Browsing by Subject "Inventory policy--Mathematical models."

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Open Access
Analyzing the effect of consumer returns in a multi-period inventory system
(2012) Erikçi, İsmail
Return of a sold item by a customer becomes tremendously common situation in many industries. Increase in the amount of returned items promotes return information to be a critical factor for inventory control. Undoubtedly another critical parameter for an inventory system is the length of the review period. Effect of the review period or length of the time-bucket is amplified with returned items, because available return information at a decision point is related to the frequency of the review. In this study, we analyze the effects of these two parameters over a multiperiod inventory system where the length of a time horizon is fixed. Dynamic programming approach is used to calculate the optimal inventory positions. In dynamic programming, it is assumed that a fixed proportion of sold items are returned. Computational results are obtained to compare the effects of return information under different return proportions and period lengths. These results are used to conduct various analyses to explore the level of the advantage gained by using return information.
Open Access
A dynamic pricing policy for perishables with stochastic demand
(2001) Yıldırım, Gonca
III this study, we consider the pricing of perishables in an inventory system where items have a fixi'd lifetime. Unit demands come from a Poisson Process with a price-dependent rate. The instances at which an item is withdrawn from inventory due to demand constitute decision epochs for setting the sales price; the time elapsed between two such consecutive instances is called a period. The sales price at each decision epoch is taken to be a lunction of Tj denoting the remaining lifetime when tin' inventory level drops to z, i = 1,...,Q. The objective is to determine the optimal pricing policy (under the proposed class) and the optimal initial stocking level to maximize the discounted expected profit. A Dynamic Programming approach is used the solve the problem numerically. Using the backward recursion, the optimal price paths are determined for the discounted expected profit for various combinations of remaining lifetimes. Our numerical studies indicate that a single price policy results in significantly lower profits when compared with our formulation.
Open Access
Finite perturbation analysis methods for optimization of periodic (s, S) inventory control systems
(2008) Mert, Erdinç
We are dealing with single item inventory systems where the period time is constant and the unsatisfied demands are backordered. The demands are independent and identically distributed random variables, but the distribution of those variables are not known. The total cost of a period consists of; ordering cost "K" which is independent of the ordering quantity, holding cost "h" for each item that remains in stock, and penalty cost "p" for the each backordered item. In the considered system, it is known that when the parameters of an (s,S) inventory policy are chosen appropriate, then the expected period cost can be minimized. There are some exact methods or heuristics for finding the optimal s and S parameters in the literature for the case where the demand distribution is known. In our study, we introduce a perturbation analysis based method for finding the optimal s and S parameters where the demand distribution is not known. Our method anticipates the sensitivity of (s,S) parameters to the period cost for the observed demand quantities. This method's performance is compared with a method that uses Integer Programming with the past data and with a method that calculates the mean and standard variation values with the past data and feeds them to the Ehrhardt's Heuristic.
Open Access
Integrated machine-scheduling and inventory planning of door manufacturing operations at OYAK Renault factory
(2012) Bozkaya, Nurcan
A car passes through press, body shell, painting and assembly stages during its manufacturing process. Due to the increased competition among car manufacturers, they aim to continuously advance and improve their processes. In this study, we analyze planning operations for the production of front/back and left/right doors in body shell department of Bursa Oyak-Renault factory and propose heuristic algorithms to improve their planning processes. In this study, we present four different mathematical models and two heuristics approaches which decrease the current costs of the company particularly with respect to inventory carrying and setup perspectives. In the body shell department of the company, there are two parallel manufacturing cells which produces doors to be assembled on the consumption line. The effective planning and scheduling of the jobs on these lines requires solving the problem of integrated machine-scheduling and inventory planning subject to inclusive eligibility constraints and sequence independent setup times with job availability in flexible manufacturing cells of the body shell department. The novelty in the models lie in the integration of inventory planning and production scheduling decisions with the aim of streamlining operations of the door manufacturing cells with the consumption line. One of the proposed heuristic approaches is Rolling Horizon Algorithm (RHA) which divides the planning horizon into sub-intervals and solves the problem by rolling the solutions through sub-intervals. The other proposed algorithm is Two-Pass Algorithm which divides the planning horizon into sub-intervals and solves each sub-problem in each sub-interval to optimality for two times by maintaining the starting and ending inventory levels feasible. These approaches are implemented with Gurobi optimization software and Java programming language and applied within a decision support system that supports daily planning activities.
Open Access
An inventory model for randomly perishing goods
(2000) Yüksel, Banu
In this study, we consider an (s, S) ordering policy with backordering for a continuous review inventory system, where the items have a random shelflife. Assuming zero lead time and no decay until the shelflife, we derive the exact expressions for both unit and random batch demand cases with renewal demand arrivals. We present some analytical results on the cost rate function for unit demand case. A detailed numerical analysis is also provided to investigate the performance of the model which incorporates the random shelflife and comparisons with flxed shelflife are given.