4,248 research outputs found
ON THE EXPRESSIVE POWER OF THE RELATIONAL MODEL: A DATABASE DESIGNER\u27S POINT OF VIEW
The purpose of this paper is to introduce a framework for assessing the expressive power of data models and to apply this framework to the relational model of data. From a designer\u27s point of view, a data model such as the relational model should not only be formally defined and easy to understand, but should also provide a powerful set of constructs to model real-world phenomena. The expressive power of a data model, defined as the degree to which its constructs match with constructs encountered in reality, can be judged by two complementary principles: the interpretation principle and the representation principle. It is asserted that database designers attempt to minimize the number of ad hoc database constraints, and that a data model faithful to the two principles supports this design strategy. Subsequently, this constraint minimization strategy is used to assess the expressive power of a particular data model, i.e., the relational data model. The authors take the position that the expressive power of the relational model is not optimal, due to a lack of adherence to both the interpretation principle and the representation principle. The paper amplifies this position by means of a number of examples, all based on publications by Codd and Date
The social cognition of medical knowledge, with special reference to childhood epilepsy
This paper arose out of an engagement in medical communication courses at a Gulf university. It deploys a theoretical framework derived from a (critical) sociocognitive approach to discourse analysis in order to investigate three aspects of medical discourse relating to childhood epilepsy: the cognitive processes that are entailed in relating different types of medical knowledge to their communicative context; the types of medical knowledge that are constituted in the three different text types analysed; and the relationship between these different types of medical knowledge and the discursive features of each text type. The paper argues that there is a cognitive dimension to the human experience of understanding and talking about one specialized from of medical knowledge. It recommends that texts be studied in medical communication courses not just in terms of their discrete formal features but also critically, in terms of the knowledge which they produce, transmit and reproduce
Long-Term Flux Monitoring of LSI +61 303 at 2.25 and 8.3 GHz
LSI +61 303 is an exotic binary system consisting of a ~10 Msun B star and a
compact object which is probably a neutron star. The system is associated with
the interesting radio source GT0236+610 that exhibits bright radio outbursts
with a period of 26.5 days. We report the results of continuous daily radio
interferometric observations of GT0236+610 at 2.25 and 8.3 GHz from 1994
January to 1996 February. The observations cover 25 complete (and 3 partial)
cycles with multiple observations each day. We detect substantial
cycle-to-cycle variability of the radio emission characterized by a rapid onset
of the radio flares followed by a more gradual decrease of the emission. We
detect a systematic change of the radio spectral index alpha which typically
becomes larger than zero at the onset of the radio outbursts. This behavior is
suggestive of expansion of material initially optically thick to radio
frequencies, indicating either that synchrotron or inverse Compton cooling are
important or that the free-free optical depth to the source is rapidly
changing. After two years of observations, we see only weak evidence for the
proposed 4-year periodic modulation in the peak flux of the outbursts. We
observe a secular trend in the outburst phases according the the best published
ephemeris. This trend indicates either orbital period evolution, or a drift in
outburst orbital phase in response to some other change in the system.Comment: 23 pages, LaTex, 7 figures, to appear in ApJ, v491, Dec 10th issue,
for associated info and preprints see
http://www.srl.caltech.edu/personnel/paulr/lsi.htm
Assessing the accuracy of blending Landsat-MODIS surface reflectances in two landscapes with contrasting spatial and temporal dynamics: A framework for algorithm selection
Blending algorithms model land cover change by using highly resolved spatial data from one sensor and highly resolved temporal data from another. Because the data are not usually observed concurrently, unaccounted spatial and temporal variances cause error in blending algorithms, yet, to date, there has been no definitive assessment of algorithm performance against spatial and temporal variances. Our objectives were to: (i) evaluate the accuracy of two advanced blending algorithms (STARFM and ESTARFM) and two simple benchmarking algorithms in two landscapes with contrasting spatial and temporal variances; and (ii) synthesise the spatial and temporal conditions under which the algorithms performed best. Landsat-like images were simulated on 27 dates in total using the nearest temporal cloud-free Landsat-MODIS pairs to the simulation date, one before and one after. RMSD, bias, and r2 estimates between simulated and observed Landsat images were calculated, and overall variance of Landsat and MODIS datasets were partitioned into spatial and temporal components. Assessment was performed over the whole study site, and for specific land covers. Results addressing objective (i) were that: ESTARFM did not always produce lower errors than STARFM; STARFM and ESTARFM did not always produce lower errors than simple benchmarking algorithms; and land cover spatial and temporal variances were strongly associated with algorithm performance. Results addressing objective (ii) indicated ESTARFM was superior where/when spatial variance was dominant; and STARFM was superior where/when temporal variance was dominant. We proposed a framework for selecting blending algorithms based on partitioning variance into the spatial and temporal components and suggested that comparing Landsat and MODIS spatial and temporal variances was a practical method to determine if, and when, MODIS could add value for blending
The first COMPTEL Source Catalogue
The imaging Compton telescope COMPTEL aboard NASA's Compton Gamma-Ray
Observatory has opened the MeV gamma-ray band as a new window to astronomy.
COMPTEL provided the first complete all-sky survey in the energy range 0.75 to
30 MeV. The catalogue, presented here, is largely restricted to published
results. It contains firm as well as marginal detections of continuum and line
emitting sources and presents upper limits for various types of objects. The
numbers of the most significant detections are 32 for steady sources and 31 for
gamma-ray bursters. Among the continuum sources, detected so far, are spin-down
pulsars, stellar black-hole candidates, supernova remnants, interstellar
clouds, nuclei of active galaxies, gamma-ray bursters, and the Sun during solar
flares. Line detections have been made in the light of the 1.809 MeV 26Al line,
the 1.157 MeV 44Ti line, the 847 and 1238 keV 56Co lines, and the neutron
capture line at 2.223 MeV. For the identification of galactic sources, a
modelling of the diffuse galactic emission is essential. Such a modelling at
this time does not yet exist at the required degree of accuracy. Therefore, a
second COMPTEL source catalogue will be produced after a detailed and accurate
modelling of the diffuse interstellar emission has become possible.Comment: 50 pages including 4 figures; accepted for publication in A&A
Supplement
COMPTEL observations of the inner galaxy
This paper presents a first global study of COMPTEL observations of the inner Galaxy in the energy range 0.75–10 MeV. Preliminary findings demonstrate COMPTEL’s capabilities for mapping the observed gamma radiation and disentangling the contributions from point sources and diffuse emission
Participatory Militias: An Analysis of an Armed Movement's Online Audience
Armed groups of civilians known as "self-defense forces" have ousted the
powerful Knights Templar drug cartel from several towns in Michoacan. This
militia uprising has unfolded on social media, particularly in the "VXM"
("Valor por Michoacan," Spanish for "Courage for Michoacan") Facebook page,
gathering more than 170,000 fans. Previous work on the Drug War has documented
the use of social media for real-time reports of violent clashes. However, VXM
goes one step further by taking on a pro-militia propagandist role, engaging in
two-way communication with its audience. This paper presents a descriptive
analysis of VXM and its audience. We examined nine months of posts, from VXM's
inception until May 2014, totaling 6,000 posts by VXM administrators and more
than 108,000 comments from its audience. We describe the main conversation
themes, post frequency and relationships with offline events and public
figures. We also characterize the behavior of VXM's most active audience
members. Our work illustrates VXM's online mobilization strategies, and how its
audience takes part in defining the narrative of this armed conflict. We
conclude by discussing possible applications of our findings for the design of
future communication technologies.Comment: Participatory Militias: An Analysis of an Armed Movement's Online
Audience. Saiph Savage, Andres Monroy-Hernandez. CSCW: ACM Conference on
Computer-Supported Cooperative Work 201
Meeting the Challenge of Scientific Dissemination in the Era of COVID-19:Toward a Modular Approach to Knowledge-Sharing for Radiation Oncology
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