481,273 research outputs found
Combining causal model and focus group discussions experiences learned from a socio-anthropological research on the differing perceptions of caretakers and health professionals on children's health (Bolivia/Peru)
The paper discusses the utility of constructing causal models in focus groups. This was experienced as a complement to an in-depth ethnographic research on the differing perceptions of caretakers and health professionals on child's growth and development in Peru and Bolivia. The rational, advantages, difficulties and necessary adaptations of combining the two techniques are discussed on the basis of concrete examples. Authors conclude that the building of a causal model in a focus group session can be useful in comparing lay etiologies of diseases as perceived by different categories of caretakers and health professionals and in identifying specific health risks faced by children. Causal model building in a focus group can help renew discussions and participants'interest but its use is only justified when the study concerns the perception of the causality of a given phenomenon
Dynamics versus replicas in the random field Ising model
In a previous article we have shown, within the replica formalism, that the
conventional picture of the random field Ising model breaks down, by the effect
of singularities in the interactions between fields involving several replicas,
below dimension eight. In the zero-replica limit several coupling constants
have thus to be considered, instead of just one. As a result we had found that
there is no stable fixed point in the vicinity of dimension six. It is natural
to reconsider the problem in a dynamical framework, which does not require
replicas, although the equilibrium properties should be recovered in the large
time limit. Singularities in the zero-replica limit are a priori not visible in
a dynamical picture. In this note we show that in fact new interactions are
also generated in the stochastic approach. Similarly these interactions are
found to be singular below dimension eight. These critical singularities
require the introduction of a time origin at which initial data are
given. The dynamical properties are thus dependent upon the waiting time. It is
shown here that one can indeed find a complete correspondence between the
equilibrium singularities in the limit, and the singularities in the
dynamics when the initial time goes to minus infinity, with replaced
by . There is thus complete coherence between the two
approaches.Comment: 8 pages, latex, no figur
Wave polarizations for a beam-like gravitational wave in quadratic curvature gravity
We compute analytically the tidal field and polarizations of an exact
gravitational wave generated by a cylindrical beam of null matter of finite
width and length in quadratic curvature gravity. We propose that this wave can
represent the gravitational wave that keep up with the high energy photons
produced in a gamma ray burst (GRB) source.Comment: 5 pages, 3 figures, minor corrections, to appear in CQ
Twist Free Energy in a Spin Glass
The field theory of a short range spin glass with Gaussian random
interactions, is considered near the upper critical dimension six. In the
glassy phase, replica symmetry breaking is accompanied with massless Goldstone
modes, generated by the breaking of reparametrization invariance of a Parisi
type solution. Twisted boundary conditions are thus imposed at two opposite
ends of the system in order to study the size dependence of the twist free
energy. A loop-expansion is performed to first order around a twisted
background. It is found, as expected but it is non trivial, that the theory
does renormalize around such backgrounds, as well as for the bulk. However two
main differences appear, in comparison with simple ferromagnetic transitions :
(i) the loop expansion yields a (negative) anomaly in the size dependence of
the free energy, thereby lifting the lower critical dimension to a value
greater than two given by (ii) the free energy is lowered
by twisting the boundary conditions. This sign may reflect a spontaneous
spatial non-uniformity of the order parameter.Comment: 15 pages, latex, no figur
Twist Free Energy
One may impose to a system with spontaneous broken symmetry, boundary
conditions which correspond to different pure states at two ends of a sample.
For a discrete Ising-like broken symmetry, boundary conditions with opposite
spins in two parallel limiting planes, generate an interface and a cost in free
energy per unit area of the interface. For continuum symmetries the order
parameter interpolates smoothly between the end planes carrying two different
directions of the order parameter. The cost in free energy is then proportional
to for a system of characteristic size L. The power of is related
to the lower critical dimension, and the coefficient of this additional free
energy vanishes at the critical temperature. In this note it is shown within a
loop expansion that one does find the expected behavior of this twist free
energy. This is a preamble to the study of situations where the broken
continuum symmetry is believed to be more complex, as in Parisi's ansatz for
the Edwards-Anderson spin glass.Comment: 15 pages, latex, no figur
Controlled Data Sharing for Collaborative Predictive Blacklisting
Although sharing data across organizations is often advocated as a promising
way to enhance cybersecurity, collaborative initiatives are rarely put into
practice owing to confidentiality, trust, and liability challenges. In this
paper, we investigate whether collaborative threat mitigation can be realized
via a controlled data sharing approach, whereby organizations make informed
decisions as to whether or not, and how much, to share. Using appropriate
cryptographic tools, entities can estimate the benefits of collaboration and
agree on what to share in a privacy-preserving way, without having to disclose
their datasets. We focus on collaborative predictive blacklisting, i.e.,
forecasting attack sources based on one's logs and those contributed by other
organizations. We study the impact of different sharing strategies by
experimenting on a real-world dataset of two billion suspicious IP addresses
collected from Dshield over two months. We find that controlled data sharing
yields up to 105% accuracy improvement on average, while also reducing the
false positive rate.Comment: A preliminary version of this paper appears in DIMVA 2015. This is
the full version. arXiv admin note: substantial text overlap with
arXiv:1403.212
Elastic-Net Regularization in Learning Theory
Within the framework of statistical learning theory we analyze in detail the
so-called elastic-net regularization scheme proposed by Zou and Hastie for the
selection of groups of correlated variables. To investigate on the statistical
properties of this scheme and in particular on its consistency properties, we
set up a suitable mathematical framework. Our setting is random-design
regression where we allow the response variable to be vector-valued and we
consider prediction functions which are linear combination of elements ({\em
features}) in an infinite-dimensional dictionary. Under the assumption that the
regression function admits a sparse representation on the dictionary, we prove
that there exists a particular ``{\em elastic-net representation}'' of the
regression function such that, if the number of data increases, the elastic-net
estimator is consistent not only for prediction but also for variable/feature
selection. Our results include finite-sample bounds and an adaptive scheme to
select the regularization parameter. Moreover, using convex analysis tools, we
derive an iterative thresholding algorithm for computing the elastic-net
solution which is different from the optimization procedure originally proposed
by Zou and HastieComment: 32 pages, 3 figure
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