3Independent demand is uncertain. Dependent demand is certain. B(4)C(2)D(2)E(1)D(3)F(2)Dependent DemandIndependent demand is uncertain. Dependent demand is certain.Inventory: a stock or store of goods
4Types of Inventories Raw materials & purchased parts Partially completed goods called work in progressFinished-goods inventories(manufacturing firms) or merchandise (retail stores)
5Types of Inventories (Cont’d) Replacement parts, tools, & suppliesGoods-in-transit to warehouses or customers
6Functions of Inventory To meet anticipated demandTo smooth production requirementsTo decouple operationsTo protect against stock-outs
7Functions of Inventory (Cont’d) To take advantage of order cyclesTo help hedge against price increasesTo permit operationsTo take advantage of quantity discounts
8Objective of Inventory Control To achieve satisfactory levels of customer service while keeping inventory costs within reasonable boundsLevel of customer serviceCosts of ordering and carrying inventory
9Effective Inventory Management A system to keep track of inventoryA reliable forecast of demandKnowledge of lead timesReasonable estimates ofHolding costsOrdering costsShortage costsA classification system
10Inventory Counting Systems Periodic SystemPhysical count of items made at periodic intervalsPerpetual Inventory System System that keeps track of removals from inventory continuously, thus monitoring current levels of each item
11Inventory Counting Systems (Cont’d) Two-Bin System - Two containers of inventory; reorder when the first is emptyUniversal Bar Code - Bar code printed on a label that has information about the item to which it is attached
12Key Inventory TermsLead time: time interval between ordering and receiving the orderHolding (carrying) costs: cost to carry an item in inventory for a length of time, usually a yearOrdering costs: costs of ordering and receiving inventoryShortage costs: costs when demand exceeds supply
13ABC Classification System Figure 11.1Classifying inventory according to some measure of importance and allocating control efforts accordingly.A - very importantB - mod. importantC - least importantAnnual$ valueof itemsABCHighLowFewManyNumber of Items
14Cycle Counting A physical count of items in inventory Cycle counting managementHow much accuracy is needed?When should cycle counting be performed?Who should do it?
15Economic Order Quantity Models Economic production modelQuantity discount model
16Assumptions of EOQ Model Only one product is involvedAnnual demand requirements knownDemand is even throughout the yearLead time does not varyEach order is received in a single deliveryThere are no quantity discounts
17Profile of Inventory Level Over Time The Inventory CycleFigure 11.2Profile of Inventory Level Over TimeQuantityon handQReceiveorderPlaceLead timeReorderpointUsagerateTime
19Cost Minimization Goal Figure 11.4CThe Total-Cost Curve is U-ShapedAnnual CostOrdering CostsOrder Quantity (Q)QO(optimal order quantity)
20Deriving the EOQUsing calculus, we take the derivative of the total cost function and set the derivative (slope) equal to zero and solve for Q.
21Minimum Total CostThe total cost curve reaches its minimum where the carrying and ordering costs are equal.
22Economic Production Quantity (EPQ) Production done in batches or lotsCapacity to produce a part exceeds the part’s usage or demand rateAssumptions of EPQ are similar to EOQ except orders are received incrementally during production
23Economic Production Quantity Assumptions Only one item is involvedAnnual demand is knownUsage rate is constantUsage occurs continuallyProduction rate is constantLead time does not varyNo quantity discounts
25Total Costs with Purchasing Cost AnnualcarryingcostPurchasingTC =+Q2HDSorderingPD
26Total Costs with PD Figure 11.7 Cost EOQTC with PDTC without PDPDQuantityAdding Purchasing cost doesn’t change EOQ
27Total Cost with Constant Carrying Costs Figure 11.9OCEOQQuantityTotal CostTCaTCcTCbDecreasingPriceCC a,b,c
28When to Reorder with EOQ Ordering Reorder Point - When the quantity on hand of an item drops to this amount, the item is reorderedSafety Stock - Stock that is held in excess of expected demand due to variable demand rate and/or lead time.Service Level - Probability that demand will not exceed supply during lead time.
29Determinants of the Reorder Point The rate of demandThe lead timeDemand and/or lead time variabilityStockout risk (safety stock)
30Safety Stock Figure 11.12 Quantity Maximum probable demand LTTimeExpected demandduring lead timeMaximum probable demandROPQuantitySafety stockSafety stock reduces risk ofstockout during lead time
31Reorder Point Figure 11.13 The ROP based on a normal Risk ofa stockoutService levelProbability ofno stockoutExpecteddemandSafetystockzQuantityz-scaleThe ROP based on a normalDistribution of lead time demand
32Fixed-Order-Interval Model Orders are placed at fixed time intervalsOrder quantity for next interval?Suppliers might encourage fixed intervalsMay require only periodic checks of inventory levelsRisk of stockout
33Fixed-Interval Benefits Tight control of inventory itemsItems from same supplier may yield savings in:OrderingPackingShipping costsMay be practical when inventories cannot be closely monitored
34Fixed-Interval Disadvantages Requires a larger safety stockIncreases carrying costCosts of periodic reviews
35Single Period ModelSingle period model: model for ordering of perishables and other items with limited useful livesShortage cost: generally the unrealized profits per unitExcess cost: difference between purchase cost and salvage value of items left over at the end of a period
36Single Period Model Continuous stocking levels Identifies optimal stocking levelsOptimal stocking level balances unit shortage and excess costDiscrete stocking levelsService levels are discrete rather than continuousDesired service level is equaled or exceeded
37Operations Strategy Too much inventory Wise strategy Tends to hide problemsEasier to live with problems than to eliminate themCostly to maintainWise strategyReduce lot sizesReduce safety stock