مدیریت موجودی Inventory Management

Transcription

1 مدیریت موجودی Inventory Management Inventory One of the most expensive assets of many companies representing as much as 50% of total invested capital Operations managers must balance inventory investment and customer service مدیریت تولید - محمدصالح احمدی 171 Types of Inventory Raw material Purchased but not processed Work-in-process Undergone some change but not completed A function of cycle time for a product Maintenance/repair/operating (MRO) Necessary to keep machinery and processes productive Finished goods Completed product awaiting shipment 1

2 The Material Flow Cycle Cycle time 95% 5% Input Wait for Wait to Move Wait in queue Setup Run Output inspection be moved time for operator time time ABC Analysis Divides inventory into three classes based on annual cost Class A - high annual cost Class B - medium annual cost Class C - low annual cost Used to establish policies that focus on the few critical parts and not the many trivial ones 2

3 ABC Analysis Item Stock Number Percent of Number of Items Stocked Annual Volume (units) x Unit Cost = Annual cost Percent of Annual cost # % 1,000 $ $ 90, % A 72% # , % A # , , % B # % , % 23% B # , , % B Class ABC Analysis Item Stock Number Percent of Number of Items Stocked Annual Volume (units) x Unit Cost = Annual cost Percent of Annual cost # $ $ 8, % C # , ,200.5% C # % % 5% C # , % C # % C 8,550 $232, % Class 3

4 Percent of annual dollar usage 11/30/2015 ABC Analysis A Items B Items C Items Percent of inventory items ABC Analysis Policies employed may include More emphasis on supplier development for A items Tighter physical inventory control for A items More care in forecasting A items 4

5 Holding, Ordering, and Shortage Costs Holding costs - the costs of holding or carrying inventory over time Ordering costs - the costs of placing an order and receiving goods Shortage costs (Stockout cost)- costs incurred when an item is out of stock. These costs include the lost Contribution Margin on sales plus lost customer goodwill, or extra cost for obtaining the items by unusual methods. Holding Costs Cost (and range) as a Percent of Category Inventory Value Housing costs (building rent or 6% (3-10%) depreciation, operating costs, taxes, insurance) Material handling costs (equipment lease or 3% (1-3.5%) depreciation, power, operating cost) Labor cost 3% (3-5%) Investment costs (borrowing costs, taxes, 11% (6-24%) and insurance on inventory) Pilferage, space, and obsolescence 3% (2-5%) Overall carrying cost 26% 5

6 Holding Costs Cost (and range) as a Percent of Category Inventory Value Housing costs (building rent or 6% (3-10%) depreciation, operating costs, taxes, insurance) Material handling costs (equipment lease or 3% (1-3.5%) depreciation, power, operating cost) Labor cost 3% (3-5%) Investment costs (borrowing costs, taxes, 11% (6-24%) and insurance on inventory) Pilferage, space, and obsolescence 3% (2-5%) Overall carrying cost 26% Inventory Models for Independent Demand Need to determine when and how much to order Basic economic order quantity Production order quantity Quantity discount model 6

7 Annual ordering cost 11/30/2015 Basic EOQ Model Important assumptions 1. Demand is known, constant, and independent 2. Lead time is known and constant 3. Receipt of inventory is instantaneous and complete 4. Quantity discounts are not possible 5. Only variable costs are setup and holding 6. Stockouts can be completely avoided Annual ordering cost D : Annual demand Q : Order Quantity D Q : Number of orders per year C o : Ordering cost (cost of every order) D Q C o : Annual ordering cost Q 7

8 Inventory level Annual cost 11/30/2015 Annual holding cost Order quantity = Q (maximum inventory level) Usage rate Average inventory on hand Q 2 C h : Holding cost (cost of holing per unit per year) Minimum inventory 0 Time Q 2 C h : Annual holding cost Minimizing Costs Curve for total cost of holding and setup Objective is to minimize total costs Minimum total cost TC = D Q C o + Q 2 C h Holding cost curve TC Q = D Q 2 C o+ 1 2 C h=0 Optimal order quantity (Q*) Setup (or order) cost curve Order quantity Q = 2DC o C h 8

9 An EOQ Example Determine optimal number of needles to order D = 1,000 units C o = $10 per order C h = $.50 per unit per year Q = Q* = 2DC o C h 2(1,000)(10) 0.50 = 40,000 = 200 units Expected number of orders Demand Order quantity = N = = D Q* N = 1, = 5 orders per year Expected time between orders = T = Number of working days per year N T = = 50 days between orders TC = D Q C o + Q 2 C h TC = (5)($10) + (100)($.50) = $50 + $50 = $100 9

10 T C T C = 1 2 ( Q Q + Q Q ) For example if the order quantity is changed to 400 (instead of 200): مدیریت تولید - محمدصالح احمدی 189 Reorder Points EOQ answers the how much question The reorder point (ROP) tells when to order ROP = = d x L Demand per day Lead time for a new order in days d = D Number of working days in a year 10

11 Inventory level (units) 11/30/2015 Reorder Point Curve Q* Slope = units/day = d ROP (units) Lead time = L Time (days) Probabilistic Models and Safety Stock Used when demand is not constant or certain Use safety stock to achieve a desired service level and avoid stockouts ROP = d x L + ss Annual stockout costs = the sum of the units short x the probability x the stockout cost/unit x the number of orders per year 11

12 Safety Stock Example ROP = 50 units Orders per year = 6 Stockout cost = $40 per frame Carrying cost = $5 per frame per year Number of Units Probability ROP Safety Stock Example ROP = 50 units Orders per year = 6 Stockout cost = $40 per frame Carrying cost = $5 per frame per year Safety Stock Additional Holding Cost Stockout Cost Total Cost 20 (20)($5) = $100 $0 $ (10)($5) = $ 50 (10)(.1)($40)(6) = $240 $290 0 $ 0 (10)(.2)($40)(6) + (20)(.1)($40)(6) = $960 $960 A safety stock of 20 frames gives the lowest total cost ROP = = 70 frames 12

13 Inventory level 11/30/2015 Probabilistic Demand Minimum demand during lead time Maximum demand during lead time Mean demand during lead time ROP = safety stock of 16.5 = ROP Normal distribution probability of demand during lead time Expected demand during lead time (350 kits) Safety stock 16.5 units 0 Lead time Time Place order Receive order Probabilistic Demand Probability of no stockout 95% of the time Risk of a stockout (5% of area of normal curve) Mean demand 350 Safety stock ROP =? kits 0 z Quantity Number of standard deviations 13

14 Probabilistic Demand Use prescribed service levels to set safety stock when the cost of stockouts cannot be determined ROP = demand during lead time + Zs dlt where Z = number of standard deviations s dlt = standard deviation of demand during lead time 14