Modelos de evolução do peso de animais em ambiente aleatório

Após uma breve revisão dos métodos usualmente utilizados para modelar o crescimento de animais, propõe-se como modelos descritivos gerais para a evolução do peso de animais em ambiente aleatório equações diferenciais estocásticas da forma: dX(t)=f(X(t))dt+sdW(t) (1) onde X(t) representa o peso (ou uma potência do peso) do animal na idade t, s mede a intensidade dos efeitos das perturbações aleatórias do ambiente sobre o crescimento, W(t) é o processo de Wiener e x(0) é o peso à nascença (que supomos conhecido). Partindo do modelo de Bertalanffy-Richards, foi considerado f(X(t))=b(A-X(t)), onde os parâmetros A e b representam, respectivamente, o peso assintótico (ou peso na maturidade) e a velocidade com que o animal dele se aproxima. Deste modo, (1) apresenta a forma do conhecido modelo de Vasicek utilizado na modelação da dinâmica das taxas de juro. A partir da solução de (1), é apresentada uma expressão explícita para a função de máxima verosimilhança. O modelo foi aplicado a dados de crescimento de bovinos mertolengos da estirpe rosilho. São apresentadas as estimativas dos parâmetros e intervalos de confiança assintóticos

Out in the dark measuring the gay and lesbian wage gap

Data insufficiency hampered the academic research of discrimination based on sexual orientation. This is a particular concern in Portugal, a country that in spite of the strong legal recognition of homosexuals still scores low in their acceptance. Resorting to a selfdesigned web-survey, this study provides for the first time academic investigation of homosexuals in Portugal and contributes with an evaluation of wage discrimination in the primary employment. The empirical results point in the direction of absence of discrimination, but there is imprecise and small evidence that some homosexual individuals may be subject to discrimination

Modelling Individual Growth in Random Environments

We have considered, as general models for the evolution of animal size in a random environment, stochastic differential equations of the form dY(t)=b( A-Y(t))dt+\sigma dW(t), where Y(t)=g(X(t)), X(t) is the size of an animal at time t, g is a strictly increasing function, A=g(a) where a is the asymptotic size, b>0 is a rate of approach to A, s measures the effect of random environmental fluctuations on growth, and W(t) is the Wiener process. The transient and stationary behaviours of this stochastic differential equation model are well-known. We have considered the stochastic Bertalanffy-Richards model (g(x)=x^c with c>0) and the stochastic Gompertz model (g(x)=ln x). We have studied the problems of parameter estimation for one path and also considered the extension of the estimation methods to the case of several paths, assumed to be independent. We used numerical techniques to obtain the parameters estimates through maximum likelihood methods as well as bootstrap methods. The data used for illustration is the weight of "mertolengo" cattle of the "rosilho" strand

A SDE growth model: Nonparametric Estimation of the Drift and the Diffusion Coefficients

We study a stochastic differential equation (SDE) growth model to describe individual growth in random environments. In particular, in this work, we discuss the estimation of the drift and the diffusion coefficients using non-parametric methods. We illustrate the methodology by using bovine growth data. Considering the diffusion process X_{t}, describing the weight of an animal at age t, characterized by the stochastic differential equation dX(t)=a(X(t))dt+b(X(t))dW(t), with W(t) being the Wiener process, we estimate the infinitesimal coefficients a(x) and b(x) nonparametrically. Our goal was to analyse which of the parametric models (with specific functional forms for a(x) and b(x)) previously used by us to describe the evolution of bovine weight were good choices and also to see whether some alternative specific parametrized functional forms of a(x) and b(x) might be suggested for further parametric analysis of this data

Modelling individual animal growth in random environments

We have considered, as general models for the evolution of animal size in a random environment, stochastic differential equations of the form dY(t)=b( A-Y(t))dt+sdW(t), where Y(t)=g(X(t)), X(t) is the size of an animal at time t, g is a strictly increasing function, A=g(a) where a is the asymptotic size, s measures the effect of random environmental fluctuations on growth, and W(t) is the Wiener process. We have considered the stochastic Bertalanffy-Richards model (g(x)=x^c with c>0) and the stochastic Gompertz model (g(x)=ln x). We have studied the problems of parameter estimation for one path and also considered the extension to several paths. We also used bootstrap methods. Results and methods are illustrated using bovine growth data

ISI

This paper will consider stochastic models for animal growth that take into account the effect on growth of the random fluctuations in the animal’s environment. Let X(t) be the body weight or size of the animal. The traditional deterministic models assume the form of a differential equation dY(t)=b(g(a)-Y(t))dt, where g is a strictly increasing function, Y(t)=g(X(t)), a is the asymptotic size or size at maturity of the animal, and b is the rate of approach to maturity. For instance, the Bertalanffy-Richards model corresponds to g being a power function and the Gompertz model to g being a logarithmic function. In early work we have considered, for animals growing in a random environment, stochastic differential equations models dY(t)=b(g(a)-Y(t))dt+sdW(t), where W(t) is a Wiener process and s measures the intensity of the random environmental fluctuations. We have considered the problems of parameter estimation and prediction for one path. Here we study the extension to several paths, in which case we have data at several time instants coming from several animals. The results and methods are applied to bovine growth data provided by Carlos Roquete (ICAM-University of Évora

Animal growth in random environments: estimation with several paths

Refereed scientific paper on stochastic differential equation models of individual animal growth (from birth to maturity) in random environments with estimation methods for several trajectories (several animals). The paper is in press in the Bulletin of ISI containing the Proceedings of the 56th Session of the ISI (2007). An electronic version is available

How authentic leadership promotes individual performance: Mediating role of organizational citizenship behavior and creativity

Purpose – The purpose of this paper is to provide a more comprehensive understanding of how authentic leadership (AL) can affect individual performance through creativity and organizational citizenship behavior (OCB)’s mediating roles. Design/methodology/approach – The sample included 177 leader-follower dyads from 26 private and small and medium-sized organizations. Followers reported their perceptions of AL, and leaders assessed each follower’s level of creativity, individual performance and OCB. Findings – The findings show that AL has a positive impact on OCB (i.e. altruism, sportsmanship, civic virtue, conscientiousness and courtesy), employee creativity, and individual performance. Creativity partially mediates the relationship between AL and individual performance. Some dimensions of OCB, namely, altruism, civic virtue and courtesy, also play a mediating role in this relationship. Research limitations/implications – Additional studies with larger samples are needed to determine more clearly not only AL’s influence on individual performance but also other psychosocial variables affecting that relationship. Practical implications – Organizations can increase employees’ creativity, OCB and individual performance by encouraging managers to adopt more AL styles. Originality/value – This study is the first to integrate AL, creativity, OCB and individual performance into a single research model, thereby extending previous research. The study also used a double-source method to collect data (i.e. leader-follower dyads) to minimize the risk of introducing common-method variance.info:eu-repo/semantics/publishedVersio

Conceções e práticas de professores de geologia sobre trabalho de campo

O trabalho de campo (TC) é uma estratégia que tem vindo a ser cada vez mais adotada, de forma a proporcionar uma aprendizagem integrada da Geologia. Nesta investigação procurou saber-se qual o tipo e a frequência da implementação do TC por parte de um grupo de professores de Geologia, bem como compreender a importância que estes atribuem ao TC no ensino e na aprendizagem. Para o efeito foi concebido e aplicado um questionário que foi respondido por 16 professores do ensino secundário. Os resultados indicam que a maioria dos professores recorre ao TC uma a três vezes por ano, e considera que o TC mais adequado para este nível de ensino é do tipo Orientado para a Resolução de Problemas. No entanto, as descrições de algumas atividades efetuadas por estes professores revelam que o TC mais implementado é do tipo Observação Dirigida

Profit optimization for cattle growing in a randomly fluctuating environment

A class of stochastic differential equation models was applied to describe the evolution of the weight of Mertolengo cattle. We have determined the optimal mean profit obtained by selling an animal at the cattle market, using two approaches. One consists in determining the optimal selling age (independently of the weight) and the other consists in selling the animal when a fixed optimal weight is achieved for the first time (independently of the age). The profit probability distribution can be computed for such optimal age/weight. For typical market values, we observed that the second approach achieves a higher optimal mean profit compared with the first one, and, in most cases, even provides a lower standard deviation