1,672 research outputs found
Pairwise similarity of TopSig document signatures
This paper analyses the pairwise distances of signatures produced by the TopSig retrieval model on two document collections. The distribution of the distances are compared to purely random signatures. It explains why TopSig is only competitive with state of the art retrieval models at early precision. Only the local neighbourhood of the signatures is interpretable. We suggest this is a common property of vector space models
Document Clustering with K-tree
This paper describes the approach taken to the XML Mining track at INEX 2008
by a group at the Queensland University of Technology. We introduce the K-tree
clustering algorithm in an Information Retrieval context by adapting it for
document clustering. Many large scale problems exist in document clustering.
K-tree scales well with large inputs due to its low complexity. It offers
promising results both in terms of efficiency and quality. Document
classification was completed using Support Vector Machines.Comment: 12 pages, INEX 200
TopSig: Topology Preserving Document Signatures
Performance comparisons between File Signatures and Inverted Files for text
retrieval have previously shown several significant shortcomings of file
signatures relative to inverted files. The inverted file approach underpins
most state-of-the-art search engine algorithms, such as Language and
Probabilistic models. It has been widely accepted that traditional file
signatures are inferior alternatives to inverted files. This paper describes
TopSig, a new approach to the construction of file signatures. Many advances in
semantic hashing and dimensionality reduction have been made in recent times,
but these were not so far linked to general purpose, signature file based,
search engines. This paper introduces a different signature file approach that
builds upon and extends these recent advances. We are able to demonstrate
significant improvements in the performance of signature file based indexing
and retrieval, performance that is comparable to that of state of the art
inverted file based systems, including Language models and BM25. These findings
suggest that file signatures offer a viable alternative to inverted files in
suitable settings and from the theoretical perspective it positions the file
signatures model in the class of Vector Space retrieval models.Comment: 12 pages, 8 figures, CIKM 201
K-tree: Large Scale Document Clustering
We introduce K-tree in an information retrieval context. It is an efficient
approximation of the k-means clustering algorithm. Unlike k-means it forms a
hierarchy of clusters. It has been extended to address issues with sparse
representations. We compare performance and quality to CLUTO using document
collections. The K-tree has a low time complexity that is suitable for large
document collections. This tree structure allows for efficient disk based
implementations where space requirements exceed that of main memory.Comment: 2 pages, SIGIR 200
Random Indexing K-tree
Random Indexing (RI) K-tree is the combination of two algorithms for
clustering. Many large scale problems exist in document clustering. RI K-tree
scales well with large inputs due to its low complexity. It also exhibits
features that are useful for managing a changing collection. Furthermore, it
solves previous issues with sparse document vectors when using K-tree. The
algorithms and data structures are defined, explained and motivated. Specific
modifications to K-tree are made for use with RI. Experiments have been
executed to measure quality. The results indicate that RI K-tree improves
document cluster quality over the original K-tree algorithm.Comment: 8 pages, ADCS 2009; Hyperref and cleveref LaTeX packages conflicted.
Removed clevere
Ehrhart quasi-polynomials of almost integral polytopes
A lattice polytope translated by a rational vector is called an almost
integral polytope. In this paper we investigate Ehrhart quasi-polynomials of
almost integral polytopes. We study the relationship between the shape of the
polytopes and algebraic properties of the Ehrhart quasi-polynomials. In
particular, we prove that lattice zonotopes and centrally symmetric lattice
polytopes are characterized by Ehrhart quasi-polynomials of their rational
translations.Comment: ver 3: revisions on presentation
Перспективы развития рынка еврокапитала в современных условиях
Clinical and research staff who work around magnetic resonance imaging (MRI) scanners are exposed to the static magnetic stray fields of these scanners. Although the past decade has seen strong developments in the assessment of occupational exposure to electromagnetic fields from MRI scanners, there is insufficient insight into the exposure variability that characterizes routine MRI work practice. However, this is an essential component of risk assessment and epidemiological studies. This paper describes the results of a measurement survey of shift-based personal exposure to static magnetic fields (SMF) (B) and motion-induced time-varying magnetic fields (dB/dt) among workers at 15 MRI facilities in the Netherlands. With the use of portable magnetic field dosimeters, >400 full-shift and partial shift exposure measurements were collected among various jobs involved in clinical and research MRI. Various full-shift exposure metrics for B and motion-induced dB/dt exposure were calculated from the measurements, including instantaneous peak exposure and time-weighted average (TWA) exposures. We found strong correlations between levels of static (B) and time-varying (dB/dt) exposure (r = 0.88–0.92) and between different metrics (i.e. peak exposure, TWA exposure) to express full-shift exposure (r = 0.69–0.78). On average, participants were exposed to MRI-related SMFs during only 3.7% of their work shift. Average and peak B and dB/dt exposure levels during the work inside the MRI scanner room were highest among technical staff, research staff, and radiographers. Average and peak B exposure levels were lowest among cleaners, while dB/dt levels were lowest among anaesthesiology staff. Although modest exposure variability between workplaces and occupations was observed, variation between individuals of the same occupation was substantial, especially among research staff. This relatively large variability between workers with the same job suggests that exposure classification based solely on job title may not be an optimal grouping strategy for epidemiological purposes
Access to out-of-hospital emergency care in Africa : consensus conference recommendations
Abstract: Out-of-hospital emergency care (OHEC) should be accessible to all who require it. However available data suggests that there are a number of barriers to such access in Africa, mainly centred around challenges in public knowledge, perception and appropriate utilisation of OHEC. Having reached consensus in 2013 on a two-tier system of African OHEC, the African Federation for Emergency Medicine (AFEM) OHEC Group sought to gain further consensus on the narrower subject of access to OHEC in Africa. The objective of this paper is to report the outputs and statements arising from the AFEM OHEC access consensus meeting, held in Cape Town, South Africa in April 2015. The discussion was structured around six dimensions of access to care (awareness, availability, accessibility, accommodation, affordability and acceptability) and tackled both Tier-1 (community first responder) and Tier-2 (formal prehospital services and Emergency Medical Services) OHEC systems. In Tier-1 systems, the role of community involvement and support was emphasised, along with the importance of a first responder system acceptable to the community in which it is embedded in order to optimise access. In Tier-2 systems, the consensus group highlighted the primacy of a single toll-free emergency number , matching of Emergency Medical Services resource demand and availability through appropriate planning and the cost-free nature of Tier-2 emergency care, among other factors that impact accessibility. Much work is still needed in prioritising the steps and clarifying the tools and metrics that would enable the ideal of optimal access to OHEC in Africa
Molecular Line Profile Fitting with Analytic Radiative Transfer Models
We present a study of analytic models of starless cores whose line profiles
have ``infall asymmetry,'' or blue-skewed shapes indicative of contracting
motions. We compare the ability of two types of analytical radiative transfer
models to reproduce the line profiles and infall speeds of centrally condensed
starless cores whose infall speeds are spatially constant and range between 0
and 0.2 km s-1. The model line profiles of HCO+ (J=1-0) and HCO+ (J=3-2) are
produced by a self-consistent Monte Carlo radiative transfer code. The analytic
models assume that the excitation temperature in the front of the cloud is
either constant (``two-layer'' model) or increases inward as a linear function
of optical depth (``hill'' model). Each analytic model is matched to the line
profile by rapid least-squares fitting.
The blue-asymmetric line profiles with two peaks, or with a blue shifted peak
and a red shifted shoulder, can be well fit by the ``HILL5'' model (a five
parameter version of the hill model), with an RMS error of 0.02 km s-1. A peak
signal to noise ratio of at least 30 in the molecular line observations is
required for performing these analytic radiative transfer fits to the line
profiles.Comment: 48 pages, 20 figures, accepted for publication in Ap
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