Nested Archimedean copulas: a new class of nonparametric tree structure estimators

Any nested Archimedean copula is defined starting from a rooted phylogenetic tree, for which a new class of nonparametric estimators is presented. An estimator from this new class relies on a two-step procedure where first a binary tree is built and second is collapsed if necessary to give an estimate of the target tree structure. Several examples of estimators from this class are given and the performance of each of these estimators, as well as of the only known comparable estimator, is assessed by means of a simulation study involving target structures in various dimensions, showing that the new estimators, besides being faster, usually offer better performance as well. Further, among the given examples of estimators from the new class, one of the best performing one is applied on three datasets: 482 students and their results to various examens, 26 European countries in 1979 and the percentage of workers employed in different economic activities, and 104 countries in 2002 for which various health-related variables are available. The resulting estimated trees offer valuable insights on the analyzed data. The future of nested Archimedean copulas in general is also discussed

Nonparametric estimation of the tree structure of a nested Archimedean copula

One of the features inherent in nested Archimedean copulas, also called hierarchical Archimedean copulas, is their rooted tree structure. A nonparametric, rank-based method to estimate this structure is presented. The idea is to represent the target structure as a set of trivariate structures, each of which can be estimated individually with ease. Indeed, for any three variables there are only four possible rooted tree structures and, based on a sample, a choice can be made by performing comparisons between the three bivariate margins of the empirical distribution of the three variables. The set of estimated trivariate structures can then be used to build an estimate of the target structure. The advantage of this estimation method is that it does not require any parametric assumptions concerning the generator functions at the nodes of the tree.Comment: 25 pages, 9 figure

Learning to Reach Agreement in a Continuous Ultimatum Game

It is well-known that acting in an individually rational manner, according to the principles of classical game theory, may lead to sub-optimal solutions in a class of problems named social dilemmas. In contrast, humans generally do not have much difficulty with social dilemmas, as they are able to balance personal benefit and group benefit. As agents in multi-agent systems are regularly confronted with social dilemmas, for instance in tasks such as resource allocation, these agents may benefit from the inclusion of mechanisms thought to facilitate human fairness. Although many of such mechanisms have already been implemented in a multi-agent systems context, their application is usually limited to rather abstract social dilemmas with a discrete set of available strategies (usually two). Given that many real-world examples of social dilemmas are actually continuous in nature, we extend this previous work to more general dilemmas, in which agents operate in a continuous strategy space. The social dilemma under study here is the well-known Ultimatum Game, in which an optimal solution is achieved if agents agree on a common strategy. We investigate whether a scale-free interaction network facilitates agents to reach agreement, especially in the presence of fixed-strategy agents that represent a desired (e.g. human) outcome. Moreover, we study the influence of rewiring in the interaction network. The agents are equipped with continuous-action learning automata and play a large number of random pairwise games in order to establish a common strategy. From our experiments, we may conclude that results obtained in discrete-strategy games can be generalized to continuous-strategy games to a certain extent: a scale-free interaction network structure allows agents to achieve agreement on a common strategy, and rewiring in the interaction network greatly enhances the agents ability to reach agreement. However, it also becomes clear that some alternative mechanisms, such as reputation and volunteering, have many subtleties involved and do not have convincing beneficial effects in the continuous case

Challenges in food safety as part of food security : lessons learnt on food safety in a globalized world

Food safety should accomplish food and nutrition security. A key challenge to scale up food safety globally is to better leverage existing capacity and research working towards evidence-based decisions. At Ghent University since 2009 an annual 3-months international Intensive Training Program on Food Safety, Quality Assurance and Risk Analysis has been organized (www.itpfoodsafety.UGent.be). The trainees were asked to express their opinion on food safety concerns in their country and to select a case study to work on throughout the course. Main food safety issues had to do with bacterial pathogens, pesticide residues and mycotoxins which were challenged by lack of food safety knowledge and appropriate legislation and enforcement by government. They welcomed education and training on these topics in particular to elaborate on control measures including good hygienic practices, implementation of certified food safety management systems and setting of appropriate criteria. A number of topics are highlighted here in particular as these topics were shown to have a common ground of interest by several participants in several countries and throughout the years. These topics include among others safety of street foods, safe milk and cheese production, and risk assessment to control Salmonella and pathogenic E. coli in meat (and other foods). Although some recurring food safety issues could be identified, other topics are of particular concern in selected countries because of specific cultural appropriate eating habits. The world is changing fast. Problems change and the information stream is very intense. Leaders in food security should be aware about food safety as well, and will have to develop an attitude of continuous learning, critical thinking and be given the right tools("know how") to develop local solutions to address the emerging societal and environmental challenges to provide sufficient, safe, healthy, nutritious and sustainable produced food to the world's population