3,464 research outputs found
Small-sample corrections for score tests in Birnbaum-Saunders regressions
In this paper we deal with the issue of performing accurate small-sample
inference in the Birnbaum-Saunders regression model, which can be useful for
modeling lifetime or reliability data. We derive a Bartlett-type correction for
the score test and numerically compare the corrected test with the usual score
test, the likelihood ratio test and its Bartlett-corrected version. Our
simulation results suggest that the corrected test we propose is more reliable
than the other tests.Comment: To appear in the Communications in Statistics - Theory and Methods,
http://www.informaworld.com/smpp/title~content=t71359723
Improved maximum likelihood estimators in a heteroskedastic errors-in-variables model
This paper develops a bias correction scheme for a multivariate
heteroskedastic errors-in-variables model. The applicability of this model is
justified in areas such as astrophysics, epidemiology and analytical chemistry,
where the variables are subject to measurement errors and the variances vary
with the observations. We conduct Monte Carlo simulations to investigate the
performance of the corrected estimators. The numerical results show that the
bias correction scheme yields nearly unbiased estimates. We also give an
application to a real data set.Comment: 12 pages. Statistical Paper
Situação da Sigatoka-Negra da bananeira no Brasil.
A bananicultura é uma das atividades de grande importância no agronegócio brasileiro, embora o País não seja um grande exportador de banana. A produção é da ordem de sete milhões de toneladas anuais, com vendas externas em torno de 2% da produção, ou seja, o mercado interno é o mais importante para a comercialização da fruta, que, em grande parte vem da agricultura familiar
Avaliação de resistência à Sigatoka-amarela em genótipos de bananeira.
A Sigatoka-amarela da bananeira, causada por Mycosphaerella musicola, é a doença mais importante dessa cultura no Brasil, dada a sua dispersão no país e as perdas que causa na produção, estimadas em 50%. Os prejuízos causados são resultantes da morte precoce das folhas e do consequente enfraquecimento da planta, refletindo negativamente na produção e na qualidade do fruto
Avaliação de resistência à Sigatoka-amarela em genótipos de bananeira.
A bananeira é produzida em cerca de 150 países em todos os continentes. Em 2011 a produção brasileira de banana chegou a 7.023.396 toneladas/ano, com aumento em relação aos anos anteriores. Entre os problemas fitossanitários da bananeira, a Sigatoka-amarela, causada por Mycosphaerella musicola Leach, é um dos mais importantes, haja vista a sua dispersão no país com adaptabilidade a diferentes condições climáticas e causa perdas estimadas em 50% na produção, com possibilidade de atingir 100% em alguns microclimas
[Carotid atherosclerosis and white matter hypodensities: a controversial relationship]
INTRODUCTION:
White matter hypodensities of presumed vascular origin, are recognized as an important cause of morbidity with established clinical and cognitive consequences. Nonetheless, many doubts remain on its physiopathology. Our goal is to clarify the potential role of carotid atherosclerosis and other vascular risk factors in the development of white matter hypodensities of presumed vascular origin.
MATERIAL AND METHODS:
We included patients that underwent CT brain scan and neurosonologic evaluation within a one-month period. Full assessment of vascular risks factors was performed. We seek to find independent associations between white matter hypodensities of presumed vascular origin, carotid intima-media thickness and vascular risk factors.
RESULTS:
472 patients were included, mean age was 67.32 (SD: 14.75), 274 (58.1%) were male. The independent predictors of white matter hypodensities of presumed vascular origin were age (OR: 1.067, 95% IC: 1.049 - 1.086, p < 0.001) and hypertension (OR: 1.726, 95% IC: 1.097 - 2.715, p = 0.018). No association was found between IMT (OR: 2.613, 95% IC: 0.886 - 7.708, p = 0.082) or carotid artery stenosis (OR: 1.021, 95% IC: 0.785 - 1.328, p = 0.877) and white matter hypodensities of presumed vascular origin.
DISCUSSION:
Only age and hypertension proved to have an independent association with white matter hypodensities of presumed vascular origin. Carotid atherosclerosis, evaluated by IMT and the degree of carotid artery stenosis, showed no association with white matter hypodensities of presumed vascular origin. Since atherosclerosis is a systemic pathology, these results suggest that alternative mechanisms are responsible for the development of white matter hypodensities of presumed vascular origin.
CONCLUSION:
Age and hypertension seem to be the main factors in the development of white matter hypodensities of presumed vascular origin. No association was found between carotid atherosclerosis and white matter hypodensities of presumed vascular origin
Vortex and gap generation in gauge models of graphene
Effective quantum field theoretical continuum models for graphene are
investigated. The models include a complex scalar field and a vector gauge
field. Different gauge theories are considered and their gap patterns for the
scalar, vector, and fermion excitations are investigated. Different gauge
groups lead to different relations between the gaps, which can be used to
experimentally distinguish the gauge theories. In this class of models the
fermionic gap is a dynamic quantity. The finite-energy vortex solutions of the
gauge models have the flux of the "magnetic field" quantized, making the
Bohm-Aharonov effect active even when external electromagnetic fields are
absent. The flux comes proportional to the scalar field angular momentum
quantum number. The zero modes of the Dirac equation show that the gauge models
considered here are compatible with fractionalization
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