ISEL - Matemática - Comunicações
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Browsing ISEL - Matemática - Comunicações by Author "Aleixo, Sandra"
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- Generalized models from Beta(p,2) densities with strong allee effect: dynamical approachPublication . Aleixo, Sandra; Rocha, J. LeonelA dynamical approach to study the behaviour of generalized populational growth models from Bets(p, 2) densities, with strong Allee effect, is presented. The dynamical analysis of the respective unimodal maps is performed using symbolic dynamics techniques. The complexity of the correspondent discrete dynamical systems is measured in terms of topological entropy. Different populational dynamics regimes are obtained when the intrinsic growth rates are modified: extinction, bistability, chaotic semistability and essential extinction.
- Modeling Allee Effect from Beta(p, 2) DensitiesPublication . Rocha, J. Leonel; Aleixo, SandraIn this work we develop and investigate generalized populational growth models, adjusted from Beta(p, 2) densities, with Allee effect. The use of a positive parameter leads the presented generalization, which yields some more flexible models with variable extinction rates. An Allee limit is incorporated so that the models under study have strong Allee effect.
- A spatio-temporal auto-regressive model for generating savings calls to a health linePublication . Paula Simões; M. Lucília Carvalho; Aleixo, Sandra; Gomes, Sérgio; Natário, IsabelUrgency admission is one of the most important factors regarding hospital costs, which can possibly be mitigated by the use of national health lines such as the Portuguese Saúde24 line (S24). Aiming future development of decision support indicators in a hospital savings context, based on the economic impact of the use of S24 rather than hospital urgency services, this study investigates spatio-temporal dependencies of the number of S24 calls generating savings in each Portuguese municipality, over the period 2010–2016, under an autoregressive approach. An econometric analysis of the savings obtained by the use of S24 is also carried out considering a savings index. Combining insights from classical spatial econometrics and from the analysis of spatio-temporal data, novel Bayesian Poisson spatio-temporal lag models are presented and applied in this paper. This extends to time the ideas of a Bayesian Poisson spatial lag model, considering both a parametric and a non-parametic structure for time and space-time effects. The results obtained for the savings index reveal that, over the last seven years, there has been a more comprehensive spatial effectiveness of the S24 line in solving the non-urgent emergency situations, that could be handled by primary health care services or in a self care basis.
- Weighted coefficients to measure agreement among several sets of ranks emphasizing top and bottom ranks at the same timePublication . Aleixo, Sandra; Teles, JúliaIn this paper, two new weighted coefficients of agreement to measure the concordance among several (more than two) sets of ranks, putting more weight in the lower and upper ranks simultaneously, are presented. These new coefficients, the signed Klotz and the signed Mood, generalize the correspondent rank-order coefficients to measure the agreement between two sets of ranks previously proposed by the authors [1]. Under the null hypothesis of no agreement or no association among the rankings, the asymptotic distribution of these new coefficients was derived. To illustrate the worth of these measures, an example is presented to compare them with the Kendall’s coefficient and the van der Waerden correlation coefficient.
- Weighting lower and upper ranks simultaneously through rank-order correlation coefficientsPublication . Aleixo, Sandra; Teles, JuliaTwo new weighted correlation coefficients, that allow to give more weight to the lower and upper ranks simultaneously, are proposed. These indexes were obtained computing the Pearson correlation coefficient with a modified Klotz and modified Mood scores. Under the null hypothesis of independence of the two sets of ranks, the asymptotic distribution of these new coefficients was derived. The exact and approximate quantiles were provided. To illustrate the value of these measures an example, that could mimic several biometrical concerns, is presented. A Monte Carlo simulation study was carried out to compare the performance of these new coefficients with other weighted coefficient, the van der Waerden correlation coefficient, and with two non-weighted indexes, the Spearman and Kendall correlation coefficients. The results show that, if the aim of the study is the detection of correlation or agreement between two sets of ranks, putting emphasis on both lower and upper ranks simultaneously, the use of van der Waerden, signed Klotz and signed Mood rank-order correlation coefficients should be privileged, since they have more power to detect this type of agreement, in particular when the concordance was focused on a lower proportion of extreme ranks. The preference for one of the coefficients should take into account the weight one wants to assign to the extreme ranks.