Person: ÇÖMEZ DOLGAN, Nagihan
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Nagihan
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ÇÖMEZ DOLGAN
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ArticlePublication Metadata only Capacitated strategic assortment planning under explicit demand substitution(Elsevier, 2021-11-01) Çömez-Dolgan, Nagihan; Fescioglu-Unver, N.; Cephe, E.; Şen, A.; Management Information Systems; ÇÖMEZ DOLGAN, NagihanBuyers have easier access to a variety of products with the rise of multi-channel distribution strategies and the increase in new product introductions. On the other hand, firms experience greater pressure in offering the correct product variety given that the manufacturing infrastructure often imposes physical and financial constraints in attaining variety. This study examines a firm's optimal assortment planning problem under an exogenous demand model, where each customer has a predetermined preference for each product from a potential set. Proportional demand substitutions are allowed from out-of-assortment products to those available. We show that the problem is NP-complete. We also show that an optimal assortment is composed of some number of the highest margin products, if one product having a higher margin than another implies that the former product has a lower demand rate than the latter. The firm's assortment capacity is fully utilized at the optimum if the customers’ substitution ratio does not exceed a particular threshold. We also introduce several approximate assortment policies that can be easily implemented, and test these policies through extensive numerical analyses. The results reveal that some of the policies can provide less than a 1% profit gap with an optimal solution for a 20-product set. The policy's performance highly depends on the firm's assortment capacity-to-product set size ratio. Moreover, we provide performance bounds for two of these well-performing approximate policies.ArticlePublication Metadata only A buyer-vendor system with untimely delivery costs: Traditional coordination vs. VMI with consignment stock(Elsevier, 2021-04) Çömez-Dolgan, Nagihan; Moussawi-Haidar, L.; Jaber, M. Y.; Management Information Systems; ÇÖMEZ DOLGAN, NagihanThis paper investigates the impact of coordinating a two-level supply chain that consists of a single-vendor and a single-buyer in the presence of untimely delivery costs. Specifically, early and late deliveries outside an agreed-upon delivery window are penalized. We investigate the replenishment policies of the vendor and the buyer with and without coordination. We show that untimely deliveries increase the replenishment cycle of the buyer, whether it coordinates with the vendor or not. More importantly, under untimely deliveries, coordination has a noticeable impact on aligning the decisions of both players, which is shown to be much more valuable in decreasing total costs. Implementing a coordinated solution becomes beneficial when the vendor's penalty and holding costs are high, the delivery window is narrow, and the buyer's holding and ordering costs are low. Next, we compare the traditional replenishment coordination mechanism with a vendor-managed-inventory (VMI) mechanism with consignment stock (CS) under untimely deliveries. We compare two coordination mechanisms, VMI-CS and traditional, and show that VMI-CS outperforms the other when the delivery times are random, but not so when the conditions are deterministic.ArticlePublication Metadata only Capacitated assortment planning of a multi-location system under transshipments(Elsevier, 2022-09) Çömez-Dolgan, Nagihan; Moussawi-Haidar, L.; Jaber, M. Y.; Cephe, E.; Management Information Systems; ÇÖMEZ DOLGAN, NagihanWe analyze a joint assortment optimization problem for multiple locations of a firm, where each of those locations has an assortment capacity. When one location does not keep a product in its assortment, it transships the requested product from another location when needed. If this is not possible, then demand may be fulfilled by a substitutable (second choice) product from the current location or another demand location through transshipment. We show that the resulting assortment planning problem is NP-complete. We provide structural properties of the optimal assortments that would simplify and speed up the search for the optimal solution. We specifically obtain the optimal product assortment in polynomial time when no substitutions are present. When both transshipments and substitutions are considered, the products adopted by all locations form a common assortment that includes the most popular products among customers, i.e., a popular set, which may not be valid for the individual assortment of each location. We derive an upper limit on customers' substitution probability for each location, such that at optimality, the location's capacity is utilized in full when the substitution probability is below this threshold value. Moreover, we show that when all locations have the same assortment capacity, utilizing theirs to the maximum reduces costly transshipment opportunities. Extensive numerical analyses are also conducted that illustrate the sensitivity of the optimal assortments to system characteristics and the benefits of allowing transshipment and demand substitution in multi-location assortment planning.ArticlePublication Metadata only Multi-plant manufacturing assortment planning in the presence of transshipments(Elsevier, 2023-11-01) Çömez-Dolgan, Nagihan; Dağ, H.; Fescioglu-Unver, N.; Şen, A.; Management Information Systems; ÇÖMEZ DOLGAN, NagihanIn this study, we consider the assortment planning problem of a manufacturing firm with multiple plants. Making a plant capable of producing a product is costly, therefore the firm cannot manufacture every product in every plant. In case a customer's order in a particular region is not available in the closest plant, another plant can ship the product using transshipment, but at an extra transportation cost. If a demanded product is not produced in any plant, substitution from first choice to a second choice is also considered, which can be either satisfied by the closest plant, or by transshipment. The problem is to jointly determine assortments in all plants such that total profit after assortment and transshipment costs is maximized. The resulting problem is complex as transshipments and substitutions are intertwined to affect assortment decisions. We show that the optimal assortments are nested, i.e., the assortment of a plant with a smaller market share is a subset of the assortment of a plant with a larger share. The common assortment of all locations is shown to be in the popular set (i.e., no leapfrogging in product popularities), and a sufficient condition on substitution rate is derived for each individual assortment to be in the popular set. We conduct an extensive numerical study to understand the effects of allowing transshipments on resulting assortments. Moreover, we introduce approximate assortment planning algorithms that benefit from the derived structural properties, which are shown to generate near-optimal assortments in a broad range of instances tested.