1,720,976 research outputs found
Estimating seemingly unrelated regression models with vector autoregressive disturbances
No full textThe numerical solution of seemingly unrelated regression (SUR) models with vector autoregressive disturbances is considered. Initially, an orthogonal transformation is applied to reduce the model to one with smaller dimensions. The transformed model is expressed as a reduced-size SUR model with stochastic constraints. The generalized QR decomposition is used as the main computational tool to solve this model. An iterative estimation algorithm is proposed when the variance-covariance matrix of the disturbances and the matrix of autoregressive coefficients are unknown. Strategies to compute the orthogonal factorizations of the non-dense-structured matrices which arise in the estimation procedure are presented. Experimental results demonstrate the computational efficiency of the proposed algorithm. © 2002 Elsevier Science B.V. All rights reserved
Seemingly unrelated regression model with unequal size observations: Computational aspects
No full textThe computational solution of the seemingly unrelated regression model with unequal size observations is considered. Two algorithms to solve the model when treated as a generalized linear least-squares problem are proposed. The algorithms have as a basic tool the generalized QR decomposition (GQRD) and efficiently exploit the block-sparse structure of the matrices. One of the algorithms reduces the computational burden of the estimation procedure by not computing explicitly the RQ factorization of the GQRD. The maximum likelihood estimation of the model when the covariance matrix is unknown is also considered. © 2002 Elsevier Science B.V. All rights reserved
Duality in conjugate gradient methods
No full textIn this paper we show that if the step (displacement) vectors generated by the preconditioned conjugate gradient algorithm are scaled appropriately they may be used to solve equations whose coefficient matrices are the preconditioning matrices of the original equations. The dual algorithms thus obtained are shown to be equivalent to the reverse algorithms of Hegedüs and are subsequently generalised to their block forms. It is finally shown how these may be used to construct dual (or reverse) algorithms for solving equations involving nonsymmetric matrices using only short recurrences, and reasons are suggested why some of these algorithms may be more numerically stable than their primal counterparts
Computationally efficient methods for solving SURE models
No full textComputationally efficient and numerically stable methods for solving Seemingly Unrelated Regression Equations (SURE) models are proposed. The iterative feasible generalized least squares estimator of SURE models where the regression equations have common exogenous variables is derived. At each iteration an estimator of the SURE model is obtained from the solution of a generalized linear least squares problem. The proposed methods, which have as a basic tool the generalized QR decomposition, are also found to be efficient in the general case where the number of linear independent regressors is smaller than the number of observations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Models estimate pressure losses in gas-condensate systems
No full textThe efficiency of currently available correlations for two-phase gas-condensate flows is undermined by the presence of water in these flow systems. Water radically modifies the pressure-loss mechanism. Five models have been developed for estimating the global pressure loss of such pipelines in actual operation. The results of these calculations are compared with field data contained in the American Gas Association's Multiphase Pipeline Data Bank which concerns different measurements relative to three lines of two-phase gas-condensate mixtures in the presence of water. The available correlations greatly underestimate pressure losses even if the water concentration is low compared to the entire liquid phase. In fact, the two-phase condensate-gas system becomes a three-phase water-condensate-gas flow in which the physical properties of liquid phases are quite different from each other
- …
