146 research outputs found
Clustering and Sharing Incentives in BitTorrent Systems
Peer-to-peer protocols play an increasingly instrumental role in Internet
content distribution. Consequently, it is important to gain a full
understanding of how these protocols behave in practice and how their
parameters impact overall performance. We present the first experimental
investigation of the peer selection strategy of the popular BitTorrent protocol
in an instrumented private torrent. By observing the decisions of more than 40
nodes, we validate three BitTorrent properties that, though widely believed to
hold, have not been demonstrated experimentally. These include the clustering
of similar-bandwidth peers, the effectiveness of BitTorrent's sharing
incentives, and the peers' high average upload utilization. In addition, our
results show that BitTorrent's new choking algorithm in seed state provides
uniform service to all peers, and that an underprovisioned initial seed leads
to the absence of peer clustering and less effective sharing incentives. Based
on our observations, we provide guidelines for seed provisioning by content
providers, and discuss a tracker protocol extension that addresses an
identified limitation of the protocol
Carbon-nitrogen interactions in European forests and semi-natural vegetation - Part 2: Untangling climatic, edaphic, management and nitrogen deposition effects on carbon sequestration potentials
The effects of atmospheric nitrogen deposition (N) on carbon (C) sequestration in forests have often been assessed by relating differences in productivity to spatial variations of N across a large geographic domain. These correlations generally suffer from covariation of other confounding variables related to climate and other growth-limiting factors, as well as large uncertainties in total (dry+wet) reactive nitrogen (N) deposition.We propose a methodology for untangling the effects of N from those of meteorological variables, soil water retention capacity and stand age, using a mechanistic forest growth model in combination with eddy covariance CO exchange fluxes from a Europe-wide network of 22 forest flux towers. Total N deposition rates were estimated from local measurements as far as possible. The forest data were compared with data from natural or semi-natural, non-woody vegetation sites. The response of forest net ecosystem productivity to nitrogen deposition (dNEP= dN) was estimated after accounting for the effects on gross primary productivity (GPP) of the co-correlates by means of a meta-modelling standardization procedure, which resulted in a reduction by a factor of about 2 of the uncorrected, apparent dGPP/dN value. This model-enhanced analysis of the C and N flux observations at the scale of the European network suggests a mean overall dNEP/dN response of forest lifetime C sequestration to N of the order of 40–50 g C per g N, which is slightly larger but not significantly different from the range of estimates published in the most recent reviews. Importantly, patterns of gross primary and net ecosystem productivity versus N were non-linear, with no further growth responses at high N levels (N >2.5–3 gNm yr) but accompanied by increasingly large ecosystem N losses by leaching and gaseous emissions. The reduced increase in productivity per unit N deposited at high N levels implies that the forecast increased N emissions and increased Ndep levels in large areas of Asia may not positively impact the continent’s forest CO sink. The large level of unexplained variability in observed carbon sequestration efficiency (CSE) across sites further adds to the uncertainty in the dC/dN response
Diagnosis of aortic graft infection : a case definition by the management of aortic graft infection collaboration (MAGIC)
Objective/Background
The management of aortic graft infection (AGI) is highly complex and in the absence of a universally accepted case definition and evidence-based guidelines, clinical approaches and outcomes vary widely. The objective was to define precise criteria for diagnosing AGI.
Methods
A process of expert review and consensus, involving formal collaboration between vascular surgeons, infection specialists, and radiologists from several English National Health Service hospital Trusts with large vascular services (Management of Aortic Graft Infection Collaboration [MAGIC]), produced the definition.
Results
Diagnostic criteria from three categories were classified as major or minor. It is proposed that AGI should be suspected if a single major criterion or two or more minor criteria from different categories are present. AGI is diagnosed if there is one major plus any criterion (major or minor) from another category. (i) Clinical/surgical major criteria comprise intraoperative identification of pus around a graft and situations where direct communication between the prosthesis and a nonsterile site exists, including fistulae, exposed grafts in open wounds, and deployment of an endovascular stent-graft into an infected field (e.g., mycotic aneurysm); minor criteria are localized AGI features or fever ≥38°C, where AGI is the most likely cause. (ii) Radiological major criteria comprise increasing perigraft gas volume on serial computed tomography (CT) imaging or perigraft gas or fluid (≥7 weeks and ≥3 months, respectively) postimplantation; minor criteria include other CT features or evidence from alternative imaging techniques. (iii) Laboratory major criteria comprise isolation of microorganisms from percutaneous aspirates of perigraft fluid, explanted grafts, and other intraoperative specimens; minor criteria are positive blood cultures or elevated inflammatory indices with no alternative source.
Conclusion
This AGI definition potentially offers a practical and consistent diagnostic standard, essential for comparing clinical management strategies, trial design, and developing evidence-based guidelines. It requires validation that is planned in a multicenter, clinical service database supported by the Vascular Society of Great Britain & Ireland
Development and validation of a questionnaire assessing volitional competencies to enhance the performance of physical activities in chronic low back pain patients
BACKGROUND: Motivation has long been emphasized as the most important determinant of action. However, there is a substantial gap between people's goals and their attainment. Patients may be motivated and yet unable to take action if their volitional competencies are insufficient. One of the important tasks of volition is goal-maintenance. Research has stressed the importance of a volitional tool, the implementation intentions. Implementation intentions indicate where, when, and how the action leading to the goal will be performed. Forming implementation intentions favours the execution of goal-directed efforts, and reinforces the relationship between intentions and behaviours. Results from various studies clearly suggest that volitional competencies and implementation intentions could play a role in low back pain (LBP) patients. However, there is at present no questionnaire allowing assessing the capacity of implementation intentions of physical activities in LBP patients. METHODS/DESIGN: This study will develop such a questionnaire, using a 3-step approach. A first qualitative step to build categories and generate items; 30 patients suffering chronic LBP will be invited to participate in semi-structured interviews; verbatim and derived items will then be submitted to a panel of experts, using a Delphi method; a second quantitative step to examine the properties of items, and determine the factorial structure of the questionnaire; 100 patients suffering chronic LBP will be recruited to respond to this phase; and third, preliminary psychometric analyses (item-scale correlations, construct validity, reliability); 180 chronic LBP patients will be recruited for this phase of the study. The relationships between implementation intentions and variables affecting physical activity on chronic LBP patients, i.e. pain, physical capacities, fear-avoidance beliefs, kinesiophobia, work status, and level of physical activity will be considered. DISCUSSION: Developing a questionnaire to assess implementation intentions would allow investigating the role of these intentions in the transition from acute to chronic LBP. The results of this study should contribute to the understanding of the psychological processes at stake in the development of chronic LBP, and in particular to the identification of factors eventually favouring patients' participation in and adherence to active physical treatments
Carbon-nitrogen interactions in European forests and semi-natural vegetation - Part 1: Fluxes and budgets of carbon, nitrogen and greenhouse gases from ecosystem monitoring and modelling
The impact of atmospheric reactive nitrogen (N) deposition on carbon (C) sequestration in soils and biomass of unfertilized, natural, semi-natural and forest ecosystems has been much debated. Many previous results of this dC/dN response were based on changes in carbon stocks from periodical soil and ecosystem inventories, associated with estimates of N deposition obtained from large-scale chemical transport models. This study and a companion paper (Flechard et al., 2020) strive to reduce uncertainties of N effects on C sequestration by linking multi-annual gross and net ecosystem productivity estimates from 40 eddy covariance flux towers across Europe to local measurement-based estimates of dry and wet N deposition from a dedicated collocated monitoring network. To identify possible ecological drivers and processes affecting the interplay between C and N inputs and losses, these data were also combined with in situ flux measurements of NO, NO and CH fluxes; soil NO̅ leaching sampling; and results of soil incubation experiments for N and greenhouse gas (GHG) emissions, as well as surveys of available data from online databases and from the literature, together with forest ecosystem (BASFOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from -70 to 826 gCm yr at total wet+dry inorganic N deposition rates (N) of 0.3 to 4.3 gNm yr and from -4 to 361 g Cm yr at N rates of 0.1 to 3.1 gNm yr in short semi-natural vegetation (moorlands, wetlands and unfertilized extensively managed grasslands). The GHG budgets of the forests were strongly dominated by CO exchange, while CH and NO exchange comprised a larger proportion of the GHG balance in short semi-natural vegetation. Uncertainties in elemental budgets were much larger for nitrogen than carbon, especially at sites with elevated N where N leaching losses were also very large, and compounded by the lack of reliable data on organic nitrogen and N losses by denitrification. Nitrogen losses in the form of NO, NO and especially NO̅ were on average 27%(range 6 %–54 %) of N at sites with N 3 gNm yr. Such large levels of N loss likely indicate that different stages of N saturation occurred at a number of sites. The joint analysis of the C and N budgets provided further hints that N saturation could be detected in altered patterns of forest growth. Net ecosystem productivity increased with N deposition up to 2–2.5 gNm yr, with large scatter associated with a wide range in carbon sequestration efficiency (CSE, defined as the NEP = GPP ratio). At elevated N levels (> 2.5 gNm yr), where inorganic N losses were also increasingly large, NEP levelled off and then decreased. The apparent increase in NEP at low to intermediate N levels was partly the result of geographical cross-correlations between N and climate, indicating that the actual mean dC/dN response at individual sites was significantly lower than would be suggested by a simple, straightforward regression of NEP vs. N
Intra-abdominal aortic graft infection: prognostic factors associated with in-hospital mortality
Inter-laboratory comparison of cryogenic water extraction systems for stable isotope analysis of soil water
For more than two decades, research groups in hydrology, ecology, soil
science, and biogeochemistry have performed cryogenic water extractions (CWEs) for
the analysis of δ2H and δ18O of soil water.
Recent studies have shown that extraction conditions (time, temperature, and
vacuum) along with physicochemical soil properties may affect extracted soil
water isotope composition. Here we present results from the first worldwide
round robin laboratory intercomparison. We test the null hypothesis that, with
identical soils, standards, extraction protocols, and isotope analyses,
cryogenic extractions across all laboratories are identical. Two standard
soils with different physicochemical characteristics along with deionized
(DI) reference water of known isotopic composition were shipped to 16
participating laboratories. Participants oven-dried and rewetted the soils to
8 and 20 % gravimetric water content (WC), using the deionized reference
water. One batch of soil samples was extracted via predefined extraction
conditions (time, temperature, and vacuum) identical to all laboratories; the
second batch was extracted via conditions considered routine in the
respective laboratory. All extracted water samples were analyzed for
δ18O and δ2H by the lead laboratory (Global
Institute for Water Security, GIWS, Saskatoon, Canada) using both a laser and an
isotope ratio mass spectrometer (OA-ICOS and IRMS, respectively). We rejected
the null hypothesis. Our results showed large differences in retrieved
isotopic signatures among participating laboratories linked to soil type and
soil water content with mean differences compared to the reference water ranging from
+18.1 to −108.4 ‰ for δ2H and +11.8 to
−14.9 ‰ for δ18O across all laboratories. In
addition, differences were observed between OA-ICOS and IRMS isotope data.
These were related to spectral interferences during OA-ICOS analysis that are
especially problematic for the clayey loam soils used. While the types of
cryogenic extraction lab construction varied from manifold systems to single
chambers, no clear trends between system construction, applied extraction
conditions, and extraction results were found. Rather, observed differences
in the isotope data were influenced by interactions between multiple factors
(soil type and properties, soil water content, system setup, extraction
efficiency, extraction system leaks, and each lab's internal accuracy). Our
results question the usefulness of cryogenic extraction as a standard for
water extraction since results are not comparable across laboratories. This
suggests that defining any sort of standard extraction procedure applicable
across laboratories is challenging. Laboratories might have to establish
calibration functions for their specific extraction system for each natural
soil type, individually.</p
Effect of antecedent conditions and fixed rock fragment coverage on soil erosion dynamics through multiple rainfall events
Influence de l’hydromorphie sur la variabilité des stocks de carbone à l’échelle de versants dans une hêtraie (forêt de Fougères — Ouest de la France)
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