141 research outputs found
A review of velocity-type PSO variants
This paper presents a review of the particular variants of particle swarm optimization, based on the velocity-type class.
The original particle swarm optimization algorithm was developed as an unconstrained optimization technique, which
lacks a model that is able to handle constrained optimization problems. The particle swarm optimization and its
inapplicability in constrained optimization problems are solved using the dynamic-objective constraint-handling
method. The dynamic-objective constraint-handling method is originally developed for two variants of the basic particle
swarm optimization, namely restricted velocity particle swarm optimization and self-adaptive velocity particle swarm
optimization. Also on the subject velocity-type class, a review of three other variants is given, specifically: (1) vertical
particle swarm optimization; (2) velocity limited particle swarm optimization; and (3) particle swarm optimization with
scape velocity. These velocity-type particle swarm optimization variants all have in common a velocity parameter which
determines the direction/movements of the particles.info:eu-repo/semantics/publishedVersio
Análise de Sobrevivência em R
info:eu-repo/semantics/publishedVersio
Parametric regression models for recurrent events analysis based on Chen distribution
In this paper, two parametric regression models based on Chen dis tribution are proposed for situations where recurrent events have the same or dif ferent risks of occurrence. Inference is based on a maximum likelihood approach,
which ensures consistent parameter estimators. However, since in recurrent event
data there is within-subject correlation, the “one step” jackknife estimator is
used. An application on a real data set is also provided for illustrative purposes.info:eu-repo/semantics/publishedVersio
Modelos para acontecimentos múltiplos
Ao longo dos últimos anos tem existido um interesse crescente em estudar o tempo até à ocorrência de vários acontecimentos, os quais podem ser observados mais do que uma vez para um mesmo indivíduo. Os dados respeitantes a acontecimentos múltiplos têm como principal característica o facto de se registar mais do que um tempo de vida para cada indivíduo, o que inviabiliza a aplicação direta do modelo de regressão de Cox. Assim, surgiu a necessidade de desenvolver novas extensões deste modelo, sendo que as mais utilizadas na prática foram sugeridas por: Prentice, Williams e Peterson (PWP); Andersen e Gill (AG); Wei, Lin e Weissfeld (WLW); e Lee, Wei e Amato (LWA). Um dos maiores obstáculos na aplicação destes modelos é a forte possibilidade de existir correlação intraindivíduos. Neste ponto, os quatro modelos referidos anteriormente são classificados como modelos marginais, uma vez que o vetor de parâmetros de regressão é estimado com base no ajustamento de um modelo que ignora a correlação entre acontecimentos. Para compensar esse facto, nestes modelos é usado um estimador robusto da matriz de covariância, o qual permite efetuar a correção necessária na estimativa da variância usual. Após realizar uma descrição detalhada de cada modelo marginal procedeu-se à respetiva implementação através do software estatístico R. Para o efeito, recorreu-se à simulação de dados relativos a acontecimentos múltiplos do mesma natureza, isto é, a acontecimentos recorrentes. Os resultados obtidos permitiram realçar e confirmar as características dos modelos estudados
A novel and simpler alkaline hydrolysis methodology for extraction of ferulic acid from brewer’s spent grain and its (partial) purification through adsorption in a synthetic resin
This work aims to develop simpler methodologies of extracting ferulic acid (FA) from
brewer’s spent grain (BSG). BSG is produced by brewing companies at high amounts all over the
year and does not possess a direct application. Thus, its use as raw material for extraction of
bioactive compounds has gained attention in the last years. FA has different interesting applications
in cosmetics, food industry, and pharmaceutics. Several studies aim for its extraction from BSG by
various methods, namely alkaline hydrolysis. In the present work, we suggest the use of autoclave
to process higher amounts of BSG in a lab scale. A simplification of the regular post-hydrolysis
procedures is also proposed to decrease the number of experimental steps and energy costs and to
simultaneously increase the extraction yield (up to 470 mg of FA per 100 g of BSG). The adsorption of
extracted FA in a synthetic resin is suggested as a partial purification method.info:eu-repo/semantics/publishedVersio
A review of Cox' s model extensions for multiple events
In longitudinal studies, it is usual that a given subject can experience several failures. To analyse multiple failure-time data, we reviewed some extensions of Cox's regression model, which were proposed by: Prentice, Williams and Peterson (PWP); Andersen and Gill (AG); Wei, Lin e Weissfeld (WLW); and Lee, Wei and Amato (LWA). Our main goal is to underline the differences between these extensions, through a brief but careful description, providing also some guidance on how to choose the proper model for each situation. The guidelines presented in this work revealed to be a useful pointer to easily choose the most suitable model. Secondarily, we used the survsim and the survival R packages to illustrate the practical implementation of these models.info:eu-repo/semantics/publishedVersio
Um olhar sobre o desenvolvimento de modelos de sobrevivência para acontecimentos recorrentes
info:eu-repo/semantics/publishedVersio
Total phenols, antioxidant potential and antimicrobial activity of walnut (Juglans regia L.) green husks
The total phenols content and antioxidant and antimicrobial activities were studied in walnut (Juglans
regia L.) green husks aqueous extracts of five different cultivars (Franquette, Mayette, Marbot, Mellanaise
and Parisienne). Total phenols content was determined by colorimetric assay and their amount ranged
from 32.61 mg/g of GAE (cv. Mellanaise) to 74.08 mg/g of GAE t (cv. Franquette). The antioxidant capacity
of aqueous extracts was assessed through reducing power assay, scavenging effects on DPPH (2,2-diphenyl-
1-picrylhydrazyl) radicals and b-carotene linoleate model system. A concentration-dependent antioxidative
capacity was verified in reducing power and DPPH assays, with EC50 values lower than 1 mg/
mL for all the tested extracts. The antimicrobial capacity was screened against Gram positive and Gram
negative bacteria, and fungi. All the extracts inhibited the growth of Gram positive bacteria, being Staphylococcus
aureus the most susceptible one with MIC of 0.1 mg/mL for all the extracts. The results obtained
indicate that walnut green husks may become important in the obtainment of a noticeable source of compounds
with health protective potential and antimicrobial activity
Modelos paramétricos flexíveis para acontecimentos recorrentes
Neste trabalho, s˜ao propostas duas classes de modelos pa ram´etricos para analisar o tempo entre acontecimentos recorrentes,
utilizando splines c´ubicos restritos. As classes de modelos diferem no
modo como se lida com a dependˆencia entre acontecimentos. Al´em
disso, ambas as classes s˜ao caracterizadas por uma transforma¸c˜ao
da fun¸c˜ao de sobrevivˆencia, atrav´es da fun¸c˜ao de liga¸c˜ao log-log ou
−logit. De forma a ilustrar a metodologia proposta, efetua-se uma
aplica¸c˜ao a um conjunto de dados reais sobre a recorrˆencia de cancro
da bexiga. Os resultados revelam que o uso de splines c´ubicos res tritos permite modelar de forma flex´ıvel e adequada o tempo entre
acontecimentos sucessivos.info:eu-repo/semantics/publishedVersio
Modelos de sobrevivência aplicados à análise de acontecimentos múltiplos
Na modela¸c˜ao de acontecimentos m´ultiplos, uma abor dagem muita utilizada consiste em desenvolver extens˜oes do modelo
semiparam´etrico de Cox. Contudo, quando se considera que o co nhecimento da distribui¸c˜ao do tempo ´e importante para o estudo,
a abordagem param´etrica revela-se mais adequada. Neste trabalho,
s˜ao apresentados dois novos modelos param´etricos baseados na dis tribui¸c˜ao de Weibull (modelos WNE e WE), como alternativas a
duas das extens˜oes do modelo de Cox para acontecimentos recorren tes (modelos AG e PWP). De forma a ilustrar as diferen¸cas entre
estas duas abordagens, ´e considerado um exemplo de aplica¸c˜ao com dados simulados. Os resultados obtidos com os modelos propostos
permitem acrescentar mais duas op¸c˜oes ao leque de escolhas de mo delos para acontecimentos recorrentes.info:eu-repo/semantics/publishedVersio
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