Dynamic Procurement of New Products with Covariate Information: The Residual Tree Method

Problem definition: We study the practice-motivated problem of dynamically procuring a new, short life-cycle product under demand uncertainty. The firm does not know the demand for the new product but has data on similar products sold in the past, including demand histories and covariate information such as product characteristics. Academic/practical relevance: The dynamic procurement problem has long attracted academic and practitioner interest, and we solve it in an innovative data-driven way with proven theoretical guarantees. This work is also the first to leverage the power of covariate data in solving this problem. Methodology:We propose a new, combined forecasting and optimization algorithm called the Residual Tree method, and analyze its performance via epi-convergence theory and computations. Our method generalizes the classical Scenario Tree method by using covariates to link historical data on similar products to construct demand forecasts for the new product. Results: We prove, under fairly mild conditions, that the Residual Tree method is asymptotically optimal as the size of the data set grows. We also numerically validate the method for problem instances derived using data from the global fashion retailer Zara. We find that ignoring covariate information leads to systematic bias in the optimal solution, translating to a 6–15% increase in the total cost for the problem instances under study. We also find that solutions based on trees using just 2–3 branches per node, which is common in the existing literature, are inadequate, resulting in 30–66% higher total costs compared with our best solution. Managerial implications: The Residual Tree is a new and generalizable approach that uses past data on similar products to manage new product inventories. We also quantify the value of covariate information and of granular demand modeling

The impact of inventory management on stock-outs of essential drugs in Sub-Saharan Africa: secondary analysis of a field experiment in Zambia

Objective: To characterize the impact of widespread inventory management policies on stock-outs of essential drugs in Zambia’s health clinics and develop related recommendations. Methods: Daily clinic storeroom stock levels of artemether-lumefantrine (AL) products in 2009–2010 were captured in 145 facilities through photography and manual transcription of paper forms, then used to determine historical stock-out levels and estimate demand patterns. Delivery lead-times and estimates of monthly facility accessibility were obtained through worker surveys. A simulation model was constructed and validated for predictive accuracy against historical stock-outs, then used to evaluate various changes potentially affecting product availability. Findings: While almost no stock-outs of AL products were observed during Q4 2009 consistent with primary analysis, up to 30% of surveyed facilities stocked out of some AL product during Q1 2010 despite ample inventory being simultaneously available at the national warehouse. Simulation experiments closely reproduced these results and linked them to the use of average past monthly issues and failure to capture lead-time variability in current inventory control policies. Several inventory policy enhancements currently recommended by USAID | DELIVER were found to have limited impact on product availability. Conclusions: Inventory control policies widely recommended and used for distributing medicines in sub-Saharan Africa directly account for a substantial fraction of stock-outs observed in common situations involving demand seasonality and facility access interruptions. Developing central capabilities in peripheral demand forecasting and inventory control is critical. More rigorous independent peer-reviewed research on pharmaceutical supply chain management in low-income countries is needed

Air Quality Measurements at a Laying Hen House: Particulate Matter Concentrations and Emissions

Particulate matter (PM) was measured in the ventilation exhaust air of a caged layer house using three tapered element oscillating microbalances (TEOMs). Diurnal patterns of PM concentration and emission were observed during 6 days in June 2002. The average daily mean (±95% c.i.) concentrations and emissions were 39±8.0, 518±74, and 1887±563 .g/m3 and 1.1±0.3, 16±3.4, and 63±15 g/d-AU for PM2.5, PM10, and total suspended particulates (TSP), respectively. Daytime (lights on) PM2.5, PM10, and TSP concentrations were 151, 108, and 136% higher (P\u3c0.05) than at night. Emissions peaked during the day when birds were most active and ventilation rates were the highest. Wide diurnal variations in PM concentration and ventilation were observed. PM emission was correlated to ventilation, ambient and exhaust temperatures, and relative humidity (P\u3c0.05)

Micro-crystalline inclusions analysis by PIXE and RBS

A characteristic feature of the nuclear microprobe using a 3 MeV proton beam is the long range of particles (around 70 \mu m in light matrices). The PIXE method, with EDS analysis and using the multilayer approach for treating the X-ray spectrum allows the chemistry of an intra-crystalline inclusion to be measured, provided the inclusion roof and thickness at the impact point of the beam (Z and e, respectively) are known (the depth of the inclusion floor is Z + e). The parameter Z of an inclusion in a mineral can be measured with a precision of around 1 \mu m using a motorized microscope. However, this value may significantly depart from Z if the analyzed inclusion has a complex shape. The parameter e can hardly be measured optically. By using combined RBS and PIXE measurements, it is possible to obtain the geometrical information needed for quantitative elemental analysis. This paper will present measurements on synthetic samples to investigate the advantages of the technique, and also on natural solid and fluid inclusions in quartz. The influence of the geometrical parameters will be discussed with regard to the concentration determination by PIXE. In particular, accuracy of monazite micro-inclusion dating by coupled PIXE-RBS will be presented

Improving HIV Early Infant Diagnosis Supply Chains in Sub-Saharan Africa: Models and Application to Mozambique

Early diagnosis of the human immunodeficiency virus (HIV) among infants born to HIV-infected mothers is critical because roughly 50% of untreated infected infants die before the age of two years. Yet most countries in sub-Saharan Africa experience significant delays in diagnosis because of operational inefficiencies in early infant diagnosis (EID) networks. We develop a two-part modeling framework relying on optimization and simulation to generate operational improvements in the assignment of clinics to laboratories and the allocation of capacity across laboratories, and to evaluate the associated impact on the number of infants initiating treatment. Applying our methodology to EID program data from Mozambique, we validate our simulation model and estimate that optimally reassigning clinics to labs would decrease the average sample turnaround time (TAT) by 11% and increase the number of infected infants starting treatment by about 4% relative to the current system. Furthermore, consolidating all diagnostic capacity in one centralized lab would decrease average TATs by an estimated 22% and increase the number of infected infants initiating treatment by 7%. Our sensitivity analysis suggests that the consolidation of capacity in a single location would remain near optimal across a wide range of laboratory utilization levels in Mozambique. However, this full consolidation solution is dominated by configurations with two or more labs for EID networks with average transportation times larger than those currently observed in Mozambique by at least 15%

Characterization of Three Carbon- and Nitrogen-Rich Particles from Comet 81P/WILD

Comets may sample the early solar system s complement of volatile-forming elements - including C and N - more fully and reliably than do the terrestrial planets or asteroids. Until recently, all elemental analyses of unambiguously cometary material were carried out remotely. The return of the Stardust mission makes it possible to analyze documented material from P81/Wild 2 in the laboratory Wild 2 particles fragmented when they stopped in the aerogel collectors. We have studied three fragments thought to be rich in C and N by using several techniques: FTIR to characterize organic matter; synchrotron-induced x-ray fluorescence (SXRF) to determine Fe and certain element/Fe ratios; SEM to image sample morphology and to detect semiquantitatively Mg, Al, Si, Ca, and Fe; and nuclear reaction analysis (NRA) to measure C, N, O, and Si

Clinical use of HIV integrase inhibitors : a systematic review and meta-analysis

Background: Optimal regimen choice of antiretroviral therapy is essential to achieve long-term clinical success. Integrase inhibitors have swiftly been adopted as part of current antiretroviral regimens. The purpose of this study was to review the evidence for integrase inhibitor use in clinical settings. Methods: MEDLINE and Web-of-Science were screened from April 2006 until November 2012, as were hand-searched scientific meeting proceedings. Multiple reviewers independently screened 1323 citations in duplicate to identify randomized controlled trials, nonrandomized controlled trials and cohort studies on integrase inhibitor use in clinical practice. Independent, duplicate data extraction and quality assessment were conducted. Results: 48 unique studies were included on the use of integrase inhibitors in antiretroviral therapy-naive patients and treatment-experienced patients with either virological failure or switching to integrase inhibitors while virologically suppressed. On the selected studies with comparable outcome measures and indication (n = 16), a meta-analysis was performed based on modified intention-to-treat (mITT), on-treatment (OT) and as-treated (AT) virological outcome data. In therapy-naive patients, favorable odds ratios (OR) for integrase inhibitor-based regimens were observed, (mITT OR 0.71, 95% CI 0.59-0.86). However, integrase inhibitors combined with protease inhibitors only did not result in a significant better virological outcome. Evidence further supported integrase inhibitor use following virological failure (mITT OR 0.27; 95% CI 0.11-0.66), but switching to integrase inhibitors from a high genetic barrier drug during successful treatment was not supported (mITT OR 1.43; 95% CI 0.89-2.31). Integrase inhibitor-based regimens result in similar immunological responses compared to other regimens. A low genetic barrier to drug-resistance development was observed for raltegravir and elvitegravir, but not for dolutegravir. Conclusion: In first-line therapy, integrase inhibitors are superior to other regimens. Integrase inhibitor use after virological failure is supported as well by the meta-analysis. Careful use is however warranted when replacing a high genetic barrier drug in treatment-experienced patients switching successful treatment