Abstract Suppliers often impose a minimum order quantity (MOQ) to ensure that production runs and... more Abstract Suppliers often impose a minimum order quantity (MOQ) to ensure that production runs and shipping quantities of each specific item are economically viable. Additional economies of scale may arise as various items share high joint costs (e.g. the cost of an overseas container shipment) and require coordination of their replenishment policies. The buyer needs to find the joint ordering interval and replenishment policies for each of the individual items to minimize the system-wide ordering and inventory costs while satisfying the quantity restrictions. We focus on the case of constant demand without backlogging. Given a fixed joint reorder interval, we characterize the optimal inventory ordering strategy for each item, which we refer to as Multi-Replenishment (MR) inventory ordering policy, and derive a closed-form expression for the optimal average inventory costs per unit of time. In contrast to the previous literature, the MR inventory ordering policy does not follow the zero-inventory-ordering (ZIO) rule and allows for orders of different sizes over time to optimally accommodate the MOQ restriction. A numerical approach is used to determine the optimal joint reorder interval, and is later extended to account for the presence of empty replenishments. An extensive computational study shows 1) the total inventory and setup cost reduction associated with the MR inventory policies and empty replenishments; 2) the impact of various parameters on cost and policy performance; and 3) the loss associated with discretizing time at different levels of granularity (weeks, days, hours, minutes, seconds) to accommodate restrictions in the timing of item deliveries.
Waiting is common in appointment-based outpatient care: patients experience delays before seeing ... more Waiting is common in appointment-based outpatient care: patients experience delays before seeing a nurse and then in the examination room before seeing the doctor. Given that service times are uncertain, the challenge for outpatient practices is to control the spacing between successive appointments to minimize waiting time while ensuring the doctor is not idle for too long. Whereas scheduling in single-doctor practices is well studied, team practice, in which one of two nurses can flexibly see the patient before the patient consults with one of two assigned doctors, is not. Optimal scheduling in a team practice is far more challenging from a computational viewpoint because the order in which patient appointments are scheduled need not be the order in which they see their doctors (i.e., patients may be allowed to crossover). In this article the authors propose a model that can solve realistic instances of this problem optimally. On the basis of analyses, the results show that (1) em...
The cost efficient management of spare parts for low-volume high-tech equipment is inherently dif... more The cost efficient management of spare parts for low-volume high-tech equipment is inherently difficult. In this on-going study, we seek to improve the OEM’s spare parts inventory management by incorporating the condition information from a large number of distributed working units in the field. For that purpose, the condition information relayed by sensors is put in context with usage parameters, preventive replacement policies, customer plans, and current economic indicators to create an aggregate forecast and inventory ordering policy. This requires a synthesis of the state of the art knowledge from multiple research streams. In this paper, we outline a simulation environment of the maintenance management of a jet engine program over its life cycle, and provide preliminary results highlighting several modules for future research to improve the performance of spare part inventory policies and assess the value of health monitoring.
ABSTRACT The solution for the Liouville equation corresponding to the problem of a two-level atom... more ABSTRACT The solution for the Liouville equation corresponding to the problem of a two-level atom in a sinusoidal external field is presented.
We investigate the effect that commercial returns have in the coordination of the supply chain an... more We investigate the effect that commercial returns have in the coordination of the supply chain and in the optimal decision variables in a two-echelon supply chain with stochastic demand in a single period. Logistics costs related to commercial returns may be incurred at both retailer and manufacturer sites. In many practical settings, the manufacturer faces the larger share. In others, such as in the apparel industry, returns are handled exclusively at the retail level and the manufacturer typically gives the retailer a certain amount of return allowance credit. We study the coordination of the supply chain in the presence of commercial returns under these two business settings.
We consider an economic lot-sizing problem with a special class of piecewise linear ordering cost... more We consider an economic lot-sizing problem with a special class of piecewise linear ordering costs, which we refer to as the class of modified all-unit discount cost functions. Such an ordering cost function represents transportation costs charged by many less-than ...
Appointments in primary care are of two types: 1) prescheduled appointments, which are booked in ... more Appointments in primary care are of two types: 1) prescheduled appointments, which are booked in advance of a given workday; and 2) same-day appointments, which are booked as calls come during the workday. The challenge for practices is to provide preferred time slots for prescheduled appointments and yet see as many same-day patients as possible during regular work hours. It is also important, to the extent possible, to match same-day patients with their own providers (so as to maximize continuity of care). In this paper, we present a mathematical framework (a stochastic dynamic program) for same-day patient allocation in multi-physician practices in which calls for same-day appointments come in dynamically over a workday. Allocation decisions have to be made in the presence of prescheduled appointments and without complete demand information. The objective is to maximize a weighted measure that includes the number of same-day patients seen during regular work hours as well as the continuity provided to these patients. Our experimental design is motivated by empirical data we collected at a 3-provider family medicine practice in Massachusetts. Our results show that the location of prescheduled appointments - i.e. where in the day these appointments are booked - has a significant impact on the number of same-day patients a practice can see during regular work hours, as well as the continuity the practice is able to provide. We find that a 2-Blocks policy which books prescheduled appointments in two clusters - early morning and early afternoon - works very well. We also provide a simple, easily implementable policy for schedulers to assign incoming same-day requests to appointment slots. Our results show that this policy provides near-optimal same-day assignments in a variety of settings.
We consider a multistage inventory system composed of a single warehouse that receives a single p... more We consider a multistage inventory system composed of a single warehouse that receives a single product from a single supplier and replenishes the inventory of n retailers through direct shipments. Fixed costs are incurred for each truck dispatched and all trucks have the same capacity limit. Costs are stationary, or more generally monotone as in . Demands for the n retailers over a planning horizon of T periods are given. The objective is to find the shipment quantities over the planning horizon to satisfy all demands at minimum system-wide inventory and transportation costs without backlogging. Using the structural properties of optimal solutions, we develop (1) an O(T 2 ) algorithm for the single-stage dynamic lot sizing problem; (2) an O(T 3 ) algorithm for the case of a one-warehouse single-retailer system; and (3) a nested shortest-path algorithm for the one-warehouse multi-retailer problem that runs in polynomial time for a given number of retailers. To overcome the computational burden when the number of retailers is large, we propose aggregated and disaggregated Lagrangian decomposition methods that make use of the structural properties and the efficient single-stage algorithm. Computational experiments show the effectiveness of these algorithms and the gains associated with coordinated versus decentralized systems. Finally, we show that the decentralized solution is asymptotically optimal.
... A classical result, due to Kawaguchi and Kyan (1986), is that the WSPT heuristic, in which jo... more ... A classical result, due to Kawaguchi and Kyan (1986), is that the WSPT heuristic, in which jobs are assigned in a nondecreasing order of their ratios pi/wi ... if all the ratios, pi/wi, are equal, that is, wi = yp for all i, the WCTP is known to be NP-complete; see Chandra and Wong (1975 ...
The aim of this paper is to understand the impact of consumer returns on the de- cision making pr... more The aim of this paper is to understand the impact of consumer returns on the de- cision making process of a two-echelon supply chain. Currently, in practice, consumer returns are largely ignored when managers make decisions regarding order quantities and prices, as evidenced in the literature and in our interaction with industry. Con- sumer returns, however, have been reported
Http Dx Doi Org 10 1080 19488300 2013 858379, Oct 1, 2013
ABSTRACT Scheduling in primary care is challenging because of the diversity of patient cases (acu... more ABSTRACT Scheduling in primary care is challenging because of the diversity of patient cases (acute versus chronic), mix of appointments (pre-scheduled versus same-day), and uncertain time spent with providers and non-provider staff (nurses/medical assistants). In this paper, we present an empirically driven stochastic integer programming model that schedules and sequences patient appointments during a work day session. The objective is to minimize a weighted measure of provider idle time and patient wait time. Key model features include: an empirically based classification scheme to accommodate different chronic and acute conditions seen in a primary care practice; adequate coordination of patient time with a nurse and a provider; and strategies for introducing slack in the schedule to counter the effects of variability in service time with providers and nurses. In our computational experiments we characterize, for each patient type in our classification, where empty slots should be positioned in the schedule to reduce waiting time. Our results also demonstrate that the optimal start times for a variety of patient-centered heuristic sequences consistently follow a pattern that results in easy to implement guidelines. Moreover, these heuristic sequences and appointment times perform significantly better than the practice's schedule. Finally, we also compare schedules suggested by our two-service-stage model (nurse and provider) with those that only consider the provider stage and find that the performance of the provider-only model is 21% worse than that of the two-service-stage model.
We study a warehouse management problem where the schedule of incoming supplies and customer orde... more We study a warehouse management problem where the schedule of incoming supplies and customer orders for a wide variety of products is known over a number of periods. In addition to storage at the warehouse, product can be kept in the shipping trailers (rolling inventory) at the warehouse yard, avoiding material handling costs but incurring trailer handling and opportunity
M Som Manufacturing Service Operations Management, 2006
1 Impact of Partial Manufacturing Flexibility on Production Variability Ana Muriel Anand Somasund... more 1 Impact of Partial Manufacturing Flexibility on Production Variability Ana Muriel Anand Somasundaram Yongmei Zhang University of Massachusetts Amherst Abstract ... Let X(D)= { } Aji Dx ij ∈ ∀ ),( be an optimal solution to this problem when demand is D=(d1,d2, dN). ...
Http Dx Doi Org 10 1080 07408170600634525, Feb 23, 2007
We study a warehouse management problem in which the schedule of incoming supplies and customer o... more We study a warehouse management problem in which the schedule of incoming supplies and customer orders for a wide variety of products is known over a number of periods. In addition to storage at the warehouse, products can be kept in the shipping trailers (rolling inventory) parked in the warehouse yard, avoiding material handling costs but incurring trailer handling and
Abstract Suppliers often impose a minimum order quantity (MOQ) to ensure that production runs and... more Abstract Suppliers often impose a minimum order quantity (MOQ) to ensure that production runs and shipping quantities of each specific item are economically viable. Additional economies of scale may arise as various items share high joint costs (e.g. the cost of an overseas container shipment) and require coordination of their replenishment policies. The buyer needs to find the joint ordering interval and replenishment policies for each of the individual items to minimize the system-wide ordering and inventory costs while satisfying the quantity restrictions. We focus on the case of constant demand without backlogging. Given a fixed joint reorder interval, we characterize the optimal inventory ordering strategy for each item, which we refer to as Multi-Replenishment (MR) inventory ordering policy, and derive a closed-form expression for the optimal average inventory costs per unit of time. In contrast to the previous literature, the MR inventory ordering policy does not follow the zero-inventory-ordering (ZIO) rule and allows for orders of different sizes over time to optimally accommodate the MOQ restriction. A numerical approach is used to determine the optimal joint reorder interval, and is later extended to account for the presence of empty replenishments. An extensive computational study shows 1) the total inventory and setup cost reduction associated with the MR inventory policies and empty replenishments; 2) the impact of various parameters on cost and policy performance; and 3) the loss associated with discretizing time at different levels of granularity (weeks, days, hours, minutes, seconds) to accommodate restrictions in the timing of item deliveries.
Waiting is common in appointment-based outpatient care: patients experience delays before seeing ... more Waiting is common in appointment-based outpatient care: patients experience delays before seeing a nurse and then in the examination room before seeing the doctor. Given that service times are uncertain, the challenge for outpatient practices is to control the spacing between successive appointments to minimize waiting time while ensuring the doctor is not idle for too long. Whereas scheduling in single-doctor practices is well studied, team practice, in which one of two nurses can flexibly see the patient before the patient consults with one of two assigned doctors, is not. Optimal scheduling in a team practice is far more challenging from a computational viewpoint because the order in which patient appointments are scheduled need not be the order in which they see their doctors (i.e., patients may be allowed to crossover). In this article the authors propose a model that can solve realistic instances of this problem optimally. On the basis of analyses, the results show that (1) em...
The cost efficient management of spare parts for low-volume high-tech equipment is inherently dif... more The cost efficient management of spare parts for low-volume high-tech equipment is inherently difficult. In this on-going study, we seek to improve the OEM’s spare parts inventory management by incorporating the condition information from a large number of distributed working units in the field. For that purpose, the condition information relayed by sensors is put in context with usage parameters, preventive replacement policies, customer plans, and current economic indicators to create an aggregate forecast and inventory ordering policy. This requires a synthesis of the state of the art knowledge from multiple research streams. In this paper, we outline a simulation environment of the maintenance management of a jet engine program over its life cycle, and provide preliminary results highlighting several modules for future research to improve the performance of spare part inventory policies and assess the value of health monitoring.
ABSTRACT The solution for the Liouville equation corresponding to the problem of a two-level atom... more ABSTRACT The solution for the Liouville equation corresponding to the problem of a two-level atom in a sinusoidal external field is presented.
We investigate the effect that commercial returns have in the coordination of the supply chain an... more We investigate the effect that commercial returns have in the coordination of the supply chain and in the optimal decision variables in a two-echelon supply chain with stochastic demand in a single period. Logistics costs related to commercial returns may be incurred at both retailer and manufacturer sites. In many practical settings, the manufacturer faces the larger share. In others, such as in the apparel industry, returns are handled exclusively at the retail level and the manufacturer typically gives the retailer a certain amount of return allowance credit. We study the coordination of the supply chain in the presence of commercial returns under these two business settings.
We consider an economic lot-sizing problem with a special class of piecewise linear ordering cost... more We consider an economic lot-sizing problem with a special class of piecewise linear ordering costs, which we refer to as the class of modified all-unit discount cost functions. Such an ordering cost function represents transportation costs charged by many less-than ...
Appointments in primary care are of two types: 1) prescheduled appointments, which are booked in ... more Appointments in primary care are of two types: 1) prescheduled appointments, which are booked in advance of a given workday; and 2) same-day appointments, which are booked as calls come during the workday. The challenge for practices is to provide preferred time slots for prescheduled appointments and yet see as many same-day patients as possible during regular work hours. It is also important, to the extent possible, to match same-day patients with their own providers (so as to maximize continuity of care). In this paper, we present a mathematical framework (a stochastic dynamic program) for same-day patient allocation in multi-physician practices in which calls for same-day appointments come in dynamically over a workday. Allocation decisions have to be made in the presence of prescheduled appointments and without complete demand information. The objective is to maximize a weighted measure that includes the number of same-day patients seen during regular work hours as well as the continuity provided to these patients. Our experimental design is motivated by empirical data we collected at a 3-provider family medicine practice in Massachusetts. Our results show that the location of prescheduled appointments - i.e. where in the day these appointments are booked - has a significant impact on the number of same-day patients a practice can see during regular work hours, as well as the continuity the practice is able to provide. We find that a 2-Blocks policy which books prescheduled appointments in two clusters - early morning and early afternoon - works very well. We also provide a simple, easily implementable policy for schedulers to assign incoming same-day requests to appointment slots. Our results show that this policy provides near-optimal same-day assignments in a variety of settings.
We consider a multistage inventory system composed of a single warehouse that receives a single p... more We consider a multistage inventory system composed of a single warehouse that receives a single product from a single supplier and replenishes the inventory of n retailers through direct shipments. Fixed costs are incurred for each truck dispatched and all trucks have the same capacity limit. Costs are stationary, or more generally monotone as in . Demands for the n retailers over a planning horizon of T periods are given. The objective is to find the shipment quantities over the planning horizon to satisfy all demands at minimum system-wide inventory and transportation costs without backlogging. Using the structural properties of optimal solutions, we develop (1) an O(T 2 ) algorithm for the single-stage dynamic lot sizing problem; (2) an O(T 3 ) algorithm for the case of a one-warehouse single-retailer system; and (3) a nested shortest-path algorithm for the one-warehouse multi-retailer problem that runs in polynomial time for a given number of retailers. To overcome the computational burden when the number of retailers is large, we propose aggregated and disaggregated Lagrangian decomposition methods that make use of the structural properties and the efficient single-stage algorithm. Computational experiments show the effectiveness of these algorithms and the gains associated with coordinated versus decentralized systems. Finally, we show that the decentralized solution is asymptotically optimal.
... A classical result, due to Kawaguchi and Kyan (1986), is that the WSPT heuristic, in which jo... more ... A classical result, due to Kawaguchi and Kyan (1986), is that the WSPT heuristic, in which jobs are assigned in a nondecreasing order of their ratios pi/wi ... if all the ratios, pi/wi, are equal, that is, wi = yp for all i, the WCTP is known to be NP-complete; see Chandra and Wong (1975 ...
The aim of this paper is to understand the impact of consumer returns on the de- cision making pr... more The aim of this paper is to understand the impact of consumer returns on the de- cision making process of a two-echelon supply chain. Currently, in practice, consumer returns are largely ignored when managers make decisions regarding order quantities and prices, as evidenced in the literature and in our interaction with industry. Con- sumer returns, however, have been reported
Http Dx Doi Org 10 1080 19488300 2013 858379, Oct 1, 2013
ABSTRACT Scheduling in primary care is challenging because of the diversity of patient cases (acu... more ABSTRACT Scheduling in primary care is challenging because of the diversity of patient cases (acute versus chronic), mix of appointments (pre-scheduled versus same-day), and uncertain time spent with providers and non-provider staff (nurses/medical assistants). In this paper, we present an empirically driven stochastic integer programming model that schedules and sequences patient appointments during a work day session. The objective is to minimize a weighted measure of provider idle time and patient wait time. Key model features include: an empirically based classification scheme to accommodate different chronic and acute conditions seen in a primary care practice; adequate coordination of patient time with a nurse and a provider; and strategies for introducing slack in the schedule to counter the effects of variability in service time with providers and nurses. In our computational experiments we characterize, for each patient type in our classification, where empty slots should be positioned in the schedule to reduce waiting time. Our results also demonstrate that the optimal start times for a variety of patient-centered heuristic sequences consistently follow a pattern that results in easy to implement guidelines. Moreover, these heuristic sequences and appointment times perform significantly better than the practice's schedule. Finally, we also compare schedules suggested by our two-service-stage model (nurse and provider) with those that only consider the provider stage and find that the performance of the provider-only model is 21% worse than that of the two-service-stage model.
We study a warehouse management problem where the schedule of incoming supplies and customer orde... more We study a warehouse management problem where the schedule of incoming supplies and customer orders for a wide variety of products is known over a number of periods. In addition to storage at the warehouse, product can be kept in the shipping trailers (rolling inventory) at the warehouse yard, avoiding material handling costs but incurring trailer handling and opportunity
M Som Manufacturing Service Operations Management, 2006
1 Impact of Partial Manufacturing Flexibility on Production Variability Ana Muriel Anand Somasund... more 1 Impact of Partial Manufacturing Flexibility on Production Variability Ana Muriel Anand Somasundaram Yongmei Zhang University of Massachusetts Amherst Abstract ... Let X(D)= { } Aji Dx ij ∈ ∀ ),( be an optimal solution to this problem when demand is D=(d1,d2, dN). ...
Http Dx Doi Org 10 1080 07408170600634525, Feb 23, 2007
We study a warehouse management problem in which the schedule of incoming supplies and customer o... more We study a warehouse management problem in which the schedule of incoming supplies and customer orders for a wide variety of products is known over a number of periods. In addition to storage at the warehouse, products can be kept in the shipping trailers (rolling inventory) parked in the warehouse yard, avoiding material handling costs but incurring trailer handling and
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Papers by Ana Muriel