Use of Promethee method to determine the best alternative for
Transcrição
Use of Promethee method to determine the best alternative for
Int J Adv Manuf Technol (2014) 70:1615–1624 DOI 10.1007/s00170-013-5405-z ORIGINAL ARTICLE Use of Promethee method to determine the best alternative for warehouse storage location assignment Marcele Elisa Fontana & Cristiano Alexandre Virgínio Cavalcante Received: 27 June 2011 / Accepted: 4 October 2013 / Published online: 27 October 2013 # Springer-Verlag London 2013 Abstract Storage includes all activities involved in holding products at a point for temporary custody and subsequent distribution. The criteria that must be considered in determining the location in a storage facility for each product are often conflicting. Thus, the main objective of this paper is to determine the best alternative for assigning a product to a warehouse storage location. A class formation and allocation model and a preference ranking organization method for enrichment evaluation (Promethee) multicriteria method are used to achieve this goal. By using the model proposed, the manager can learn of all the possible non-dominated allocations of the products in the warehouse and can then change the allocation when needed. The Promethee method is efficient at ranking alternatives which facilitates the decision process. Keywords Cube-per-order index . Warehouse storage location assignment . Time required . Promethee method 1 Introduction Storage is one of the most traditional aspects of logistics. A good storage system facilitates the identification of physical material, thereby increasing productivity and reducing the cost of manpower, because it facilitates the management of inventories. According to Daniels et al. [1], changes in demand, and the consequent reallocation of warehouse space, often require movement of stock that can cause severe disruptions to warehouse operations, especially when the warehouse is used intensively. M. E. Fontana (*) : C. A. V. Cavalcante (*) Federal University of Pernambuco, Av. Acadêmico Hélio Ramos, s/n - Cidade Universitária, UFPE-CTG, Recife, Pernambuco 50740-530, Brazil e-mail: [email protected] e-mail: [email protected] Warehouse management plays an important role for many efficient supply chains [2]. With the advent of supply chain management, the strategic role of a warehouse changed. The activities involved in processing orders in today’s warehouses are conducted at a faster pace than in the recent past. In order to satisfy clients’ demands for shorter cycle times when meeting orders, warehouse managers are interested in finding the most economical way of fulfilling orders: the one which minimizes the costs involved in terms of distance traveled or travel time [3]. An efficient warehouse should take into account, principally, the reduction in costs, space used, and distance traveled. Therefore, the objective of finding solutions for the stock location problem is to reduce the requirement for space and to minimize the total distance traveled or travel time throughout the warehousing process [4]. For many products, these criteria can be conflicting. Thus, the main objective, in this paper, is to investigate an approach that uses a multicriteria method in order to reach the best alternative for allocating a product in a warehouse, according to the decision maker’s preferences. For the decision maker (DM), the criteria are not compensatory, i.e., it is not possible for trade-offs to occur when evaluating the criteria. Therefore, the preference ranking organization method for enrichment evaluation (Promethee) multicriteria method was chosen in this paper. The specific objective of the study is to compare the results provided by the Promethee method when the DM’s preferences on the criteria used change. The paper is organized as follows. Section 2 provides a review of the literature specific to the problem studied and focuses on those papers which are fundamental to the model proposed. Section 3 presents how the problem is formulated and gives a simulated description of a warehouse, i.e., what both the criteria used and the scenarios simulated are. The results are given and discussed in Section 4. Finally, Section 5 makes some concluding remarks and suggests further lines of study. 1616 2 Warehouse storage location assignment The function of a distribution warehouse is to receive and store products and to fulfill orders from external clients which typically comprise a large number of order lines (where each order line specifies a quantity of one particular product). The number of different products in a distribution warehouse may be large, while the quantities per order line may be small, and this often results in a complex and relatively costly order picking process [5, 6]. One way to decrease handling time is by changing the operational procedures [7]. Warehouse storage decisions influence almost all the key performance indicators of a warehouse such as order picking time and the cost of using storage space, labor, etc. [8]. Typical planning issues in warehouses are inventory management and storage location assignment. Intelligent inventory management may well result in reducing warehousing costs. On the other hand, an effective policy for assigning warehouse storage location may reduce the mean travel times for storage/ retrieval and order picking [9]. Order picking—the retrieval of stock keeping units (SKUs) from a warehouse to satisfy customer orders—is a vital supply chain component for many companies, both from the production system point of view (i.e., the supply of assembly stations with assembly kits) and from the point of view of physical distribution activities (i.e., fulfilling a customer’s order [10, 11]). A crucial link between order picking and service level is that the faster the items on an order can be retrieved, the sooner they are available for shipping to the client [12]. Successful firms are those that provide the right products to the right clients, in the right place, at the right time, and for the right price [13]. The time spent on warehousing activities is an important factor in the total time spent on the request cycle. Therefore, it is essential to study viable and sustainable means to minimize this time. Different storage strategies can be used. The implementation of each storage strategy is an operational issue. The selection of an order picking method is a strategic decision, since it has a wide impact on many other decisions in the design and operation of a warehouse [14]. Since warehousing activities are frequent and numerous, even small improvements can achieve significant savings [15]. Order picking is often considered the most critical operation in warehousing. The efficiency of an order picking process greatly depends on the storage policy used, i.e., on where products are located within the warehouse [16]. According to Brynzér and Johansson [17], components have many characteristics that can be used to assign items to locations or zones in warehouses, such as frequency, size, weight, part number, supplier, etc. Thus, three storage location assignment policies were put forward by Hausman et al., in 1976, [6, 8], namely: dedicated or fixed storage, randomized or variable storage, and class-based storage. Int J Adv Manuf Technol (2014) 70:1615–1624 A dedicated storage policy prescribes a particular location for the storage of each product [5], such that no other item can be stored there, even if the space is empty. Under a dedicated storage policy, each storage area may only be used for a specific item. The materials are placed in existing open spaces. A randomized storage policy allows items to be stored anywhere in the storage area. Randomized and dedicated storage are extreme cases of a class-based storage policy: randomized storage considers a single class, and dedicated storage considers one class for each item [18]. Under class-based storage rack assignment, products having similar turnover rates are stored in the same class, and the rack is divided into several classes of different sizes [19, 20]. For the formation of classes, Hesket, in 1963, proposed the cube-per-order index (COI), which is expressed as the ratio of storage space required (cube) per SKU and the order frequency of the SKU [17], i. e., COIp =f p *[Max I t p ]/D p , where COIp is the cube-per-order index for product p ; f p is the density (area required to store one unit load of product p); I t p is the storage level in unit loads planned for product p during period t; and D p is the total demand for the product p in the planning period (in unit loads). The rule ranks the items in ascending order of the index, and then assigns them in that order to the locations nearest to the input/output (I/O) point in order to reduce the cost of order picking [21]. The location of the items must follow the ascending order of the index. Based on the COI index, there is a tendency for products with lower space requirements and high demand to be located at the front of the warehouse, i.e., next to I/O, and products with greater space requirements and low demand to be located far from the I/O. Thus, the COI tries to make storage more efficient, by minimizing the distances with a single control for order picking. A class formation and allocation model was put forward by Muppani and Adil [18; 22]. This model uses the COI index to rank items. The problem can be defined as: given P products/ items, their average demand D p and planned inventory levels I p for T periods and the layout of the storage area divided into a lattice of storage locations, establish classes of products and allocate them to storage locations so that the total cost of order picking and storage space is minimized in a single command warehouse, thus exploiting the reduction in area. Therefore, we have: Minimize "( X # ) X XX X ða :d :y Þ l l l lc X ðal :ylc Þ þ 2h: Dp xpc Z¼f ða :y Þ c c p l l l lc ð1Þ Subject to: COIp xpc ≤ COIp ; xp ;c ; l ylc ≤ l 0 yl0 c0 ∀p≠p0 and c < c0 ∀l≠l 0 and c < c0 ð2Þ ð3Þ Int J Adv Manuf Technol (2014) 70:1615–1624 " Max X t # I tp f p xpc ≤ p X xpc ¼ 1 X ðal ylc Þ ∀c; t 1617 ð4Þ p ∀p; ð5Þ c X ylc ≤ 1 ∀l; ð6Þ c xpc ; ylc ∈f1; 0g ∀p; c; l: ð7Þ where: c l p t al dl f h – (c = 1, 2, 3, . . . , C =P) for classes – (l = 1, 2, 3, . . . , L) for storage locations – (p = 1, 2, 3, . . . , P ) for products/items – (t = 1, 2, 3, . . . , T) for time periods. – Footprint area of location l (in square meters) – Distance of location l from the input/output point (in meters) – Space cost for the planning horizon (in US dollars per square meter) – Order picking cost per unit load (in US dollars per meter) Decision variables: xpc ¼ 1; if product p is assigned to class c; xpc ¼ 0; otherwise : ylc ¼ 1; if location l is assigned to class c; ylc ¼ 0; otherwise : The objective function (1) minimizes the sum of storage space cost and order picking cost over the planning horizon. Constraints (2) and (3) together ensure that if products that have a lower COI are assigned to class c and products with a higher COI are assigned to class c′, then c is located nearer to the I/O point than c ′. Constraint (4) ensures that there is adequate storage space to hold the items in class c in each planning period t. Constraint (5) ensures that each product is assigned to one and only one class. Constraint (6) ensures that is assigned to not more than one class. Constraint (7) imposes binary restrictions on the decision variables. Based on class formation and an allocation model, the three criteria needed to determine the best alternative for warehouse storage location assignment may be reached, namely: the required space, the order picking distance and the total operation cost (include just the required space cost and the order picking cost). Other criteria will be mentioned in Section 3. Moreover, due its features, the Promethee method was chosen to evaluate the alternatives by each criterion in order to reach the best alternative as per the DM’s preferences. The procedures of this method, in the order to be followed, are given below. 2.1 Promethee method The Promethee method is a relatively simple ranking method in conception and application compared with other methods for multicriteria analysis. It is well adapted for problems where a finite number of alternatives are to be ranked considering several, sometimes conflicting, criteria [23]. The method has two phases: the construction of an outranking relation (aggregating information about the alternatives and about the criteria) and the exploitation of that relation for decision aid [24]. The additional information requested to run Promethee is particularly clear and easy for both the analysts and the DMs to understand. It consists of [25] information between the criteria and information within each criterion. This approach does not provide specific guidelines for determining these weights but assumes that the DM is able to weigh the criteria appropriately, at least when the number of criteria is not too large [26]. The weights must be normalized, given j the number of criteria, where j = {1, 2, 3,…,k} and w j is the weight of criterion j, since ∑ kj = 1w j =1. The ranking of alternatives is carried out by pairwise comparison of the alternatives for each criterion. The comparison is measured using a predefined preference function [27]. The preference is expressed by a number in the interval [0, 1] (0 for no preference or indifference to 1 for strict preference). The function relating the difference in performance to preference is called the generalized criterion and is determined by the DM [23]. In order to facilitate the selection of a specific preference function, Vincke and Brans in 1985 proposed six basic types: (1) usual criterion, (2) U-shape criterion, (3) V-shape criterion, (4) level criterion, (5) V-shape with indifference criterion, and (6) Gaussian criterion. So, for each criterion, the preference function P j (.) translates the difference between the evaluations obtained by two alternatives [28]. Since g j (a) is the preference function of alternative a on criterion j, and g j (b) the preference function of alternative b also on criterion j, the difference between these is d j (a,b)=g j (a)−g j (b). The larger the deviation, the larger the is preference. For small deviations, the DM will allocate a low preference to the best alternative and even possibly indifference if he/she considers that this deviation is negligible. In some cases, it is interesting to amplify the notion of indifference for the Promethee method (see [29]). In each case (Table 1 adapted from [25]), 0, 1, or 2 parameters have to be defined so that their significance is clear [30]: “q” is a threshold or indifference; “p” is a threshold of strict preference; and “s” is an intermediate value between “q” and “p”. The indifference threshold is the largest deviation which the DM considers as negligible, while the preference threshold is the smallest deviation which is considered as sufficient to generate a full preference. The identification of a generalized 1618 Int J Adv Manuf Technol (2014) 70:1615–1624 Table 1 Types of generalized criteria (preference function) criterion is then limited to the selection of the appropriate parameters. Thereafter, a multicriteria preference index is formed for each pair of alternatives as a weighted average of the corresponding preferences computed in step (1) for each criterion. The index π(a, b) (in the interval [0, 1]) expresses the preference of alternative a over b considering all criteria. Thus, the alternatives can be ranked according to [23]: & & The sum of indices π(a, i) indicating the preference of alternative a over all the others. This is deemed as the “leaving flow” Φ +(a) and shows how “good” alternative a is. The sum of indices π(i, a) indicating the preference of all other alternatives compared to a. This is deemed as the “entering flow” Φ −(a) and shows how “inferior” alternative a is. Promethee I partial ranking provides a ranking of alternatives. In some cases, this ranking may be incomplete. This means that some alternatives cannot be compared and, therefore, cannot be included in a complete ranking. However, Promethee II provides a complete ranking of the alternatives from best to worst [26]. 3 Problem formulation In addition to the required space, distance traveled, and storage cost mentioned in papers arising from the literature review, another important criterion in warehouse operations is the time required for picking operations. Thus, for each scenario simulated, what should be determined are the following: (a) the average time taken to go to each product p; (b) the average time spent meeting a client’s orders; (c) the average time taken to meet the orders of a group of clients which has asked for the same product p. Items (b) and (c) are important since the orders for product p do not arrive at the same time and, consequently, they are not met at the same time. Thus, the average time taken to go to each product p (Time_prod) is the ratio between the sum of the time needed to pick all products only once and the quantity of different types of products (P ) there are in the warehouse, as per Eq. (8). This time is expressed in seconds. Time prod ¼ P X distp v p¼1 P ð8Þ where: distp v – Distance until product p (in meters) – Velocity of pick (in meters per second) To continue formulating the problem, there is a need to define the time spent serving all clients in the same group (Time_C p ), i.e., all clients that demand the product p, is the sum of the time needed to pick all orders for product p, as per Eq. (9). Time C p ¼ k p distp v ð9Þ where: Dp C kp ¼ CAPp CAPp ¼ CAPnominal fd kp – Number of trips needed to pick all orders for product p ð10Þ ð11Þ Int J Adv Manuf Technol (2014) 70:1615–1624 Dp C CAPp 1619 – Total demand for product p – Total number of clients who ordered product p – Total capacity of the equipment used to pick the products. The CAPp is the ratio between the nominal capacity of the equipment and the density of product p. The equipment has a specific capacity to pick each product. The capacity should be rounded towards −∞ and the number of trips (k p ) should be rounded towards +∞. Therefore, the average time spent serving each client (Time_m) is the ratio between the sum of the time taken to serve each client group and the total number of clients (N), as in Eq. (12). This time is also expressed in seconds. p X Time m ¼ Time C p p¼1 N ð12Þ Finally, the average time to meet the orders of a group of clients (Time_G) is the ratio between the sums of the time taken to serve each group of client by the total number of distinct products in warehouse, which is expressed by exp. (13). P X Time G ¼ Time C p p¼1 P ð13Þ Figure 1 shows how the model proposed finds the best alternative, using the Promethee method. Therefore, based on the characteristics in each warehouse, the alternatives by each criterion may be evaluated and the best solution for optimizing the warehouse operations by the proposal methodology may be reached. 3.1 Description of the warehouse simulated Initially, this description assumed that the picking operations are performed under a single command and all items (products) are stored and transported in identical supports. Each storage location is uniformly used, and the points awarded are distributed homogeneously in the space allotted for the class. This assumption implies that the geometric center of the class is the same as the load center. It is assumed that the inventory decision is made independently of the storage decision and all the time required in the process of storage, except the time for picking, are independent of storage allocation. The warehouse simulated is rectangular. It is divided into cells of 1.0×1.0 m, as per Fig. 2 which is adapted from [31]. It was divided into four periods, each period representing a week, totaling a month. The layout used in the warehouse considers five rows (x direction in Fig. 2). The use of space in direction y was not restricted, which does not represent a significant point in the simulation. It was found that all products are stacked to a maximum of ten levels. Storage costs are considered: US$ 1.00/m2 for used space and US$ 0.0025/m for the distance to pick all products. Other storage costs were not considered. The scenario was generated randomly. In these, the quantities requested were in the range of between 100 and 1,200 units weekly; the demand used in the calculations is the average of the four periods; and the number of clients is between 1 and 30. The inventory, i.e., the quantity of items in stock for the period, is the ratio of the average demand for the product divided by a random variable, which simulates the variations of units stored in each period. The values of the quantities demanded, the numbers of clients, and units in the inventory were rounded towards +∞, because decimal values make no sense. The space required is related to the inventory. Thus, the space used by each type of product is the multiplication between the number of units stored by their density and divided by ten (total number of levels on which each product can be stacked). Therefore, it can be seen that there is variation of the space required in the four periods. In this context, the density of the products is a value between 0 and 1 m2/un (randomly generated) and it must always be different from zero. In the simulated warehouse, there are eight distinct products and it was established that all products considered in the first period are the same as those in the subsequent periods. The values presented in the course of the simulations refer to the average, i.e., these values represented only one period (a week). The distances to picking represent the path between the point of origin (I/O) and the place that the product is stored (the round trip). Therefore, the picker always starts the process at I/O. The average speed of picking is considered 1 m/s, so the data obtained represent both the distance (in meters) and the time spent (in seconds). The criteria selected to determine the best alternative for the formation of class and locations in the warehouse were: Cr1 Cr2 Cr3 Cr4 – Space: this is the total space required to locate products, respecting the areas reserved for each class. The value is given in square meters – Picking: this is the total distance traveled when the pick is issued from a single command, or it is caught by a unit of production time to meet clients’ orders. Its value is related to fetching the product and returning it to the origin (I/O); it is given in meters walked – Total cost of picking a single command: this is the sum of the costs incurred on the space required and the distance traveled in a single command. Values are expressed in US dollars – Time to products: this is the average time to go to the product and return to the source (I/O), i.e., the round trip. Values are in seconds and also represent the distance (in meters) 1620 Int J Adv Manuf Technol (2014) 70:1615–1624 Fig. 1 Proposed model using the Promethee method Cr5 Cr6 – The average time it takes to serve a client: the average time required to meet the orders of each client in seconds – The average time it takes to serve a group: the average time required to meet the orders of a group of clients in seconds, provided the group is formed by clients who request the same type of product. Based on the search of the literature, it was realized that the choice of warehouse design, i.e., determining the type of class, formation, and location of the warehouse, depends exclusively on the scenario in which the warehouse is inserted; the costs involving its activities; and, especially, on the objectives of its management. For example, if the company’s goal is to offer a client service that takes less time, the importance of this criterion will be greater and will tend to alternatives that are more efficient at this. Another important aspect to be considered is whether the company gives priority to reducing the space used, or minimizing the distance traveled, or both. This point is influenced by the costs and the level of service that the company intends to offer. Based on these statements, this paper reports on three simulations about the distinct preferences of the DM. In the first case, the DM prefers an alternative that offers the shortest total distance traveled and needs to offer, as far as possible, the lowest average time to serve each client. This type of storage generally has a high turnover of products and therefore the total space required does not have as much impact as the picking operations do. In the second case, the manager is interested in an efficient design in terms of less time; so much attention is paid to each client and client group in terms of the average time of each product. This type of company aims to raise the level of service offered to clients and/or client group as much as possible. In the third and last case, the DM wants to lessen the space used, because he uses a rented area and also expects to minimize its total cost with storage, since clients are not as sensitive to service time and the warehouse has a low turnover Int J Adv Manuf Technol (2014) 70:1615–1624 1621 Fig. 2 Illustration of the layout of the storage area in the warehouse of products. The weights assigned to each criterion, for the three cases, are presented in Table 2. The weights presented in Table 2 are normalized, i.e., their sum is equal to 1. The data on the warehouse is the same for all three cases, since the goal is to compare the results provided by the Promethee method when the DMs change their preferences. 4 Results and discussion In total, 128 alternatives were generated by a class formation and allocation model. The dominance between alternatives when comparing the results in all criteria selected for the problem was evaluated. Only six alternatives are nonTable 2 The weights of the criteria according to the DMs’ preferences Criteria Cr1 – Space Cr2 – Picking Cr3 – Total cost Cr4 – Time to products Cr5 – The average time it takes to serve a client Cr6 – The average time it takes to serve a group Cases dominated. The non-dominated alternatives are a subset of actions which are better in at least one criterion when compared to another alternative and there is no alternative better than them in all criteria. Table 3 shows these alternatives. The simulation made here is just illustrative. The complexity and number of non-dominated alternatives will increase as the number of alternatives increases. For the purposes of Promethee method, the usual criterion for comparing alternatives was used. Thus, it is not necessary to change the range of values of the alternatives on different criteria, since the response will be only 0 or 1 (worse/same or better). The ranking results from Promethee I and II for the three simulated cases of weights can be seen in Table 4. In the first and third case, the most efficient alternative according to the weights adopted by the DM is a 2. In the second case of weight, the best alternative is a 4. In the first scenario, the DM’s objective was to get the lowest distance in picking under a single command and the lowest average time taken to serve each client. In this case, the method enables the DM to obtain the best alternative, since it offers the third alternative of shorter distance traveled and lowest average time taken of both, serve a client and a client group. Note that alternative a 2 is the best alternative considering the two criteria simultaneously (Cr2—picking and Cr5—the average time it takes to serve a client). There is incomparability between alternatives a 5 and a 6 in the ranking of Promethee I. However, these alternatives are ranked last. In the second case, the DM considers as priorities the shortest average time to reach each product and lowest average time taken to serve both, the client and the client group. The result from the Promethee method was a 4 as the best alternative. This shows the second shortest average time taken to serve a client (Cr5) and a group of client (Cr6), and the shortest average time to picking each product (Cr4). The ranking of the Promethee I is the same as Promethee II, because there were no incomparability or reversals of order in this simulated case. In the third case, the DM prioritizes using less space and incurring the lowest total cost for storage. Based on these points, the method gives a 2 as the most efficient alternative. It has the lowest total cost (Cr3) and the second smallest space Table 3 Subset of the non-dominated alternatives 1 2 3 Alternatives Cr1 Cr2 Cr3 Cr4 Cr5 Cr6 0.05 0.35 0.10 0.05 0.35 0.10 0.05 0.10 0.10 0.25 0.25 0.25 0.45 0.05 0.35 0.05 0.05 0.05 a1 a2 a3 a4 a5 a6 189.08 163.63 170.29 170.29 162.57 188.08 125.270 128.430 129.330 132.330 141.510 127.650 502.25 484.70 493.61 501.11 516.35 508.20 30.38 31.45 31.30 29.33 30.03 34.46 181.85 173.95 174.96 174.56 181.06 188.53 2773.26 2652.71 2668.07 2662.09 2761.14 2875.11 1622 Int J Adv Manuf Technol (2014) 70:1615–1624 Table 4 Ranking: Promethee I and II CASE 1 Alternatives a2 a4 a3 a1 a5 a6 CASE 2 Alternatives a4 a2 a3 a5 a1 a6 CASE 3 Alternatives a2 a3 a5 a4 a1 a6 Promethee I Φ +(a) 4.05 2.80 2.65 Table 5 Product classes formed by each alternative Φ −(a) 0.95 2.15 2.30 Promethee II Φ(a) 3.10 0.65 0.35 2.45 3.55 3.55 0.10 −2.10 −2.10 Φ −(a) 1.20 1.25 2.25 2.65 3.05 4.55 Promethee II Φ(a) 2.55 2.50 0.45 −0.30 −1.10 −4.10 2.55 1.45 1.45 Promethee I Φ +(a) 3.75 3.75 2.70 2.35 1.95 0.45 Promethee I Φ +(a) 4.25 2.80 3.00 2.65 1.20 0.65 Φ (a) 0.75 1.75 2.00 Promethee II Φ(a) 3.50 1.05 1.00 1.90 3.80 4.35 0.75 −2.60 −3.70 − required (Cr1). There is incomparability between alternatives a 3 and a 5 in the ranking of Promethee I. To sum up, in the real world, the process of order picking, in general, is complex, and it has a large number of different products and customers order. However, it is important to realize that the benefits of the proposed model are tangible in large warehouses, although the example used is a simple one. The reality of a warehouse is not static. Using the proposed model, it is possible to discover all the possible nondominated allocations for the products in a warehouse. The manager can determine, by ranking the alternatives, which alternative of the allocation is the most efficient and appropriate for the current time. In other words, when the objectives of the warehouse change, the manager has previous knowledge of the most efficient allocation for each case. The final configuration of the warehouse for all non-dominated alternatives is given in Table 5, for the example shown. The model’s gains may be significant when considering a warehouse which handles a large volume of materials. The calculations must be redone whenever Alternative a 1 Class and rank 1 2 3 4 5 Alternative a 2 Class and rank 1 2 3 – – Alternative a 3 Class and rank 1 2 3 4 – Alternative a 4 Class and rank 1 2 3 – Alternative a 5 Class and rank 1 2 – – Alternative a 6 Class and rank 1 2 3 4 Products P8 P7 P6 P5 P4 P1 P3 P2 Products P8 P7 P6 P5 P4 P1 P3 P2 – – Products P8 P7 P6 P5 P4 P1 P3 P2 – Products P8 P7 P6 P5 P4 P1 P3 P2 Products P8 P7 P6 P5 P4 P1 P3 P2 Products P8 P7 P6 P5 P4 P1 P3 P2 a change in the warehouse occurs, such as: the characteristics and quantity of different products, demand, number of clients, and available space for storage change. This is unnecessary only when the priorities of the managing change, i.e., it is not needed only when the importance of each criterion changes. Moreover, other multicriteria methods can be used to rank alternatives, but the comparison between them is not relevant in this paper. Int J Adv Manuf Technol (2014) 70:1615–1624 5 Conclusion This study made use of a methodology using a mathematical model and a simulation so that processes in different scenarios could be evaluated. The model presented is likely to be used in real cases, after adapting it to the characteristics of a given warehouse and will be helpful in the search for efficient storage at the lowest overall cost to the company. The rankings resulting from Promethee I and II were equal. This is due to the characteristics of the alternatives evaluated, where the very good alternatives, i.e., the greatest positive flow, were the alternatives with the least negative flow for that particular simulation of weights. Thus, the net flow of Promethee II resulted in a complete ranking identical to the partial ranking of Promethee I. It can be concluded that the weighting given to the criteria can completely alter the configuration chosen for the warehouse. However that may be, it can be stated that modeling a warehouse is not static and it must be reviewed when the objectives of the warehouse change. As seen, this revision is facilitated by previous knowledge of non-dominated alternatives. This is possible by applying the class formation and allocation model. In addition, the Promethee method was effective in determining the best alternative location of the items stored. One advantage of this method is the ranking of alternatives. Thus the DM can make his/her activities flexible, through the ranking, between one alternative and another among the best, when it is necessary, i.e., they consider which criterion can be left aside so full benefit may be derived from another one. 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