1,720,999 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Quantitative 3-D GPR analysis to estimate the total volume and water content of a glacier
No full textWe apply an automated picking and inversion algorithm to a 3-D GPR data set acquired on an alpine glacieret, to study its internal stratigraphy, density distribution, total volume, and water content. GPR surveys are particularly useful for glaciological studies, since the transmitted signal can propagate efficiently through the entire glacier volume, while the large number of recorded traces makes any quantitative analysis statistically sound. The applied auto-picking algorithm is designed to accurately and objectively identify the main reflections within a GPR data set, and to characterize them in terms of their peak amplitudes, travel times, and polarities. The inversion algorithm then uses these quantities to recover the subsurface stratigraphy and EM velocity distribution along each GPR profile. In air-ice mixtures, the EM velocity is linked to the density through well-known empirical formulas. Therefore, our inversion algorithm is able to recover the density distribution within a glacier, and combine it with the internal stratigraphy to estimate its water content. By applying this procedure to a 3-D GPR data set, we can obtain an accurate model of an entire glacier, while 4-D surveys can be used to monitor its temporal changes and estimate its annual and seasonal mass balances
Combined effects of temperature and humidity on the mechanical properties of polyurethane foams
No full textThe effects of temperature and environmental moisture on the viscoelastic behavior of polyurethane foams were investigated both theoretically
and experimentally. It was shown that the effect of the environmental parameters can be explained in terms of a variation of the free volume
of the solid fraction of the foams, thus allowing the use of the superposition principle to predict their influence on the viscoelastic behavior of
the materials. Dynamic mechanical analyses were performed to measure the dependence on frequency, temperature, and relative humidity of the
complex modulus of two different polyurethane foams, differing in terms of their glass transition temperature. The time-temperature-humidity
superposition principle was proved to be applicable for the tested materials. Next, the relaxation spectra and their dependence on the relative
humidity were adopted to assess its effects on the large strain behavior of the foams
Experimental Performance of the Handover Procedure in a WiFi Network
No full textAn experimental evaluation of the handover performance
in a 802.11b (WiFi) network has been obtained in terms of available
bandwidth and handover latency. The network consists of two access
points connected to a wired LAN and a measurement setup composed
by laptop PCs, equipped with WiFi cards, on which a sniffer is used
to capture all the packets transmitted on the radio link. Applications
considered are streaming and ftp-like applications. The analysis of the
handover delay is performed, showing the dependence on the network
configuration parameters and results, both at the MAC and UDP/TCP
layer are presented, to show how applications can suffer the handover
latencies. It will be shown that these latencies can vary significantly
for the same configuration of mobile stations and APs and that the
primary source of delay is the scan procedure used to discover the
new access point
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Minimum threshold for the sampling rate to prevent amplitude distortions in aliasing-free GPR surveys
No full textWe study sampling-related amplitude distortions within aliasing-free GPR data sets, and compare them with other factors which can affect the recorded signal. In particular, we analyze how much the sampled peak amplitudes can change with different sampling rates, and recommend a minimum threshold for the sampling rate in order to contain the maximum peak amplitude error within acceptable limits. The selection of the sampling rate during data acquisition is commonly based on the Nyquist-Shannon theorem, which offers practical lower limits in order to avoid aliasing effects and to accurately preserve the spectral content of the original analog signal. However, we show that the Nyquist-Shannon theorem does not prevent possible amplitude distortions, and that significant and unrecoverable data loss can occur even in aliasing-free data sets. We also show that interpolation and re-sampling offer only limited solutions, since the accuracy of the reconstructed signal depends on the implemented interpolation method, while its subsequent re-sampling simply reintroduces the initial problem. Based on our analysis, we recommend using during data acquisition a sampling rate equal to at least 12 times the signal central frequency, which is higher than the commonly adopted standards, in order to limit the maximum peak amplitude error within 5%
Quantitative Analysis of GPR Signals: Transmitted Wavelet, Amplitude Decay, and Sampling-Related Amplitude Distortions
No full textWe study the importance of accurately recording
signal amplitudes for the quantitative analysis of GPR data sets.
Specifically, we measure the peak amplitudes of signals emitted by
GPR antennas with different central frequencies and study their
amplitude decay with distance, in order to extrapolate the peak
amplitude of the wavelet initially transmitted by each antenna. The
purpose is to compare the reference and reflected amplitudes in
order to accurately estimate the subsurface EM impedance contrasts.
Moreover, we study how sampling-related amplitude
distortions can affect the quantitative analysis, and subsequently
the resulting subsurface models, even in the absence of aliasing
effects. The well-known Nyquist–Shannon theorem gives practical
lower limits for the sampling rate in order to preserve the spectral
content of a digitized signal; however, we show that it does not
prevent possible amplitude distortions. In particular, we demonstrate
that significant and unrecoverable loss of amplitude
information occurs even at sampling rates well above the Nyquist–
Shannon threshold. Interpolation may theoretically reduce such
amplitude distortions; however, its accuracy would depend on the
implemented algorithm and it is not verifiable in real data sets,
since the actual amplitude information is limited to the sampled
values. Moreover, re-sampling the interpolated signal simply
reintroduces the initial problem, when a new sampling rate is
selected. Our analysis suggests that, in order to limit the maximum
peak amplitude error within 5%, the sampling rate selected during
data acquisition must be at least 12 times the signal central frequency,
which is higher than the commonly adopted standards
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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