128 research outputs found
Fixed Price Approximability of the Optimal Gain From Trade
Bilateral trade is a fundamental economic scenario comprising a strategically
acting buyer and seller, each holding valuations for the item, drawn from
publicly known distributions. A mechanism is supposed to facilitate trade
between these agents, if such trade is beneficial. It was recently shown that
the only mechanisms that are simultaneously DSIC, SBB, and ex-post IR, are
fixed price mechanisms, i.e., mechanisms that are parametrised by a price p,
and trade occurs if and only if the valuation of the buyer is at least p and
the valuation of the seller is at most p. The gain from trade is the increase
in welfare that results from applying a mechanism; here we study the gain from
trade achievable by fixed price mechanisms. We explore this question for both
the bilateral trade setting, and a double auction setting where there are
multiple buyers and sellers. We first identify a fixed price mechanism that
achieves a gain from trade of at least 2/r times the optimum, where r is the
probability that the seller's valuation does not exceed the buyer's valuation.
This extends a previous result by McAfee. Subsequently, we improve this
approximation factor in an asymptotic sense, by showing that a more
sophisticated rule for setting the fixed price results in an expected gain from
trade within a factor O(log(1/r)) of the optimal gain from trade. This is
asymptotically the best approximation factor possible. Lastly, we extend our
study of fixed price mechanisms to the double auction setting defined by a set
of multiple i.i.d. unit demand buyers, and i.i.d. unit supply sellers. We
present a fixed price mechanism that achieves a gain from trade that achieves
for all epsilon > 0 a gain from trade of at least (1-epsilon) times the
expected optimal gain from trade with probability 1 - 2/e^{#T epsilon^2 /2},
where #T is the expected number of trades resulting from the double auction
Sistema de fixação dinĂąmica de coluna lombar Dynesys: experiĂȘncia clĂnica em 30 pacientes num perĂodo mĂ©dio de 1 ano
The Impact of Online Social Networks on Decision Support Systems
Previous research on this matter had already determined that many
concepts are encompassed by both online social networking and decision support
systems research. Due to the large number of concepts and using clustering
techniques, we were able to determine four concept clusters, namely: the
technical infrastructure, online communities, network analysis and knowledge
management. Then, we intended to gain further knowledge on how those
concepts influenced DSS related research and the contribution of each cluster to
the support of the phases of decision-making process. We also wanted to perceive
the interconnections among the concept clusters themselves, for which we used
structural equation modeling techniques.
The obtained results evidence that not only online social networks are being used
as a technical infrastructure to support the three decision making phases and to
support knowledge management and online communities, but also that the other
clusters only regard the intelligence phase of the decision process.info:eu-repo/semantics/publishedVersio
Total disc replacement surgery for symptomatic degenerative lumbar disc disease: a systematic review of the literature
The objective of this study is to evaluate the effectiveness and safety of total disc replacement surgery compared with spinal fusion in patients with symptomatic lumbar disc degeneration. Low back pain (LBP), a major health problem in Western countries, can be caused by a variety of pathologies, one of which is degenerative disc disease (DDD). When conservative treatment fails, surgery might be considered. For a long time, lumbar fusion has been the âgold standardâ of surgical treatment for DDD. Total disc replacement (TDR) has increased in popularity as an alternative for lumbar fusion. A comprehensive systematic literature search was performed up to October 2008. Two reviewers independently checked all retrieved titles and abstracts, and relevant full text articles for inclusion. Two reviewers independently assessed the risk of bias of included studies and extracted relevant data and outcomes. Three randomized controlled trials and 16 prospective cohort studies were identified. In all three trials, the total disc replacement was compared with lumbar fusion techniques. The CharitĂ© trial (designed as a non-inferiority trail) was considered to have a low risk of bias for the 2-year follow up, but a high risk of bias for the 5-year follow up. The CharitĂ© artificial disc was non-inferior to the BAKÂź Interbody Fusion System on a composite outcome of âclinical successâ (57.1 vs. 46.5%, for the 2-year follow up; 57.8 vs. 51.2% for the 5-year follow up). There were no statistically significant differences in mean pain and physical function scores. The Prodisc artificial disc (also designed as a non-inferiority trail) was found to be statistically significant more effective when compared with the lumbar circumferential fusion on the composite outcome of âclinical successâ (53.4 vs. 40.8%), but the risk of bias of this study was high. Moreover, there were no statistically significant differences in mean pain and physical function scores. The Flexicore trial, with a high risk of bias, found no clinical relevant differences on pain and physical function when compared with circumferential spinal fusion at 2-year follow up. Because these are preliminary results, in addition to the high risk of bias, no conclusions can be drawn based on this study. In general, these results suggest that no clinical relevant differences between the total disc replacement and fusion techniques. The overall success rates in both treatment groups were small. Complications related to the surgical approach ranged from 2.1 to 18.7%, prosthesis related complications from 2.0 to 39.3%, treatment related complications from 1.9 to 62.0% and general complications from 1.0 to 14.0%. Reoperation at the index level was reported in 1.0 to 28.6% of the patients. In the three trials published, overall complication rates ranged from 7.3 to 29.1% in the TDR group and from 6.3 to 50.2% in the fusion group. The overall reoperation rate at index-level ranged from 3.7 to 11.4% in the TDR group and from 5.4 to 26.1% in the fusion group. In conclusion, there is low quality evidence that the CharitĂ© is non-inferior to the BAK cage at the 2-year follow up on the primary outcome measures. For the 5-year follow up, the same conclusion is supported only by very low quality evidence. For the ProDisc, there is very low quality evidence for contradictory results on the primary outcome measures when compared with anterior lumbar circumferential fusion. High quality randomized controlled trials with relevant control group and long-term follow-up is needed to evaluate the effectiveness and safety of TDR
Rationality, Irrationality and Escalating Behavior in Lowest Unique Bid Auctions
Information technology has revolutionized the traditional structure of markets. The removal of geographical and time constraints has fostered the growth of online auction markets, which now include millions of economic agents worldwide and annual transaction volumes in the billions of dollars. Here, we analyze bid histories of a little studied type of online auctions â lowest unique bid auctions. Similarly to what has been reported for foraging animals searching for scarce food, we find that agents adopt LĂ©vy flight search strategies in their exploration of âbid spaceâ. The LĂ©vy regime, which is characterized by a power-law decaying probability distribution of step lengths, holds over nearly three orders of magnitude. We develop a quantitative model for lowest unique bid online auctions that reveals that agents use nearly optimal bidding strategies. However, agents participating in these auctions do not optimize their financial gain. Indeed, as long as there are many auction participants, a rational profit optimizing agent would choose not to participate in these auction markets
What should an ideal spinal injury classification system consist of? A methodological review and conceptual proposal for future classifications
Since Böhler published the first categorization of spinal injuries based on plain radiographic examinations in 1929, numerous classifications have been proposed. Despite all these efforts, however, only a few have been tested for reliability and validity. This methodological, conceptual review summarizes that a spinal injury classification system should be clinically relevant, reliable and accurate. The clinical relevance of a classification is directly related to its content validity. The ideal content of a spinal injury classification should only include injury characteristics of the vertebral column, is primarily based on the increasingly routinely performed CT imaging, and is clearly distinctive from severity scales and treatment algorithms. Clearly defined observation and conversion criteria are crucial determinants of classification systemsâ reliability and accuracy. Ideally, two principle spinal injury characteristics should be easy to discern on diagnostic images: the specific location and morphology of the injured spinal structure. Given the current evidence and diagnostic imaging technology, descriptions of the mechanisms of injury and ligamentous injury should not be included in a spinal injury classification. The presence of concomitant neurologic deficits can be integrated in a spinal injury severity scale, which in turn can be considered in a spinal injury treatment algorithm. Ideally, a validation pathway of a spinal injury classification system should be completed prior to its clinical and scientific implementation. This review provides a methodological concept which might be considered prior to the synthesis of new or modified spinal injury classifications
Governance and assessment insights in Information Technology: the Val IT Model
The purpose of this paper is the investigation of the relationship between Information Technology and value creation. In doing so, the paper presents a conceptual framework based on the Val IT model. This framework can assist companies in the selection, implementation and optimization of IT investments to create long-term value. The paper is based on a qualitative approach with the inclusion of a single-method approach. Research is developed according to the study of national and international literature. Following an initial analysis of existing literature on the evaluation of company investments, research has been conducted through the Val IT method to assess these intangible assets. The sources of research are secondary in nature (documents, reports, newspaper articles, papers and scientific books). This methodology emphasises the connection between the modern services created in the knowledge economy and the application of new technologies, especially in the field of Information Technology. Val IT is a governance framework for creating business value from IT investments. It allows companies to increase the possibility of selecting investments with a high potential of value creation, as well as with a greater possibility of success in executing, creating, strengthening and using these services
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