Generalized log-gamma regression models with cure fraction

In this paper, the generalized log-gamma regression model is modified to allow the possibility that long-term survivors may be present in the data. This modification leads to a generalized log-gamma regression model with a cure rate, encompassing, as special cases, the log-exponential, log-Weibull and log-normal regression models with a cure rate typically used to model such data. The models attempt to simultaneously estimate the effects of explanatory variables on the timing acceleration/deceleration of a given event and the surviving fraction, that is, the proportion of the population for which the event never occurs. The normal curvatures of local influence are derived under some usual perturbation schemes and two martingale-type residuals are proposed to assess departures from the generalized log-gamma error assumption as well as to detect outlying observations. Finally, a data set from the medical area is analyzed.

Association between particulate matter from biomass burning and respiratory diseases in the southern region of the Brazilian Amazon

Objective. To investigate the short-term effects of exposure to particulate matter from biomass burning in the Amazon on the daily demand for outpatient care due to respiratory diseases in children and the elderly. Methods. Epidemiologic study with ecologic time series design. Daily consultation records were obtained from the 14 primary health care clinics in the municipality of Alta Floresta, state of Mato Grosso, in the southern region of the Brazilian Amazon, between January 2004 and December 2005. Information on the daily levels of fine particulate matter was made available by the Brazilian National Institute for Spatial Research. To control for confounding factors ( situations in which a non-causal association between exposure and disease is observed due to a third variable), variables related to time trends, seasonality, temperature, relative humidity, rainfall, and calendar effects ( such as occurrence of holidays and weekends) were included in the model. Poisson regression with generalized additive models was used. Results. A 10 mu g/m(3) increase in the level of exposure to particulate matter was associated with increases of 2.9% and 2.6% in outpatient consultations due to respiratory diseases in children on the 6th and 7th days following exposure. Significant associations were not observed for elderly individuals. Conclusions. The results suggest that the levels of particulate matter from biomass burning in the Amazon are associated with adverse effects on the respiratory health of children.

Compressible Sherrington-Kirkpatrick spin-glass model

We introduce a Sherrington-Kirkpatrick spin-glass model with the addition of elastic degrees of freedom. The problem is formulated in terms of an effective four-spin Hamiltonian in the pressure ensemble, which can be treated by the replica method. In the replica-symmetric approximation, we analyze the pressure-temperature phase diagram, and obtain expressions for the critical boundaries between the disordered and the ordered (spin-glass and ferromagnetic) phases. The second-order para-ferromagnetic border ends at a tricritical point, beyond which the transition becomes discontinuous. We use these results to make contact with the temperature-concentration phase diagrams of mixtures of hydrogen-bonded crystals.

THE EFFECT OF CORTICO-THALAMIC CONNECTIONS ON THE DIVERSITY OF CORTICAL ACTIVATIONS AS MODELED BY THE ISING MODEL

The influence of the thalamus on the diversity of cortical activations is investigated in terms of the Ising model with respect to progressive levels of cortico-thalamic connectivity. The results show that better diversity is achieved at lower modulation levels, being higher than those obtained with counterpart network models.

On the skew-normal calibration model

In this article, we present the EM-algorithm for performing maximum likelihood estimation of an asymmetric linear calibration model with the assumption of skew-normally distributed error. A simulation study is conducted for evaluating the performance of the calibration estimator with interpolation and extrapolation situations. As one application in a real data set, we fitted the model studied in a dimensional measurement method used for calculating the testicular volume through a caliper and its calibration by using ultrasonography as the standard method. By applying this methodology, we do not need to transform the variables to have symmetrical errors. Another interesting aspect of the approach is that the developed transformation to make the information matrix nonsingular, when the skewness parameter is near zero, leaves the parameter of interest unchanged. Model fitting is implemented and the best choice between the usual calibration model and the model proposed in this article was evaluated by developing the Akaike information criterion, Schwarz`s Bayesian information criterion and Hannan-Quinn criterion.

Kinetics of autocatalytic acid hydrolysis of cellulose with crystalline and amorphous fractions

The acid hydrolysis of cellulose with crystalline and amorphous fractions is analyzed on the basis of autocatalytic model with a positive feedback of acid production from the degraded biopolymer. In the condition of low acid rate production compared with hydrolysis rate, both fraction of cellulose decrease exponentially with linear and cubic time dependence, and the normalized number of scissions per cellulose chain follows a sigmoid behavior with reaction time. The model predicts that self generated acidic compounds from cellulose accelerate the degradation of the biopolymer. However, if the acidic compounds produced are volatile species, then their release under low pressure will reduce the global rate of degradation of cellulose toward its intrinsic rate value determined by the residual acid catalyst present in the starting material.

Uncertainty Characterization Of A Conceptual Rainfall-Runoff Model By Using Grey Numbers

A procedure for characterizing global uncertainty of a rainfall-runoff simulation model based on using grey numbers is presented. By using the grey numbers technique the uncertainty is characterized by an interval; once the parameters of the rainfall-runoff model have been properly defined as grey numbers, by using the grey mathematics and functions it is possible to obtain simulated discharges in the form of grey numbers whose envelope defines a band which represents the vagueness/uncertainty associated with the simulated variable. The grey numbers representing the model parameters are estimated in such a way that the band obtained from the envelope of simulated grey discharges includes an assigned percentage of observed discharge values and is at the same time as narrow as possible. The approach is applied to a real case study highlighting that a rigorous application of the procedure for direct simulation through the rainfall-runoff model with grey parameters involves long computational times. However, these times can be significantly reduced using a simplified computing procedure with minimal approximations in the quantification of the grey numbers representing the simulated discharges. Relying on this simplified procedure, the conceptual rainfall-runoff grey model is thus calibrated and the uncertainty bands obtained both downstream of the calibration process and downstream of the validation process are compared with those obtained by using a well-established approach, like the GLUE approach, for characterizing uncertainty. The results of the comparison show that the proposed approach may represent a valid tool for characterizing the global uncertainty associable with the output of a rainfall-runoff simulation model.

Fatores de risco para morte em pacientes diabéticos e não diabéticos em tratamento hemodialítico

A mortalidade dos pacientes diabéticos, quando iniciam tratamento hemodialítico, ainda é muito elevada, significativamente maior do que a dos pacientes não diabéticos. As doenças cardíacas são a principal causa de morte nestes pacientes. O diabetes, por si só, está associado a uma alta prevalência de hipertensão, doença cardiovascular e insuficiência cardíaca, resultando em morbi-mortalidade significativas. Tradicionalmente, a mortalidade tem sido associada à cardiopatia isquêmica. A mortalidade cardiovascular, entretanto, não está relacionada apenas à isquemia, mas também à insuficiência cardíaca e à morte súbita. O objetivo deste estudo foi analisar o papel da doença cardiovascular como fator prognóstico para a morte de pacientes diabéticos e não diabéticos, que iniciam hemodiálise, levando em consideração outros fatores. Este foi um estudo prospectivo de uma coorte de 40 pacientes diabéticos e 28 não diabéticos, que iniciaram programa de hemodiálise, de agosto de 1996 a junho de 1999, em 5 hospitais de Porto Alegre, Brasil. O tempo total de acompanhamento foi de 4,25 anos. A avaliação inicial, realizada entre o 20 e o 30 mês de hemodiálise, incluiu: um questionário com características demográficas, história do diabetes e suas complicações, história de hipertensão e acidente vascular cerebral; o exame físico incluindo avaliação nutricional e exame oftalmológico; e avaliação laboratorial com medidas de parâmetros nutricionais, bioquímicos, hormonais, perfil lipídico, e controle metabólico do diabetes, além da avaliação da adequação da diálise. Para a avaliação cardiovascular foram utilizados: questionário Rose, ECG em repouso, cintilografia em repouso e sob dipiridamol, e ecocardiograma bi-dimensional e com Doppler. A mortalidade foi analisada ao final dos 51 meses, e as causas de morte, definidas pelos registros médicos, atestados de óbito ou informações do médico assistente ou familiar. Na análise estatística, foram empregados o teste t de Student, o qui-quadrado (χ2) ou teste exato de Fisher. Para a análise da sobrevida, o método de Kaplan-Meier foi utilizado, e, para identificar os principais fatores associados à mortalidade, construiu-se um modelo de regressão múltipla de Cox. O nÍvel de significância adotado foi de 5%. Ao final do estudo, os pacientes diabéticos tiveram um índice de mortalidade significativamente mais elevado do que os pacientes sem diabetes (47,5% vs. 7,1%; P=0,0013, log rank test). Na análise de Cox, o padrão pseudonormal ou restritivo de disfunção diastólica esteve associado a um risco de 3,2 (IC 95%:1,2-8,8; P=0,02), e a presença de diabetes, a um risco de 4,7 (IC 95%:1,03-21,4; P=0,04) para a morte. Concluiu-se que a disfunção diastólica do ventrículo esquerdo foi o principal preditor de mortalidade nesta coorte de pacientes que estão iniciando tratamento hemodialítico.

The polictical economy informed lobbies

We analize a discrete type version of a common agency model with informed principals of Martimort and Moreira (2005) in the context of lobby games. We begin discussing issues related to the common values nature of the model, i.e.the agent cares directly about the principal’s utility function. With this feature the equilibrium of Martimort and Moreira (2005) is not valid. We argue in favor of one solution, although we are not able to fully characterize the equilibrium in this context. We then turn to an application: a modification of the Grossman and Helpman (1994) model of lobbying for tariff protection to incoporate assimetric information (but disconsidering the problem of common values) in the lobbies objective function. We show that the main results of the original model do not hold and that lobbies may behave less agressively towards the police maker when there is private information in the lobbies valuation for the tariffs.

Corporate capital structure revisited: incomplete markets and default

After Modigliani and Miller (1958) presented their capital structure irrelevance proposition, analysis of corporate Önancing choices involving debt and equity instruments have generally followed two trends in the literature, where models either incorporate informational asymmetries or introduce tax beneÖts in order to explain optimal capital structure determination (Myers, 2002). None of these features is present in this paper, which develops an asset pricing model with the purpose of providing a positive theory of corporate capital structure by replicating main aspects of standard contractual practice observed in real markets. Alternatively, the imperfect market structure of the economy is tailored to match what is most common in corporate reality. Allowance for default on corporate debt with an associated penalty of seizure of Örmís future cash áows by creditors is introduced, for instance. In this context, a qualitative assessment of Önancial managersídecisions is carried out through numerical procedures.

Optimal auctions with multidimensional types and the desirability of exclusion

Within the context of a single-unit, independent private values auction model, we show that if bidder types are multidimensional, then under the optimal auction exclusion of some bidder types will occur. A second contribution of the paper is methodological in nature. In particular, we identify conditions under which an auction model with multidimensional types can be reduced to a model with one dimensional types without loss of generality. Reduction results of this type have achieved the status of folklore in the mechanism design literature. Here, we provide a proof of the reduction result for auctions.

Endogenous time-dependent rules and the costs of disinflation with imperfect credibility

This paper examines the output effects of monetary disinflation in a model with endogenous time-dependent pricing rules and imperfect credibility of the disinflation policy. We find that these features interact to generate an additional effect on top f the ones obtained with either endogenous time-dependent rules (Bonomo and Carvalho, 2003) or imperfect credibility (Ball, 1995) in isolation. This results in higher output costs of monetary disinflation.

Endogenous growth with deferred technological change

Lucas (2009) propôs um modelo com dois setores para explicar padrões observados em dados de crescimento. Entretanto, a análise de Lucas não envolve uma decisão intertemporal para o consumidor. O comportamento das variáveis é determinado à priori pela tecnologia escolhida. Rodriguez (2006) propôs um modelo com a tecnologia com dois setores apresentada por Lucas adicionando um processo de decisão intertemporal para o consumidor. Adicionalmente aos resultados obtidos por Rodriguez, nós caracterizamos suficiência e apresentamos exemplos esclarecedores de casos particulares do modelo. Ademais, nós fazemos um esforço para derivar novos insights e esclarecer alguns pontos técnicos. Finalmente, nós obtemos condições sob as quais a economia investe em capital humano mesmo com benefícios diferidos.

Ensaios em opções reais e investimento sob incerteza

Esta tese é composta por três artigos, nos quais são apresentadas extensões e aplicações da Teoria das Opções Reais, todas de interesse para formuladores de política econômica no Brasil. O primeiro faz uma análise original da questão da bioprospecção, ou a exploração da diversidade biológica para fins econômicos. Duas estruturas alternativas para o desenho do mecanismo de concessão, visando o uso sustentável da biodiversidade brasileira, são sugeridas: (i) um modelo de projetos de P&D com maturidade incerta, no qual a intensidade do processo de Poisson que governa o tempo de maturação é explicitamente dependente do nível da biodiversidade no local concedido; (ii) um modelo de Agente-Principal, onde o Estado delega o exercício da opção de investimento à empresa de pesquisa biotecnológica. O segundo artigo avança a analogia entre opções de venda (“put options”) e cotas de importação. Os parâmetros relevantes para apreçar as licenças são agora obtidos endogenamente, a partir da interação entre a firma importadora e os produtores domésticos. Por fim, no terceiro, é feita análise pioneira do mercado paralelo de títulos precatórios no Brasil. Um modelo para a valoração de tais títulos é construído e proposto, tendo por base o arcabouço institucional existente sobre o assunto, tanto no governo central, como nos estados e municípios.

Risk sharing and the household collective model

When the joint assumption of optimal risk sharing and coincidence of beliefs is added to the collective model of Browning and Chiappori (1998) income pooling and symmetry of the pseudo-Hicksian matrix are shown to be restored. Because these are also the features of the unitary model usually rejected in empirical studies one may argue that these assumptions are at odds with evidence. We argue that this needs not be the case. The use of cross-section data to generate price and income variation is based Oil a definition of income pooling or symmetry suitable for testing the unitary model, but not the collective model with risk sharing. AIso, by relaxing assumptions on beliefs, we show that symmetry and income pooling is lost. However, with usual assumptions on existence of assignable goods, we show that beliefs are identifiable. More importantly, if di:fferences in beliefs are not too extreme, the risk sharing hypothesis is still testable.