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Inventory Management V Lecture 8 ESD.260 Fall 2003 Caplice Inventory Policy Single Item w/Probabilistic Demand
Key Questions What are the questions I should ask to determine the type of inventory control system to use? How important is the item? Should the item be reviewed periodically or continuously? What form of inventory policy should I use? What cost or service objectives should I set? MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
How important is the item? Segmentation of Inventory Not all inventory is created equally Different classes of inventory Result in different levels of profitability / revenue Have different demand patterns and magnitudes And thus -require different control policies ABC Analysis Commonly used in practice Classify items by revenue or value Combination of usage, sales price, etc. MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
ABC Analysis Identify the SKUs that management should spend time on Prioritize SKUs by their value to firm Create logical groupings Adjust as needed Example: Samle of 20 SKUs Total of 4,677 units Total~$22k MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT Part IDPrice Annual DemandAnnual $ Value 5497J $ $ K62 $ $ $ $31.50 P001 $ $ M993 $ $ 2, HHT8 $ $ M4 $ $ KE $ $ 1, O3 $ $ K2 $ $ 8, SD3 $ $ HJQ2 $ $ LK $ $ RT $ $ JH4 $ $ 5, P $ $ $ $ JQ2 $ $ TT7 $ $ UJS2 $ $ ,677$ 21,983.84
ABC Analysis MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT A Items: 80% of Value 20% of SKUs B Items: 15% of Value 30% of SKUs C Items: 5% of Value 50% of SKUs Part IDPrice Annual Demand Annual $ Value Cum$ Value Pct Ann $ VAlue 55K2$ $ 8,347 38% 58JH4$ $5,198$ 13,54462% 2M993$ $2,723$ 16,26874% 3HHT8$ $1,342$ 17,61080% 89KE$ $1,038$ 18,64785% 5497J$ $585$19,23287% 38JQ2$ $531 $19,76390% 2340P$ $411$20,17492% 990RT$ $346$20,52093% 56M4$ $341$20,86195% P001$ $299$21,16096% 978SD3$ $186$21,34697% 45O3$ $179$21,52598% 3784$ $126$21,65198% 3K62$ $123$21,77399% 56TT7$ $64$21,83799% 78HJQ2$ $52$21,890100% 7UJS2$ $49$21,938100% 88450$ $32$21,970100% 23LK$ $14$21,984100% 4,677$ 21,984
ABC Analysis MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT Percent of Annual value Percent of SKUs Distribution By Value
ABC Analysis So, what is different between the classes? A Items Very few high impact items are included Require the most managerial attention and review Expect many exceptions to be made B Items Many moderate impact items (sometimes most) Automated control w/ management by exception Rules can be used for A (but usually too many exceptions) C Items Many if not most of the items that make up minor impact Control systems should be as simple as possible Reduce wasted management time and attention Group into common regions, suppliers, end users But – these are arbitrary classifications MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
How else to classify inventory? Criticality to Operations Profitability Usage rate * Volume Dollar Value & # Customer Transactions Dollar Value & Criticality Multiple dimensions (cluster analysis) MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
Continuous or Periodic Review? Periodic Review Know stock level only at certain times Review interval (RI) = time between consecutive reviews Continuous Review Is continuous really continuous? Transactions reporting Collecting information vs. Making decision Advantages of Periodic Review Coordination of replenishments Able to predict workload Forces a periodic review Continuous Review Continuous Review Replenishments made dynamically Cost of equipment (scanners etc.) Able to provide same level of service with less safety stock MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What form of inventory policy? Order-Point, Order-Quantity (Q,R) System Continuous Review Policy: Order Q if Inventory Position (IP) R Two-bin system Simple to set up and operate – low errors Order-Point, Order-Up-To-Level (s, S) System Continuous Review (RI=0) Policy: Order (S-IP) if IP s Min-Max system Optimal (s,S) is typically better than optimal (Q,R) Finding optimal (s,S) is difficult – try reasonable values Impact on suppliers – fixed versus variable orders MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What form of inventory policy? Periodic-Review, Order-Up-To-Level (RI, S) System Periodic Review (RI>0) Policy: Order (S-IP) every RI units of time Replenishment cycle system Higher carrying costs than Continuous System Able to coordinate related replenishments Periodic Review, Hybrid (RI, s, S) System Periodic Review (RI>0) Policy: Every RI units of time, order (S-IP) if IP s, otherwise, do not order General case for other policies Optimal (RI,s,S) is better than any other system Use of reasonable vs. optimal parameters MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What form of inventory policy? No hard and fast rules, but some rules of thumb Type of Item Continuous Review Periodic Review A Items(s,S)(RI, s, S) B Items(Q,R)(RI,S) C ItemsManual ~ (Q,R)Manual ~ (RI,S) MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What cost and service objectives? 1. Common Safety Factors Approach Simple, widely used method Apply a common metric to aggregated items Example: Time Supplies SS = 2 month supply for all items Example: Equal Safety Factors SS = k σ e for all items MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What cost and service objectives? 2. Cost Minimization Approach Requires costing of shortages Fixed cost per stockout Fractional cost per unit short Fractional cost per short per unit time Cost per customer line item short Find trade-off between relevant costs MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What cost and service objectives? 3. Customer Service Approach Establish constraint on customer service Probability of no stockouts per replenishment cycle (Service Level) Fraction of demand satisfied from shelf (Fill Rate) Fraction of time that net stock is positive (Ready Rate) Average time between stock outs (TBS) Fraction of orders completely satisfied from shelf (Order Fill Rate) Definitions in practice are fuzzy Minimize costs with respect to customer service constraints MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
What cost and service objectives? 4. Aggregate Considerations Weight specific characteristic of each item Examples Minimize expected total stockout occasions per year Equivalent to including a fixed cost per stockout Same cost applied across all products Minimize expected total value of shortages per year Equivalent to including fractional cost per unit short for each item and minimizing costs Select characteristic most essential to firm MIT Center for Transportation & Logistics - ESD © Chris Caplice, MIT
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