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Spreadsheet Models for Managers: Session 11 11/1 Copyright © 1994-2011 Richard Brenner Spreadsheet Models for Managers Session 11 Inventory Modeling Economic Order Quantity Quantity Discounts Last revised: July 6, 2011
Spreadsheet Models for Managers: Session 11 11/2 Copyright © 1994-2011 Richard Brenner Review of last time: Capital Leases II: Modeling Capital leases affect all three financial statements Cash outlays Assets Liabilities Depreciation Interest expense For streams of lease events under identical leases Create a Lease Characteristic Array Convolve with lease event stream In models with multiple identical leases, combine lease streams, then compute effects on financial statements rather than vice versa
Spreadsheet Models for Managers: Session 11 11/3 Copyright © 1994-2011 Richard Brenner Capacity models Categories of capacity questions Inventory “Do we have enough” “What’s the restock point?” “What is the best choice of restock quantity?” Servers “How many checkouts should a supermarket have open?” “How many phone lines do we need to support our business?” “How many customer service reps are enough?” “How many file servers are enough?” Space “When should we lease more?” “How much parking do we really need?” Each category of capacity problem requires a unique approach.
Spreadsheet Models for Managers: Session 11 11/4 Copyright © 1994-2011 Richard Brenner Importance of inventory Inventory management is most important in goods-oriented operations Manufacturing Mining Retail Inventory is less important when Added value is mostly not related to goods Professional services operations Software or other knowledge-driven companies Finished Goods (and their components) cannot be stocked Electric power Broadcasting But even these organizations must inventory some items Keep inventory low when holding costs are high Telecommunications Common carrier transport Education Entertainment Real estate development/rental Wholesale distribution/logistics Fulfillment
Spreadsheet Models for Managers: Session 11 11/5 Copyright © 1994-2011 Richard Brenner Kinds of inventory Raw materials and purchased parts for use in finished goods Work in process (partial assemblies) Finished goods Maintenance inventory Internally generated vs. externally acquired inventory
Spreadsheet Models for Managers: Session 11 11/6 Copyright © 1994-2011 Richard Brenner Inventory cost factors Costs related to the number of orders Decision to order Telephone, postage, other communications Labor Computation and record-keeping (accounting, tracking, control) Price verification (external only) Receiving and handling Freight Setup and stowage Costs related to the total value and bulk of inventory Handling and storage Interest costs Opportunity costs relative to alternative inventoried components Insurance, taxes, space, equipment Heating, cooling, humidity control Shelf life factors: spoilage, theft, obsolescence
Spreadsheet Models for Managers: Session 11 11/7 Copyright © 1994-2011 Richard Brenner Inventory cost factors (2) Costs of stockouts Lost sales Lost customers Idle time (no parts -> idle assembly lines) Idle workpieces in process back-order/expedite cycle Size of order quantity discounts freight insurance security Knowing the true cost of inventory is important even when you outsource it.
Spreadsheet Models for Managers: Session 11 11/8 Copyright © 1994-2011 Richard Brenner Effects of demand On inventory strategy Constant demand vs. variable demand Constant demand is easier to manage Constant demand is easier to model Constant demand is more and more rare because: The world is changing rapidly Repeated orders of identical products are less and less common Variable demand is best modeled statistically First understand the constant demand problem
Spreadsheet Models for Managers: Session 11 11/9 Copyright © 1994-2011 Richard Brenner Representing constant demand 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Jan Feb Mar Apr May Jun Jul AugSep Oct NovDec Jan Feb Mar Apr May Jun Jul AugSep Oct NovDec Jan Date Inventory Units Average Inventory: 2 units
Spreadsheet Models for Managers: Session 11 11/10 Copyright © 1994-2011 Richard Brenner Constant demand data This is the data for the graphical example on the previous slide Inventory is replenished every fourth month The data show inventory at the start of each month Notice: no zeros in the table, but the restock point is zero. Average inventory for constant demand: If : I reorder = 0 + I max = I average I reorder 2 = I average I max 2
Spreadsheet Models for Managers: Session 11 11/11 Copyright © 1994-2011 Richard Brenner Constant demand On an annual basis: For constant demand, use evenly spaced, equal sized orders where D = Annual demand (units per year) Q = Quantity in one order (units) C 0 = Cost to place one order (dollars) C c = Carrying cost of inventory (dollars per unit per year) (if restock point is 0) = —– DQDQ C 0 + —– Q2Q2 C c = —– DQDQ C 0 + —— 2 C c I max Total Cost ( ) = Cost Related to No. Orders ( ) Cost Related to Inventory Size ( ) +
Spreadsheet Models for Managers: Session 11 11/12 Copyright © 1994-2011 Richard Brenner Economic order quantity EOQ: Lowest-cost order quantity for constant demand It can be shown mathematically that EOQ is the value of Q for which Ordering Costs = Carrying Costs. Readings: Economic Order Quantity 2DC 0 √ CcCc
Spreadsheet Models for Managers: Session 11 11/13 Copyright © 1994-2011 Richard Brenner Quantity discounts When there is a quantity discount, there is an additional term in the cost equation: Here we are computing not only the inventory costs, but the buying costs as well. To minimize total costs, you have to take an experimental approach: compute costs for each break in the quantity schedule. = —– DQDQ C 0 + —– Q2Q2 C c + D u(Q) Total Cost u(Q) = unit price as a function of order size
Spreadsheet Models for Managers: Session 11 11/14 Copyright © 1994-2011 Richard Brenner Examples You are a buyer of dyes for an apparel manufacturer. The annual demand for dyes of all colors is 64,000 liters. Price is $10 per liter. The cost to hold an item in inventory is 20% of the value of the item per year. It costs $10 to place an order (you use email). What is the EOQ? Make plots showing the two components of total cost as functions of order quantity. You expect volume to pick up next year, so you negotiated a quantity discount. If you buy more than 1000 liters at a time, you can get a price of $8 per liter. If you buy more than 3000 liters at a time, the price is only $7 per liter. Now what is the optimum order quantity? Example
Spreadsheet Models for Managers: Session 11 11/15 Copyright © 1994-2011 Richard Brenner Readings Readings: Economic Order Quantity
Spreadsheet Models for Managers: Session 11 11/16 Copyright © 1994-2011 Richard Brenner Preview of next time: Service Systems A Service System is a facility containing servers, customer entry and exit facilities, and a waiting facility The simplest system is single-stage, single-server When arrivals are Poisson distributed, and service completion is exponentially distributed, we can predict system performance
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