Jaime Catalán

A Combinational Auction Improves School Meals in Chile

Chiles school system is using mathematical modeling to assign catering contracts in a singleround sealed-bid combinational auction. The Chilean state spends around US

Optimizing Long-Term Production Plans in Underground and Open-Pit Copper Mines

180 million a year to feed 1,300,000 students from low income families, making this one of the largest state auctions. To improve the quality of the assignment in the auction process, we constructed an integer linear programming model to decide contract awards optimally among different concession holders. The model completely changed the nature of the process in three crucial aspects. First, it gave transparency and objectivity to the complete process, generating competition among firms. Second, it allowed the companies to build flexible territorial bids to include their scale economies, leading to efficient resource allocation. Finally, the model indeed found an optimal solution, which is not easy because the assignment problem was NP-complete with more than 10,000 binary variables. This new methodology improved the price-quality ratio of the meals with yearly savings of around US

Solving multiple scenarios in a combinatorial auction

40 million--equivalent to the cost of feeding 300,000 children during one year.

A Strategic Empty Container Logistics Optimization in a Major Shipping Company

We present a methodology for long-term mine planning based on a general capacitated multicommodity network flow formulation. It considers underground and open-pit ore deposits sharing multiple downstream processing plants over a long horizon. The purpose of the model is to optimize several mines in an integrated fashion, but real size instances are hard to solve due to the combinatorial nature of the problem. We tackle this by solving the relaxation of a tight linear formulation, and we round the resulting near-integer solution with a customized procedure. The model has been implemented at Codelco, the largest copper producer in the world. Since 2001, the system has been used on a regular basis and has increased the net present value of the production plan for a single mine by 5%. Moreover, integrating multiple mines provided an additional increase of 3%. The system has allowed planners to evaluate more scenarios. In particular, the model was used to study the option of delaying by four years the conversion of Chiquicamata, Codelcos largest open-pit mine, to underground operations.

Fluid Extravasation Related to Hip Arthroscopy A Prospective Computed Tomography–Based Study

As part of its social policy, the government of Chile provides more than 1.8 million meals daily to public schoolchildren under the authority of Junta Nacional de Auxilio Escolar y Becas (JUNAEB), the state agency responsible for the program, at an annual cost of 360 million dollars. The service is provided by private firms chosen through an annual public auction. In order to capture economies of scale, a combinatorial auction design is implemented, allowing suppliers to bid on different sets of geographical units within the country. The bid evaluation process must solve multiple scenarios of a difficult combinatorial optimization model. To date, more than 2 billion dollars have been awarded under this methodology. In this paper, we describe the 2006 auction process and report that solution times can be significantly improved if the scenarios are solved in an appropriate order and the optimal solution to one scenario is employed as the initial solution of another. Results reflecting these improvements are given for real instances of the 2006 auction.

Primary Meningococcal Type C Arthritis: A Case Report and Literature Review

In this paper, we present a system that Compania Sud Americana de Vapores (CSAV), one of the worlds largest shipping companies, developed to support its decisions for repositioning and stocking empty containers. CSAVs main business is shipping cargo in containers to clients worldwide. It uses a fleet of about 700,000 TEU containers of different types, which are carried by both CSAV-owned and third-party ships. Managing the container fleet is complex; CSAV must make thousands of decisions each day. In particular, imbalances exist among the regions. For example, China often has a deficit of empty containers and is a net importer; Saudi Arabia often has a surplus and is a net exporter. CSAV and researchers from the University of Chile developed the Empty Container Logistics Optimization System (ECO) to manage this imbalance. ECOs multicommodity, multiperiod model manages the repositioning problem, whereas an inventory model determines the safety stock required at each location. CSAV uses safety stock to ensure high service levels despite uncertainties, particularly in the demand for containers. A hybrid forecasting system supports both the inventory and the multicommodity network flow model. Major improvements in data gathering, real-time communications, and automation of data handling were needed as input to the models. A collaborative Web-based optimization framework allows agents from different zones to interact in decision making. The use of ECO led to direct savings of

An asymmetric multi-item auction with quantity discounts applied to Internet service procurement in Buenos Aires public schools

81 million for CSAV, a reduction in inventory stock of 50 percent, and an increase in container turnover of 60 percent. Moreover, the system helped CSAV to become more efficient and to overcome the 2008 economic crisis.

A combinatorial auction improves school meals in Chile: a case of OR in developing countries

Background: Complications related to hip arthroscopy are rare, with a current rate of <2%. Some complications are related to fluid extravasation, which has been associated with life-threatening conditions such as abdominal compartment syndrome, cardiopulmonary arrest, hypothermia, and atelectasis. Purpose: To identify risk factors for fluid volume extravasation in hip arthroscopy and to determine the relationship between anatomical location on computed tomography (CT) and extravasated volume. Study Design: Case series; Level of evidence, 4. Methods: We performed a prospective cohort study of 40 consecutive arthroscopies for femoroacetabular impingement. Patient demographics and procedures performed (ie, acetabuloplasty and its size, femoral osteoplasty, and psoas tenotomy) were recorded. The extravasated volume was estimated by the difference between the infused volume and the intraoperatively collected volume. Within 12 hours after the procedure, the pelvis was scanned by CT. We created a 3-stage radiological classification based on progressive involvement of anatomical structures attributed to liquid extravasation. Statistical analyses were performed with a 95% CI and a significance level of 5%. Results: No relevant clinical symptoms related to fluid extravasation were recorded. The mean extravasated volume was 3.06 L at a rate of 1.05 L/h, corresponding to nearly 10% of the infused volume. There was a trend toward greater extravasated volume with longer operative time and longer time in the peripheral compartment (without axial traction); however, there was no statistical significance. The anatomical classification on CT imaging was directly related to the extravasated volume and compromised the thigh, gluteus, and retroperitoneum and intraperitoneal spaces. There was a 6-fold greater probability of female patients having an advanced stage extravasation on CT classification. Conclusion: In our series, 10% of the infused volume was extravasated in uncomplicated hip arthroscopy. Risk factors for extravasation were related to operative time, particularly during peripheral compartment intervention (ie, without traction). The anatomical classification proposed in this study reflects the progressive involvement of the thigh, gluteus, iliopsoas, and retroperitoneal and intraperitoneal spaces after increase in extravasated volume. Compared with males, females were associated with more severe anatomical compromise for the same extravasated volume.

Bone Tumors Around the Knee: Risks and Benefits of Arthroscopic Procedures

Acute septic arthritis is a common clinical problem in emergency departments. Primary meningococcal arthritis (PMA) is very rare and few cases are reported in literature. D. B. M. consulted the emergency department for knee pain and fever; analysis showed that the cause was a Neisseria meningitidis type C infection. He received a treatment consisting of 2 arthroscopies and 5 weeks of antibiotics. At five weeks he returned to work and at 2 months he resumed sports (jogging and soccer) without complaints. Primary arthritis of the knee caused by Neisseria meningitidis is very rare. It has a very good response to antibiotics and arthroscopy procedure. Short-term follow-up and functional results are often good or excellent.

NO SUCH THING AS A FREE LUNCH

This article studies a multi-item auction characterized by asymmetric bidders and quantity discounts. We report a practical application of this type of auction in the procurement of Internet services to the 709 public schools of Buenos Aires. The asymmetry in this application is due to firms’ existing technology infrastructures, which affect their ability to provide the service in certain areas of the city. A single round first-price sealed-bid auction, it required each participating firm to bid a supply curve specifying a price on predetermined graduated quantity intervals and to identify the individual schools it would supply. The maximal intersections of the sets of schools each participant has bid on define regions we call competition units. A single unit price must be quoted for all schools supplied within the same quantity interval, so that firms cannot bid a high price where competition is weak and a lower one where it is strong. Quantity discounts are allowed so that the bids can reflect returns-to-scale of the suppliers and the auctioneer may benefit of awarding bundles of units instead of separate units. The winner determination problem in this auction poses a challenge to the auctioneer. We present an exponential formulation and a polynomial formulation for this problem, both based on integer linear programming. The polynomial formulation proves to find the optimal set of bids in a matter of seconds. Results of the real-world implementation are reported.