New Network Goods

New horizontally-differentiated goods involving product-specific network effects are quite prevalent. Consumers market-wide preference for each of these goods typically is initially unknown. Later, as sales data begin to accumulate, agents learn market-wide preferences, which thus become common knowledge. We study such a market, pinpointing the factors which determine whether the market-wide preferred firm reinforces its lead as time elapses, penetration and under-cost pricing prevail, and first- or last-mover effects in market-wide preferences occur.Network effects, horizontal differentiation

A probabilistic formulation for Empirical Population Synthesis: Sampling methods and tests

We present a probabilistic formulation of the classical problem of synthesizing spectral properties of a galaxy using a base of star clusters. The problem consists of estimating the population vector x, composed by the contributions of n_star base elements to the integrated spectrum of a galaxy, and the extinction A_V, given a set of absorption line equivalent widths and continuum colors. The formalism is applied to the n_star = 12 base defined by Schmidt etal and subsequently used in several studies. The 13-D parameter space is explored with a Markov chain Monte Carlo sampling scheme based on the Metropolis algorithm, which produces a smooth and efficient mapping of the P(x,A_V) probability distribution. This version of Empirical Population Synthesis is used to investigate the ability to recover the detailed history of star-formation and chemical evolution using this spectral base. This is studied as a function of (1) the magnitude of the measurement errors and (2) the set of observables used in the synthesis. Only for extremely high S/N all 12 base proportions can be accurately recovered, though the observables are very precisely reproduced for any S/N. Furthermore, the individual mean x components are biased in the sense that components which carry a large fraction of the light tend to share their contribution preferably among components of same age. This compensation effect is linked to noise-induced linear dependences in the base, which very effectively redistribute the likelihood in x-space. The age distribution, however, can be satisfactorily recovered for realistic data quality. (abridged)Comment: 22 pages, 12 figs. MNRAS accepte

The Evolution of Stars and Gas in starburst Galaxies

In systems undergoing starbursts the evolution of the young stellar population is expected to drive changes in the emission line properties. This evolution is usually studied theoretically, with a combination of evolutionary synthesis models for the spectral energy distribution of starbursts and photoionization calculations. In this paper we present a more empirical approach to this issue. We apply empirical population synthesis techniques to samples of Starburst and HII galaxies in order to measure their evolutionary state and correlate the results with their emission line properties. A couple of useful tools are introduced which greatly facilitate the interpretation of the synthesis: (1) an evolutionary diagram, whose axis are the strengths of the young, intermediate age and old components of the stellar population mix, and (2) the mean age of stars associated with the starburst, \ov{t}_{SB}. These tools are tested with grids of theoretical galaxy spectra and found to work very well even when only a small number of observed properties (absorption line equivalent widths and continuum colors) is used in the synthesis. Starburst nuclei and HII galaxies are found to lie on a well defined sequence in the evolutionary diagram. Using the empirically defined mean starburst age in conjunction with emission line data we have verified that the equivalent widths of H$\beta$ and [OIII] decrease for increasing \ov{t}_{SB}. The same evolutionary trend was identified for line ratios indicative of the gas excitation, although no clear trend was identified for metal rich systems. All these results are in excellent agreement with long known, but little tested, theoretical expectations.Comment: 15 pages, 12 figures. MNRAS accepte

Essays in industrial economics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Economics, 2008.Includes bibliographical references.This dissertation consists of three chapters in Industrial Economics. Chapter 1 is the product of joint work with Filippo Balestrieri. In chapter 1 we examine the use of lotteries among horizontal differentiated goods as a mechanism to price discriminate consumers. We use the linear city model to represent a market with two differentiated goods. We show that the optimal selling strategy for a multi product monopolist implies offering at least one lottery with probability 1/2. This result is in stark contrast with the the optimal selling strategy of a single product monopolist that attempts to price discriminate consumers based on the probability of delivering the good. Riley and Zeckhauser (1983) show that the single good monopolist does not offer lotteries. We then examine the use of lotteries among the differentiated goods in the case of a market with two competing firms, each one producing one good. We define two different cases depending on whether the market is fully covered. We call fully covered market the case in which the equilibrium prices of the two firms are such that all consumers buy one good. We show that if the market is fully covered, no lotteries are offered in equilibrium. However, when the market is not fully covered the optimal selling strategy may include offering lotteries. With more than two firms selling differentiated goods, even in a fully covered market, lotteries can be used in equilibrium. In this case, firms might be worse off than in the case where no lotteries are provided. Chapter 2 examines firms optimal pricing policy when they sell a storable good of repeated consumption to time-inconsistent consumers. Consumers with time-inconsistent preferences might struggle to make optimal consumption decisions over time. Sophisticate consumers, aware of their time-inconsistent preferences, often try to limit their consumption of certain goods by strategically rationing the quantities they purchase.(cont.) On the other hand, naive consumers, unaware of their time-inconsistent preferences, may stockpile tempting goods at "home" not realizing that the higher availability of the good might lead them to overconsume the good.It is shown that if consumers are time-consistent, quantity discounts don't increase the firms' profit. In contrast, if firms face naive time-inconsistent consumers, the optimal pricing policy is to use small quantity discounts as a device to increase sales. These consumers take advantage of quantity discount with the intention of saving on future purchases. However, after buying the good they can not resist and overconsume it. We also show that even if consumers are sophisticated, firms still use quantity discounts. Sophisticated time-inconsistent consumers realize that increasing the quantity purchased often leads them to overconsume the good. Hence, they require a significant quantity discount to increase the quantity purchased. Offering a quantity discount leads them to stockpile the good "at home" and hence promotes overconsumption. Chapter 3 analyzes the use of exclusive dealing agreements to prevent the entry of rival firms. An exclusive dealing agreement is a contract between a buyer and a seller where the buyer commits to buy a good exclusively from the seller. Exclusive dealing agreements are one of the most common vertical restraints used by firms. Aghion and Bolton (1987) were the first to show that an incumbent seller may want to use exclusive dealing agreement that prevents the entry of a rival seller. They argue that an incumbent seller and a buyer sign an exclusive dealing contract in order to extract surplus from a more efficient entrant seller.(cont.) We propose an alternative explanation for the use of exclusive dealing agreements to prevent entry when the buyer is a downstream distributor that also faces the threat of entry. The idea is that the entry of more efficient upstream seller, by decreasing the market power of the upstream firms, makes entry in the downstream market more attractive. This can lead to further entry in the downstream market and to an increase in the competition faced by the downstream firms. Since part of the bigger surplus created by the entry of a more efficient seller is captured by the downstream entrant firms, entry in the upstream market does not necessarily benefits the incumbent downstream firms.by Joao Leao.Ph.D

Informatizaçao em clínica de marcapasso

Os autores discutem a necessidade atual de se estabelecer um projeto, de característica universal, para a informatizaçao das clínicas de marcapasso (diagnósticos, implantes e avaliaçoes), em funçao da rápida evoluçao tecnológica por que passa a especialidade. Nao há no Brasil um padrao instituído, que possa ser seguido por todos. Os autores apresentam sua experiência pessoal na formaçao de uma equipe mista, com profissionais médicos e da área de informática, que elegeu o Sistema constituído, na sua essência, por tabelas de múltipla escolha, balizando essa conduta: a) na necessidade de manipulaçao de um complexo volume de informaçoes médicas de maneira padronizada, b) na agilidade para atualizaçao do Sistema, composto por componentes independentes, c) na facilidade para realizaçao de pesquisas e, d) na maior economia do método para o armazenamento de grandes volumes de dados

Spectral classification and properties of the O Vz stars in the Galactic O-Star Spectroscopic Survey (GOSSS)

On the basis of the Galactic O-Star Spectroscopic Survey (GOSSS), a detailed systematic investigation of the O Vz stars is presented. The currently used spectral classification criteria are rediscussed, and the Vz phenomenon is recalibrated through the addition of a quantitative criterion based on the equivalent widths of the He I 4471, He II 4542, and He II 4686 spectral lines. The GOSSS O Vz and O V populations resulting from the newly adopted spectral classification criteria are comparatively analyzed. The locations of the O Vz stars are probed, showing a concentration of the most extreme cases toward the youngest star forming regions. The occurrence of the Vz spectral peculiarity in a solar-metallicity environment, as predicted by the fastwind code, is also investigated, confirming the importance of taking into account several processes for the correct interpretation of the phenomenon.Comment: Accepted for publication in The Astronomical Journa

Rationale, study design, and analysis plan of the Alveolar Recruitment for ARDS Trial (ART): Study protocol for a randomized controlled trial

Background: Acute respiratory distress syndrome (ARDS) is associated with high in-hospital mortality. Alveolar recruitment followed by ventilation at optimal titrated PEEP may reduce ventilator-induced lung injury and improve oxygenation in patients with ARDS, but the effects on mortality and other clinical outcomes remain unknown. This article reports the rationale, study design, and analysis plan of the Alveolar Recruitment for ARDS Trial (ART). Methods/Design: ART is a pragmatic, multicenter, randomized (concealed), controlled trial, which aims to determine if maximum stepwise alveolar recruitment associated with PEEP titration is able to increase 28-day survival in patients with ARDS compared to conventional treatment (ARDSNet strategy). We will enroll adult patients with ARDS of less than 72 h duration. The intervention group will receive an alveolar recruitment maneuver, with stepwise increases of PEEP achieving 45 cmH(2)O and peak pressure of 60 cmH2O, followed by ventilation with optimal PEEP titrated according to the static compliance of the respiratory system. In the control group, mechanical ventilation will follow a conventional protocol (ARDSNet). In both groups, we will use controlled volume mode with low tidal volumes (4 to 6 mL/kg of predicted body weight) and targeting plateau pressure <= 30 cmH2O. The primary outcome is 28-day survival, and the secondary outcomes are: length of ICU stay; length of hospital stay; pneumothorax requiring chest tube during first 7 days; barotrauma during first 7 days; mechanical ventilation-free days from days 1 to 28; ICU, in-hospital, and 6-month survival. ART is an event-guided trial planned to last until 520 events (deaths within 28 days) are observed. These events allow detection of a hazard ratio of 0.75, with 90% power and two-tailed type I error of 5%. All analysis will follow the intention-to-treat principle. Discussion: If the ART strategy with maximum recruitment and PEEP titration improves 28-day survival, this will represent a notable advance to the care of ARDS patients. Conversely, if the ART strategy is similar or inferior to the current evidence-based strategy (ARDSNet), this should also change current practice as many institutions routinely employ recruitment maneuvers and set PEEP levels according to some titration method.Hospital do Coracao (HCor) as part of the Program 'Hospitais de Excelencia a Servico do SUS (PROADI-SUS)'Brazilian Ministry of Healt