客户服务水平之间的关系

PITFALLS OF INVENTORY CONTROL库存控制的缺陷

• Control by edict• No one person is responsible• The fallacy of 100% safety stock protection• The “Who needs a forecast?” attitude

WEIGHTED CUSTOMER SERVICE LEVELS USING ABC ANALYSIS应用 ABC 分析法加权客户服务水平

TIME-BASED COMPETITION基于时间的竞争• Enhances customer value• Reduce inventory investment• Improves profitability• Responds to changing market needs• Provides competitive/differential

advantage• Eliminates "cost added"/duplicate

functions

TOTAL SUPPLY CHAIN INVENTORY总供应链库存

Logistics questions regarding inventory有关库存的物流问题

• What level of customer service is required?

• What level of disrupted service is acceptable?

• How centralized an operation?• What is cost of a wrong decision?• How sophisticated an inventory system is

required?

Variables considered by the logistics manager物流经理考虑的变量

FACTORS INFLUENCING SAFETY STOCKS影响安全库存的因素

• Forecast error• Exposure to stock-out• Lead time• Service level requirements

Some Safety Stock Strategies一些安全库存策略

• Specified fill rate (demand filled from stock) or Specified service level (probability a stock-out will not occur)

• Maximize $ demand filled from a given investment.

• Set SS based on specified number of Sigmas (Std.Dev., MAD, etc.)

Some Safety Stock Strategies (cont.)一些安全库存策略（续）

• Set SS based on specified time supply.• Minimize shortage occurrences for a given

investment (# of orders with a problem.)• Minimize transaction shortages for a given

investment (# of problems in orders.)

Materials Management Systems;R.G. Brown 1977, Pg. 178