International Journal of Production Research, 2004
This research focuses on solving a common wafer test scheduling problem in semiconductor manufact... more This research focuses on solving a common wafer test scheduling problem in semiconductor manufacturing. During wafer testing, a series of test processes are conducted on wafers using computer-controlled test stations at various temperatures. The test processes are conducted in a specified order on a wafer lot, resulting in precedence constraints for the schedule. Furthermore, the assignment of the wafer lots
We consider a container terminal loading and unloading containers to and from a set of ships, and... more We consider a container terminal loading and unloading containers to and from a set of ships, and storing the containers in the terminal yard. Each ship is served by multiple quay cranes, which load and unload containers to and from ships. Containers are moved between the ships and the yard using a fleet of vehicles, each with unit capacity. The
ABSTRACT We propose a novel analytic approach for the comparative statics analysis of multiproduc... more ABSTRACT We propose a novel analytic approach for the comparative statics analysis of multiproduct multiresource newsvendor networks under responsive pricing. Our approach involves exploiting the properties of the primal mathematical programming formulation and of the dual variables and linking those properties to the concept of convex orders and to properties of the underlying demand function. The use of convex orders allows us to establish our main results without restriction to a specific demand distribution. A major strength of our approach is that it is "scalable," i.e., it applies to newsvendor networks with any number of "nonindependent" (i.e., demand or resource sharing) products and resources, without an exponential increase in effort as problem size increases. This is unlike the current approaches commonly used in the operations management literature, which typically involve a parametric analysis of the recourse problem, followed by the use of Jacobians and the implicit function theorem. Providing a rigorous framework for comparative statics analysis, which can be applied to other problems that are not amenable to traditional parametric analysis, is our main contribution. We demonstrate this approach on the optimal capacity decision problem in multiproduct newsvendor networks under responsive pricing, formulated as a two-stage stochastic programming problem with recourse: The firm determines the resource capacities ex ante, in the first stage, when demand intercepts are uncertain, and makes the pricing and production decisions ex post, in the second stage, when demand intercepts (e.g., market conditions) are fully observed. This particular problem and its variants are well studied in the operations management literature. A comparative statics analysis is integral to the study of the capacity investment decision, as it allows answers to important questions such as the following: "Does the firm acquire more or less of the different resources available as demand uncertainty increases? Does the firm benefit from an increase in demand uncertainty?" Using our proposed approach, we establish comparative statics results on how the newsvendor's expected profit and optimal capacity decision change with demand risk in multiproduct multiresource newsvendor networks. We also extend our analysis to the study of demand dependence in two-product networks.
The ability to effectively match supply and demand can lead to significant revenue benefits in th... more The ability to effectively match supply and demand can lead to significant revenue benefits in the airline industry. Airline supply management deals with assigning the right resources (i.e., aircraft and crew) to the right routes in the flight network. Due to certain crew regulations, operating characteristics, and constraints of the airline companies, these supply management decisions need to be made well in advance of departures, at a time when demand is highly uncertain. However, demand forecasts improve markedly over time, as more information on demand patterns is gathered.
IIE Transactions on Healthcare Systems Engineering, 2011
... International Journal of Production Research , 29(8): 16451659. [Taylor & Francis On... more ... International Journal of Production Research , 29(8): 16451659. [Taylor & Francis Online], [Web of Science ®], [CSA] View all references; Pepe, 200461. Pepe, MS 2004. ... [Taylor & Francis Online], [Web of Science ®], [CSA] View all references, and the references therein). ...
Health care during hospitalization has become more complex. With this complexity comes additional... more Health care during hospitalization has become more complex. With this complexity comes additional patient risk that may lead to patient safety events. These risks usually occur infrequently, but when they do, they can be catastrophic for patients. Common approaches for analyzing serious or sentinel events in health care include root cause analysis (RCA), which is retrospective in nature and has a number of limitations. Hospitals are also required to prospectively perform a failure modes and effects analysis (FMEA) annually on one high risk event. The FMEA process is also not without its own limitations. This paper describes an approach to prospectively analyze potential serious and sentinel events by using a tool known as socio-technical probabilistic risk assessment (ST-PRA). This tool, used in several other industries, has seen some moderate success in health care for analyzing risks related to blood transfusions, medication errors, and infections. The major benefits of this tool,...
Babesia microti causes transfusion-transmitted babesiosis (TTB); currently, blood donor screening... more Babesia microti causes transfusion-transmitted babesiosis (TTB); currently, blood donor screening assays are unlicensed but used investigationally. We developed a decision tree model assessing the comparative- and cost-effectiveness of B. microti blood donation screening strategies in endemic areas compared to the status quo (question regarding a history of babesiosis), including testing by: 1) universal antibody (Ab), 2) universal polymerase chain reaction (PCR), 3) universal Ab/PCR, and 4) recipient risk-targeted Ab/PCR. The model predicted the number of TTB cases, complicated TTB cases, cases averted, and quality-adjusted life years (QALYs). Economic outcomes included each strategy's per-donation cost, waste (number of infection-free units incorrectly discarded), and waste index (number wasted units/number true positives). Sensitivity analyses examined uncertainty in transmission probabilities, prevalence rates, and other key model inputs. Universal PCR in four endemic states would prevent 24 to 31 TTB cases/100,000 units transfused (pht) at an incremental cost-effectiveness ratio (ICER) of $26,000 to $44,000/QALY (transmission probability dependent) and waste index of zero. Universal Ab/PCR would prevent 33 to 42 TTB cases pht at an ICER of $54,000 to $83,000/QALY and waste index of 0.05. The questionnaire is most wasteful (99.62 units wasted pht; 208.62 waste index), followed by the risk-targeted strategy (76.27 units wasted pht; 0.68 waste index). The model predicted zero cases of TTB or complicated TTB with universal Ab/PCR (versus [33, 42] and [13, 18] pht, respectively [no screening]). Results are highly sensitive to transmission probabilities. Universal PCR in endemic states is an effective blood donation screening strategy at a threshold of $50,000/QALY. Using a higher cost-effectiveness ratio, universal Ab/PCR is the most effective strategy.
To understand how structural and process elements may affect the risk for surgical site infection... more To understand how structural and process elements may affect the risk for surgical site infections (SSIs) in the ambulatory surgery center (ASC) environment, the researchers employed a tool known as socio-technical probabilistic risk assessment (ST-PRA). ST-PRA is particularly helpful for estimating risks in outcomes that are very rare, such as the risk of SSI in ASCs. Study objectives were to (1) identify the risk factors associated with SSIs resulting from procedures performed at ASCs and (2) design an intervention to mitigate the likelihood of SSIs for the most common risk factors that were identified by the ST-PRA for a particular surgical procedure. ST-PRA was used to study the SSI risk in the ASC setting. Both quantitative and qualitative data sources were utilized, and sensitivity analysis was performed to ensure the robustness of the results. The event entitled "fail to protect the patient effectively" accounted for 51.9% of SSIs in the ambulatory care setting. Critical components of this event included several failure risk points related to skin preparation, antibiotic administration, staff training, proper response to glove punctures during surgery, and adherence to surgical preparation rules related to the wearing of jewelry, watches, and artificial nails. Assuming a 75% reduction in noncompliance on any combination of 2 of these 5 components, the risk for an SSI decreased from 0.0044 to between 0.0027 and 0.0035. An intervention that targeted the 5 major components of the major risk point was proposed, and its implications were discussed.
The Socio-Technical Probabilistic Risk Assessment, a proactive risk assessment tool imported from... more The Socio-Technical Probabilistic Risk Assessment, a proactive risk assessment tool imported from high-risk industries, was used to identify risks for surgical site infections (SSIs) associated with the ambulatory surgery center setting and to guide improvement efforts. This study had 2 primary objectives: (1) to identify the critical risk factors associated with SSIs resulting from procedures performed at ambulatory surgery centers and (2) to design an intervention to mitigate the probability of SSI for the highest risk factors identified. Inputs included quantitative and qualitative data sources from the evidence-based literature and from health care providers. The Socio-Technical Probabilistic Risk Assessment ranked the failure points (events) on the basis of their contribution to an SSI. The event, entitled "Failure to protect the patient effectively," which included several failure points, was the most critical unique event with the highest contribution to SSI risk. A total of 51.87% of SSIs in this setting were caused by this failure. Consequently, we proposed an intervention aimed at all 5 major components of this failure. The intervention targets improvements in skin preparation; proper administration of antibiotics; staff training in infection control principles, including practices for the prevention of glove punctures; and procedures to ensure the removal of watches, jewelry, and artificial nails.
Consider a set of product variants that are differentiated by some secondary attributes such as f... more Consider a set of product variants that are differentiated by some secondary attributes such as flavor, color, or size. The retailer's problem is to jointly determine the set of variants to include in her product line ("assortment"), together with their prices and inventory levels, so as to maximize her expected profit. We model the consumer choice process using a multinomial logit choice model and consider a newsvendor type inventory setting. We derive the structure of the optimal assortment for some important special cases, including the case of horizontally differentiated items, and propose a dominance relationship for the general case that simplifies the search for an optimal assortment. We also discuss structural properties of the optimal prices. Finally, motivated by our analytical results, we propose a heuristic solution procedure, which is shown to be quite effective through a numerical study.
Identifying a globally optimal solution for an intractable discrete optimization problem is often... more Identifying a globally optimal solution for an intractable discrete optimization problem is often cost prohibitive. Therefore, solutions that are within a predetermined threshold are often acceptable in practice. This dissertation introduces the concept of β-acceptable solutions where β is a predetermined threshold for the objective function value.
We consider a container terminal discharging and uploading containers to and from ships. The disc... more We consider a container terminal discharging and uploading containers to and from ships. The discharged containers are stored at prespecified storage locations in the terminal yard. Containers are moved between the ship area and the yard using a fleet of vehicles, each of which can carry one container at a time. The problem is to dispatch vehicles to the containers so as to minimize the total time it takes to serve a ship, which is the total time it takes to discharge all containers from the ship and upload new containers onto the ship. We develop easily implementable heuristic algorithms and identify both the absolute and asymptotic worst-case performance ratios of these heuristics. In simple settings, most of these algorithms are optimal, while in more general settings, we show, through numerical experiments, that these algorithms obtain near-optimal results for the dispatching problem.
ABSTRACT Blood products, derived from donated blood, are essential for many medical treatments, a... more ABSTRACT Blood products, derived from donated blood, are essential for many medical treatments, and their safety, in terms of being free of Transfusion-Transmitted Infections (TTIs)—i.e., infectious agents that can be spread through their use—is crucial. However, blood screening tests are not perfectly reliable and may produce false negative or false-positive results. Currently, blood donations are tested using a same-for-all testing scheme, where a single test set is used on all blood donations. This article studies differential testing schemes, which may involve multiple test sets, each applied to a randomly selected fraction of the donated blood. Thus, although each blood donation is still tested by a single test set, multiple test sets may be used by the Blood Center. This problem is modeled within an optimization framework and a novel solution methodology is provided that allows important structural properties of such testing schemes to be characterized. It is shown that an optimal differential testing scheme consists of at most two test sets, and such a dual-test scheme can significantly reduce the TTI risk over the current same-for-all testing. The presented analysis leads to an efficient greedy algorithm that generates the optimal differential test sets for a range of budgets to inform the decision-maker (e.g., Blood Center). The differential model is extended to the case where different test sets can be used on sub-sets of donations defined by donation characteristics (e.g., donor demographics, seasonality, or region) that differentiate the sub-set’s TTI prevalence rates. The risk reduction potential of differential testing is quantified through two case studies that use published data from Sub-Saharan Africa and the United States. The study generates key insight into public policy decision making on the design of blood screening schemes.
The residual risk (RR) of transfusion-transmitted infections, including the human immunodeficienc... more The residual risk (RR) of transfusion-transmitted infections, including the human immunodeficiency virus and hepatitis B and C viruses, is typically estimated by the incidence/window period model, which relies on the following restrictive assumptions: Each screening test, with probability 1, (1) detects an infected unit outside of the test's window period; (2) fails to detect an infected unit within the window period; and (3) correctly identifies an infection-free unit. These assumptions need not hold in practice due to random or systemic errors and individual variations in the window period. We develop a probability model that accurately estimates the RR by relaxing these assumptions, and quantify their impact using a published cost-effectiveness study and also within an optimization model. These assumptions lead to inaccurate estimates in cost-effectiveness studies and to sub-optimal solutions in the optimization model. The testing solution generated by the optimization model translates into fewer expected infections without an increase in the testing cost.
International Journal of Production Research, 2004
This research focuses on solving a common wafer test scheduling problem in semiconductor manufact... more This research focuses on solving a common wafer test scheduling problem in semiconductor manufacturing. During wafer testing, a series of test processes are conducted on wafers using computer-controlled test stations at various temperatures. The test processes are conducted in a specified order on a wafer lot, resulting in precedence constraints for the schedule. Furthermore, the assignment of the wafer lots
We consider a container terminal loading and unloading containers to and from a set of ships, and... more We consider a container terminal loading and unloading containers to and from a set of ships, and storing the containers in the terminal yard. Each ship is served by multiple quay cranes, which load and unload containers to and from ships. Containers are moved between the ships and the yard using a fleet of vehicles, each with unit capacity. The
ABSTRACT We propose a novel analytic approach for the comparative statics analysis of multiproduc... more ABSTRACT We propose a novel analytic approach for the comparative statics analysis of multiproduct multiresource newsvendor networks under responsive pricing. Our approach involves exploiting the properties of the primal mathematical programming formulation and of the dual variables and linking those properties to the concept of convex orders and to properties of the underlying demand function. The use of convex orders allows us to establish our main results without restriction to a specific demand distribution. A major strength of our approach is that it is "scalable," i.e., it applies to newsvendor networks with any number of "nonindependent" (i.e., demand or resource sharing) products and resources, without an exponential increase in effort as problem size increases. This is unlike the current approaches commonly used in the operations management literature, which typically involve a parametric analysis of the recourse problem, followed by the use of Jacobians and the implicit function theorem. Providing a rigorous framework for comparative statics analysis, which can be applied to other problems that are not amenable to traditional parametric analysis, is our main contribution. We demonstrate this approach on the optimal capacity decision problem in multiproduct newsvendor networks under responsive pricing, formulated as a two-stage stochastic programming problem with recourse: The firm determines the resource capacities ex ante, in the first stage, when demand intercepts are uncertain, and makes the pricing and production decisions ex post, in the second stage, when demand intercepts (e.g., market conditions) are fully observed. This particular problem and its variants are well studied in the operations management literature. A comparative statics analysis is integral to the study of the capacity investment decision, as it allows answers to important questions such as the following: "Does the firm acquire more or less of the different resources available as demand uncertainty increases? Does the firm benefit from an increase in demand uncertainty?" Using our proposed approach, we establish comparative statics results on how the newsvendor's expected profit and optimal capacity decision change with demand risk in multiproduct multiresource newsvendor networks. We also extend our analysis to the study of demand dependence in two-product networks.
The ability to effectively match supply and demand can lead to significant revenue benefits in th... more The ability to effectively match supply and demand can lead to significant revenue benefits in the airline industry. Airline supply management deals with assigning the right resources (i.e., aircraft and crew) to the right routes in the flight network. Due to certain crew regulations, operating characteristics, and constraints of the airline companies, these supply management decisions need to be made well in advance of departures, at a time when demand is highly uncertain. However, demand forecasts improve markedly over time, as more information on demand patterns is gathered.
IIE Transactions on Healthcare Systems Engineering, 2011
... International Journal of Production Research , 29(8): 16451659. [Taylor & Francis On... more ... International Journal of Production Research , 29(8): 16451659. [Taylor & Francis Online], [Web of Science ®], [CSA] View all references; Pepe, 200461. Pepe, MS 2004. ... [Taylor & Francis Online], [Web of Science ®], [CSA] View all references, and the references therein). ...
Health care during hospitalization has become more complex. With this complexity comes additional... more Health care during hospitalization has become more complex. With this complexity comes additional patient risk that may lead to patient safety events. These risks usually occur infrequently, but when they do, they can be catastrophic for patients. Common approaches for analyzing serious or sentinel events in health care include root cause analysis (RCA), which is retrospective in nature and has a number of limitations. Hospitals are also required to prospectively perform a failure modes and effects analysis (FMEA) annually on one high risk event. The FMEA process is also not without its own limitations. This paper describes an approach to prospectively analyze potential serious and sentinel events by using a tool known as socio-technical probabilistic risk assessment (ST-PRA). This tool, used in several other industries, has seen some moderate success in health care for analyzing risks related to blood transfusions, medication errors, and infections. The major benefits of this tool,...
Babesia microti causes transfusion-transmitted babesiosis (TTB); currently, blood donor screening... more Babesia microti causes transfusion-transmitted babesiosis (TTB); currently, blood donor screening assays are unlicensed but used investigationally. We developed a decision tree model assessing the comparative- and cost-effectiveness of B. microti blood donation screening strategies in endemic areas compared to the status quo (question regarding a history of babesiosis), including testing by: 1) universal antibody (Ab), 2) universal polymerase chain reaction (PCR), 3) universal Ab/PCR, and 4) recipient risk-targeted Ab/PCR. The model predicted the number of TTB cases, complicated TTB cases, cases averted, and quality-adjusted life years (QALYs). Economic outcomes included each strategy's per-donation cost, waste (number of infection-free units incorrectly discarded), and waste index (number wasted units/number true positives). Sensitivity analyses examined uncertainty in transmission probabilities, prevalence rates, and other key model inputs. Universal PCR in four endemic states would prevent 24 to 31 TTB cases/100,000 units transfused (pht) at an incremental cost-effectiveness ratio (ICER) of $26,000 to $44,000/QALY (transmission probability dependent) and waste index of zero. Universal Ab/PCR would prevent 33 to 42 TTB cases pht at an ICER of $54,000 to $83,000/QALY and waste index of 0.05. The questionnaire is most wasteful (99.62 units wasted pht; 208.62 waste index), followed by the risk-targeted strategy (76.27 units wasted pht; 0.68 waste index). The model predicted zero cases of TTB or complicated TTB with universal Ab/PCR (versus [33, 42] and [13, 18] pht, respectively [no screening]). Results are highly sensitive to transmission probabilities. Universal PCR in endemic states is an effective blood donation screening strategy at a threshold of $50,000/QALY. Using a higher cost-effectiveness ratio, universal Ab/PCR is the most effective strategy.
To understand how structural and process elements may affect the risk for surgical site infection... more To understand how structural and process elements may affect the risk for surgical site infections (SSIs) in the ambulatory surgery center (ASC) environment, the researchers employed a tool known as socio-technical probabilistic risk assessment (ST-PRA). ST-PRA is particularly helpful for estimating risks in outcomes that are very rare, such as the risk of SSI in ASCs. Study objectives were to (1) identify the risk factors associated with SSIs resulting from procedures performed at ASCs and (2) design an intervention to mitigate the likelihood of SSIs for the most common risk factors that were identified by the ST-PRA for a particular surgical procedure. ST-PRA was used to study the SSI risk in the ASC setting. Both quantitative and qualitative data sources were utilized, and sensitivity analysis was performed to ensure the robustness of the results. The event entitled "fail to protect the patient effectively" accounted for 51.9% of SSIs in the ambulatory care setting. Critical components of this event included several failure risk points related to skin preparation, antibiotic administration, staff training, proper response to glove punctures during surgery, and adherence to surgical preparation rules related to the wearing of jewelry, watches, and artificial nails. Assuming a 75% reduction in noncompliance on any combination of 2 of these 5 components, the risk for an SSI decreased from 0.0044 to between 0.0027 and 0.0035. An intervention that targeted the 5 major components of the major risk point was proposed, and its implications were discussed.
The Socio-Technical Probabilistic Risk Assessment, a proactive risk assessment tool imported from... more The Socio-Technical Probabilistic Risk Assessment, a proactive risk assessment tool imported from high-risk industries, was used to identify risks for surgical site infections (SSIs) associated with the ambulatory surgery center setting and to guide improvement efforts. This study had 2 primary objectives: (1) to identify the critical risk factors associated with SSIs resulting from procedures performed at ambulatory surgery centers and (2) to design an intervention to mitigate the probability of SSI for the highest risk factors identified. Inputs included quantitative and qualitative data sources from the evidence-based literature and from health care providers. The Socio-Technical Probabilistic Risk Assessment ranked the failure points (events) on the basis of their contribution to an SSI. The event, entitled "Failure to protect the patient effectively," which included several failure points, was the most critical unique event with the highest contribution to SSI risk. A total of 51.87% of SSIs in this setting were caused by this failure. Consequently, we proposed an intervention aimed at all 5 major components of this failure. The intervention targets improvements in skin preparation; proper administration of antibiotics; staff training in infection control principles, including practices for the prevention of glove punctures; and procedures to ensure the removal of watches, jewelry, and artificial nails.
Consider a set of product variants that are differentiated by some secondary attributes such as f... more Consider a set of product variants that are differentiated by some secondary attributes such as flavor, color, or size. The retailer's problem is to jointly determine the set of variants to include in her product line ("assortment"), together with their prices and inventory levels, so as to maximize her expected profit. We model the consumer choice process using a multinomial logit choice model and consider a newsvendor type inventory setting. We derive the structure of the optimal assortment for some important special cases, including the case of horizontally differentiated items, and propose a dominance relationship for the general case that simplifies the search for an optimal assortment. We also discuss structural properties of the optimal prices. Finally, motivated by our analytical results, we propose a heuristic solution procedure, which is shown to be quite effective through a numerical study.
Identifying a globally optimal solution for an intractable discrete optimization problem is often... more Identifying a globally optimal solution for an intractable discrete optimization problem is often cost prohibitive. Therefore, solutions that are within a predetermined threshold are often acceptable in practice. This dissertation introduces the concept of β-acceptable solutions where β is a predetermined threshold for the objective function value.
We consider a container terminal discharging and uploading containers to and from ships. The disc... more We consider a container terminal discharging and uploading containers to and from ships. The discharged containers are stored at prespecified storage locations in the terminal yard. Containers are moved between the ship area and the yard using a fleet of vehicles, each of which can carry one container at a time. The problem is to dispatch vehicles to the containers so as to minimize the total time it takes to serve a ship, which is the total time it takes to discharge all containers from the ship and upload new containers onto the ship. We develop easily implementable heuristic algorithms and identify both the absolute and asymptotic worst-case performance ratios of these heuristics. In simple settings, most of these algorithms are optimal, while in more general settings, we show, through numerical experiments, that these algorithms obtain near-optimal results for the dispatching problem.
ABSTRACT Blood products, derived from donated blood, are essential for many medical treatments, a... more ABSTRACT Blood products, derived from donated blood, are essential for many medical treatments, and their safety, in terms of being free of Transfusion-Transmitted Infections (TTIs)—i.e., infectious agents that can be spread through their use—is crucial. However, blood screening tests are not perfectly reliable and may produce false negative or false-positive results. Currently, blood donations are tested using a same-for-all testing scheme, where a single test set is used on all blood donations. This article studies differential testing schemes, which may involve multiple test sets, each applied to a randomly selected fraction of the donated blood. Thus, although each blood donation is still tested by a single test set, multiple test sets may be used by the Blood Center. This problem is modeled within an optimization framework and a novel solution methodology is provided that allows important structural properties of such testing schemes to be characterized. It is shown that an optimal differential testing scheme consists of at most two test sets, and such a dual-test scheme can significantly reduce the TTI risk over the current same-for-all testing. The presented analysis leads to an efficient greedy algorithm that generates the optimal differential test sets for a range of budgets to inform the decision-maker (e.g., Blood Center). The differential model is extended to the case where different test sets can be used on sub-sets of donations defined by donation characteristics (e.g., donor demographics, seasonality, or region) that differentiate the sub-set’s TTI prevalence rates. The risk reduction potential of differential testing is quantified through two case studies that use published data from Sub-Saharan Africa and the United States. The study generates key insight into public policy decision making on the design of blood screening schemes.
The residual risk (RR) of transfusion-transmitted infections, including the human immunodeficienc... more The residual risk (RR) of transfusion-transmitted infections, including the human immunodeficiency virus and hepatitis B and C viruses, is typically estimated by the incidence/window period model, which relies on the following restrictive assumptions: Each screening test, with probability 1, (1) detects an infected unit outside of the test's window period; (2) fails to detect an infected unit within the window period; and (3) correctly identifies an infection-free unit. These assumptions need not hold in practice due to random or systemic errors and individual variations in the window period. We develop a probability model that accurately estimates the RR by relaxing these assumptions, and quantify their impact using a published cost-effectiveness study and also within an optimization model. These assumptions lead to inaccurate estimates in cost-effectiveness studies and to sub-optimal solutions in the optimization model. The testing solution generated by the optimization model translates into fewer expected infections without an increase in the testing cost.
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Papers by Ebru Bish