Computer aids for process model-building

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Échantillonnage préférentiel et méta-modèles : méthodes bayésiennes optimale et défensive

National audienceThis paper considers the problem of the choice of an instrumental distribution for the estimation by preferential sampling of an integral $\int h(x) \pi(x)\, {\rm d}x$, where the function $h$ is expensive to evaluate. To build such an instrumental distribution, we adopt a Bayesian approach where we introduce a prior about $h$, which makes it possible to construct a Bayes-optimal instrumental distribution, given evaluation results of $h$. This instrumental distribution minimizes the expectation (with respect to the prior distribution about $h$) of the (frequentist) variance of estimation. However, the variance of estimation obtained using this instrumental distribution can be larger than that of the Monte Carlo estimator in some cases. We present a "defensive" correction of the Bayes-optimal instrumental distribution to address this issue. We illustrate the approach on a problem of estimation of a probability of failure, based on an industrial numerical simulator from the domain of structural reliability.Cet article considère le problème du choix d'une loi instrumentale pour l'estimation par échantillonnage préférentiel d'une intégrale du type $\int h(x) \pi(x)\, {\rm d}x$, la fonction $h$ étant coûteuse à évaluer. Pour construire une telle loi instrumentale, nous adoptons une approche bayésienne consistant à introduire un a priori sur $h$, ce qui permet, étant donnés des résultats d'évaluations de $h$, de construire une loi instrumentale optimale au sens bayésien. Cette loi instrumentale, bien que permettant de réduire l'espérance (par rapport à la loi a priori) de la variance fréquentiste d’estimation, peut conduire dans certains cas à une variance supérieure à celle de l'estimateur de Monte Carlo. Nous proposons une correction, dite "défensive", de la loi optimale bayésienne pour remédier à ce problème. Une application à l'estimation d'une probabilité de défaillance, à partir d'un code industriel issu du domaine de la fiabilité des structures, illustre l'approche proposée

Dynamics of long-term colonization of respiratory tract by Haemophilus influenzae in cystic fibrosis patients shows a marked increase in hypermutable strains

The persistence and variability of 188 Haemophilus influenzae isolates in respiratory tract of 30 cystic fibrosis (CF) patients over the course of 7 years was studied. Antibiotic susceptibility testing, DNA fingerprinting, and analysis of outer membrane protein profiles were performed on all isolates. A total of 115 distinct pulsed-field gel electrophoresis profiles were identified. Ninety percent of patients were cocolonized with two or more clones over the studied period. A third of the patients were cross-colonized with one or two H. influenzae strains; 11% of the clones persisted for 3 or more months. Biotype, outer membrane protein profiles, and resistance profiles showed variation along the studied period, even in persisting clones. Four isolates (2.1%) recovered from 3 patients were type f capsulate, with three of them belonging to the same clone. beta-Lactamase production was detected in 23.9% of isolates while 7% of the beta-lactamase-negative isolates presented diminished susceptibility to ampicillin (beta-lactamase-negative ampicillin resistance phenotype). Remarkably, 21.3% of the H. influenzae isolates presented decreased susceptibility to ciprofloxacin, which was mainly observed in persisting clones. Of the H. influenzae isolates from CF patients, 18 (14.5%) were found to be hypermutable in comparison with 1 (1.4%) from non-CF patients (P < 0.0001). Ten patients (33.3%) were colonized by hypermutable strains over the study period. A multiresistance phenotype and long-term clonal persistence were significantly associated in some cases for up to 7 years. These results suggest that H. influenzae bronchial colonization in CF patients is a dynamic process, but better-adapted clones can persist for long periods of time.This work was supported by a research grant from the Fondo de Investigaciones Sanitarias, Ministerio de Sanidad y Consumo, Madrid, Spain (95-98/0364). We are grateful to E. Moguel for his technical assistance and to C. García (Hospital Lozano Blesa, Zaragoza) and J. C. Alados (Hospital Virgen de las Nieves, Granada) for their cooperation

New Concepts in the Invasive and Non Invasive Evaluation of Remodelling of the Right Ventricle and Pulmonary Vasculature in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a rare fatal disease defined as a sustained elevation of pulmonary arterial pressure to more than 25 mmHg at rest, with a mean pulmonary-capillary wedge pressure and left ventricular enddiastolic pressure of less than 15 mmHg at rest. Histopathology of PAH is founded on structural modifications on the vascular wall of small pulmonary arteries characterized by thickening of all its layers. These changes, named as vascular remodelling, include vascular proliferation, fibrosis, and vessel obstruction. In clinical practice the diagnosis of PAH relies on measurements of pulmonary vascular pressure and cardiac output, and calculation of pulmonary vascular resistances. Direct evaluation of pulmonary vascular structure is not routinely performed in pulmonary hypertension since current imaging techniques are limited and since little is known about the relationship between structural changes and functional characteristics of the pulmonary vasculature. Intravascular ultrasound studies in patients with pulmonary hypertension have shown a thicker middle layer, increased wall-thickness ratio and diminished pulsatility than in control patients. Optical Coherence Tomography, a new high resolution imaging modality that has proven its superiority over intravascular ultrasound (IVUS) for the detection and characterization of coronary atherosclerotic plaque composition, may potentially be a useful technique for the in vivo study of the pulmonary arterial wall. In addition current progress in Echo Doppler technique will quantify right ventricular function with parameters independent of loading conditions and not requiring volumetric approximations of the complex geometry of the right ventricle. This would allow the in vivo study of right ventricular and pulmonary artery remodelling in PAH

Elastic properties of graphene suspended on a polymer substrate by e-beam exposure

A method for fabricating multiple free-standing structures on the same sheet of graphene is demonstrated. Mechanically exfoliated mono- and bilayer graphene sheets were sandwiched between two layers of polymethyl-methacrylate. Suspended areas were defined by e-beam exposure allowing precise control over their shape and position. Mechanical characterization of suspended graphene sheets was performed by nanoindentation with an atomic force microscopy tip. The obtained built-in tensions of 12 nN are significantly lower than those in suspended graphene exfoliated on an SiO2 substrate, and therefore permit access to the intrinsic properties of this material system

Quantification et réduction de l'incertitude concernant les propriétés de monotonie d'un code de calcul coûteux à évaluer

National audienceNous nous intéressons à l'estimation de propriétés de monotonie d'un modèle numérique dont la sortie est supposée scalaire --- par exemple, un modèle de type éléments finis associé à un post-traitement. Plusieurs indicateurs quantitatifs de monotonie sont considérés (extrema et taux de positivité des dérivées partielles). L'évaluation de la sortie du modèle numérique étant usuellement coûteuse, par exemple en temps de calcul, l'estimation de ces indicateurs doit pouvoir être conduite avec un budget réduit d'évaluations. Nous adoptons dans cet article une démarche bayésienne, dans laquelle le modèle numérique est lui-même modélisé par un processus gaussien, et nous estimons au moyen de simulations conditionnelles les lois a posteriori des indicateurs proposés. Cette démarche permet d'envisager une planification séquentielle d'expériences supplémentaires, visant à réduire l'incertitude sur certains des indicateurs de monotonie. Nous appliquons cette approche à un modèle numérique d'un composant passif dans une centrale électrique

Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey

Kinematic distances to 750 molecular clouds identified in the 13CO J=1-0 Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS) are derived assuming the Clemens rotation curve of the Galaxy. The kinematic distance ambiguity is resolved by examining the presence of HI self-absorption toward the 13CO emission peak of each cloud using the Very Large Array Galactic Plane Survey (VGPS). We also identify 21 cm continuum sources embedded in the GRS clouds in order to use absorption features in the HI 21 cm continuum to distinguish between near and far kinematic distances. The Galactic distribution of GRS clouds is consistent with a four-arm model of the Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS clouds match star count data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) star-count data. We conclude that molecular clouds must form in spiral arms and be short-lived (lifetimes < 10 Myr) in order to explain the absence of massive, 13CO bright molecular clouds in the inter-arm space