70,785 research outputs found
Salient Regions for Query by Image Content
Much previous work on image retrieval has used global features such as colour and texture to describe the content of the image. However, these global features are insufficient to accurately describe the image content when different parts of the image have different characteristics. This paper discusses how this problem can be circumvented by using salient interest points and compares and contrasts an extension to previous work in which the concept of scale is incorporated into the selection of salient regions to select the areas of the image that are most interesting and generate local descriptors to describe the image characteristics in that region. The paper describes and contrasts two such salient region descriptors and compares them through their repeatability rate under a range of common image transforms. Finally, the paper goes on to investigate the performance of one of the salient region detectors in an image retrieval situation
An approach for jointly modeling multivariate longitudinal measurements and discrete time-to-event data
In many medical studies, patients are followed longitudinally and interest is
on assessing the relationship between longitudinal measurements and time to an
event. Recently, various authors have proposed joint modeling approaches for
longitudinal and time-to-event data for a single longitudinal variable. These
joint modeling approaches become intractable with even a few longitudinal
variables. In this paper we propose a regression calibration approach for
jointly modeling multiple longitudinal measurements and discrete time-to-event
data. Ideally, a two-stage modeling approach could be applied in which the
multiple longitudinal measurements are modeled in the first stage and the
longitudinal model is related to the time-to-event data in the second stage.
Biased parameter estimation due to informative dropout makes this direct
two-stage modeling approach problematic. We propose a regression calibration
approach which appropriately accounts for informative dropout. We approximate
the conditional distribution of the multiple longitudinal measurements given
the event time by modeling all pairwise combinations of the longitudinal
measurements using a bivariate linear mixed model which conditions on the event
time. Complete data are then simulated based on estimates from these pairwise
conditional models, and regression calibration is used to estimate the
relationship between longitudinal data and time-to-event data using the
complete data. We show that this approach performs well in estimating the
relationship between multivariate longitudinal measurements and the
time-to-event data and in estimating the parameters of the multiple
longitudinal process subject to informative dropout. We illustrate this
methodology with simulations and with an analysis of primary biliary cirrhosis
(PBC) data.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS339 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Stability Issues for w < -1 Dark Energy
Precision cosmological data hint that a dark energy with equation of state and hence dubious stability is viable. Here we discuss for any
nucleation from to in a first-order phase
transition. The critical radius is argued to be at least of galactic size and
the corresponding nucleation rate is glacial, thus underwriting the dark
energy's stability and rendering remote any microscopic effect.Comment: 9 pages LaTeX. Significantly rewritten (including abstract
Universe Models with a Variable Cosmological "Constant" and a "Big Bounce"
We present a rich class of exact solutions which contains radiation-dominated
and matter-dominated models for the early and late universe. They include a
variable cosmological ``constant'' which is derived from a higher dimension and
manifests itself in spacetime as an energy density for the vacuum. This is in
agreement with observational data and is compatible with extensions of general
relativity to string and membrane theory. Our solutions are also typified by a
non-singular ``big bounce'' (as opposed to a singular big bang), where matter
is created as in inflationary cosmology.Comment: 17 pages, 2 figures, AASTEX. To appear in Ap
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