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Browsing by Issue Date, starting with "2016-08-01"

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  • Comparing two power supplies for fast-field cycling nuclear magnetic resonance relaxometers power losses and performance
    Publication . Lima, Marco; Pereira, Bruno; Sousa, Duarte M.; Roque, António; Margato, Elmano
    The main power supply of a Fast Field-Cycling Nuclear Magnetic Resonance (FFC-NMR) is the key element comparing the performance of different solutions. The power supply is a current source that supplies a magnet being the current controlled in order to perform adjustable and repetitive current cycles. This power supply can be based on different topologies, operating principles and controlled using distinct techniques. If for the final users of this experimental technique the current cycles of the equipment is the core feature, for the developers also the power losses distribution needs to be analyzed in order to develop efficient solutions. In this paper, the power losses and the dynamic behavior of two solutions for the FFC-NMR power supply are compared and discussed.
    2016-08-01Conference object Restricted access Show more
  • Clustering stability and ground truth: numerical experiments
    Publication . Amorim, Maria José; Cardoso, Maria Margarida
    Stability has been considered an important property for evaluating clustering solutions. Nevertheless, there are no conclusive studies on the relationship between this property and the capacity to recover clusters inherent to data ("ground truth"). This study focuses on this relationship resorting to synthetic data generated under diverse scenarios (controlling relevant factors). Stability is evaluated using a weighted cross-validation procedure. Indices of agreement (corrected for agreement by chance) are used both to assess stability and external validation. The results obtained reveal a new perspective so far not mentioned in the literature. Despite the clear relationship between stability and external validity when a broad range of scenarios is considered, within-scenarios conclusions deserve our special attention: faced with a specific clustering problem (as we do in practice), there is no significant relationship between stability and the ability to recover data clusters.
    2016-08-01Conference object Restricted access Show more