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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
391

False presences: Fanon's reinforcement of the female subaltern

Pagan, Louan Isabel 11 June 2014 (has links)
A false presence of the colonized female arises within Frantz Fanon's books Peau noire, masques blancs, L'an V de la révolution algérienne, and Les damnés de la terre. Through a close reading of these texts, this thesis aims to locate where these blind spots exist and how they are facilitated by Fanon, while also acknowledging the potential for these exclusions to be accidental. The introduction provides a brief synopsis of Fanon's life and the colonial relations of his time, namely the Algerian revolution. The first chapter deals with how Fanon's language reinforces the female subaltern through silencing, effacement, and stereotypical language. The second chapter concerns the similarities between racial and gendered power relations within his books. In the third chapter, I discuss hierarchies of power and their establishment through gendered binaries. Finally, my concluding chapter posits how addressing these false presences can lead to deeper understanding and interaction with the female subaltern in Fanon's books. / Master of Arts
392

Enhancing Pavement Surface Macrotexture Characterization

Mogrovejo Carrasco, Daniel Estuardo 30 April 2015 (has links)
One of the most important objectives for transportation engineers is to understand pavement surface properties and their positive and negative effects on the user. This can improve the design of the infrastructure, adequacy of tools, and consistency of methodologies that are essential for transportation practitioners regarding macrotexture characterization. Important pavement surface characteristics, or tire-pavement interactions, such as friction, tire-pavement noise, splash and spray, and rolling resistance, are significantly influenced by pavement macrotexture. This dissertation compares static and dynamic macrotexture measurements and proposes and enhanced method to quantify the macrotexture. Dynamic measurements performed with vehicle-mounted lasers have the advantage of measuring macrotexture at traffic speed. One drawback of these laser devices is the presence of 'spikes' in the collected data, which impact the texture measurements. The dissertation proposes two robust and innovative methods to overcome this limitation. The first method is a data-driven adaptive method that detects and removes the spikes from high-speed laser texture measurements. The method first calculates the discrete wavelet transform of the texture measurements. It then detects (at all levels) and removes the spikes from the obtained wavelet coefficients (or differences). Finally, it calculates the inverse discrete wavelet transform with the processed wavelet coefficients (without outliers) to obtain the Mean Profile Depth (MPD) from the measurements with the spikes removed. The method was validated by comparing the results with MPD measurements obtained with a Circular Texture Meter (CTMeter) that was chosen as the control device. Although this first method was able to successfully remove the spikes, it has the drawback that it depends on manual modeling of the distribution of the wavelet coefficients to correctly define an appropriate threshold. The next step of this dissertation proposes an enhanced to the spike removal methodology for macrotexture measurements taken with high-speed laser devices. This denoising methodology uses an algorithm that defines the distribution of texture measurements by using the family of Generalized Gaussian Distributions (GGD), along with the False Discovery Rate (FDR) method that controls the proportion of wrongly identified spikes among all identified spikes. The FDR control allows for an adaptive threshold selection that differentiates between valid measurements and spikes. The validation of the method showed that the MPD results obtained with denoised dynamic measurements are comparable to MPD results from the control devices. This second method is included as a crucial step in the last stage of this dissertation as explained following. The last part of the dissertation presents an enhanced macrotexture characterization index based on the Effective Area for Water Evacuation (EAWE), which: (1) Estimates the potential of the pavement to drain water and (2) Correlates better with two pavement surface properties affected by macrotexture (friction and noise) that the current MPD method. The proposed index is defined by a three-step process that: (1) removes the spikes, assuring the reliability of the texture profile data, (2) finds the enveloping profile that is necessary to delimit the area between the tire and the pavement when contact occurs, and (3) computes the EAWE. Comparisons of current (MPD) and proposed (EAWE) macrotexture characterization indices showed that the MPD overestimates the ability of the pavement for draining the surface water under a tire. / Ph. D.
393

On the Detection of False Information: From Rumors to Fake News

Ghanem, Bilal Hisham Hasan 10 January 2021 (has links)
Tesis por compendio / [ES] En tiempos recientes, el desarrollo de las redes sociales y de las agencias de noticias han traído nuevos retos y amenazas a la web. Estas amenazas han llamado la atención de la comunidad investigadora en Procesamiento del Lenguaje Natural (PLN) ya que están contaminando las plataformas de redes sociales. Un ejemplo de amenaza serían las noticias falsas, en las que los usuarios difunden y comparten información falsa, inexacta o engañosa. La información falsa no se limita a la información verificable, sino que también incluye información que se utiliza con fines nocivos. Además, uno de los desafíos a los que se enfrentan los investigadores es la gran cantidad de usuarios en las plataformas de redes sociales, donde detectar a los difusores de información falsa no es tarea fácil. Los trabajos previos que se han propuesto para limitar o estudiar el tema de la detección de información falsa se han centrado en comprender el lenguaje de la información falsa desde una perspectiva lingüística. En el caso de información verificable, estos enfoques se han propuesto en un entorno monolingüe. Además, apenas se ha investigado la detección de las fuentes o los difusores de información falsa en las redes sociales. En esta tesis estudiamos la información falsa desde varias perspectivas. En primer lugar, dado que los trabajos anteriores se centraron en el estudio de la información falsa en un entorno monolingüe, en esta tesis estudiamos la información falsa en un entorno multilingüe. Proponemos diferentes enfoques multilingües y los comparamos con un conjunto de baselines monolingües. Además, proporcionamos estudios sistemáticos para los resultados de la evaluación de nuestros enfoques para una mejor comprensión. En segundo lugar, hemos notado que el papel de la información afectiva no se ha investigado en profundidad. Por lo tanto, la segunda parte de nuestro trabajo de investigación estudia el papel de la información afectiva en la información falsa y muestra cómo los autores de contenido falso la emplean para manipular al lector. Aquí, investigamos varios tipos de información falsa para comprender la correlación entre la información afectiva y cada tipo (Propaganda, Trucos / Engaños, Clickbait y Sátira). Por último, aunque no menos importante, en un intento de limitar su propagación, también abordamos el problema de los difusores de información falsa en las redes sociales. En esta dirección de la investigación, nos enfocamos en explotar varias características basadas en texto extraídas de los mensajes de perfiles en línea de tales difusores. Estudiamos diferentes conjuntos de características que pueden tener el potencial de ayudar a discriminar entre difusores de información falsa y verificadores de hechos. / [CA] En temps recents, el desenvolupament de les xarxes socials i de les agències de notícies han portat nous reptes i amenaces a la web. Aquestes amenaces han cridat l'atenció de la comunitat investigadora en Processament de Llenguatge Natural (PLN) ja que estan contaminant les plataformes de xarxes socials. Un exemple d'amenaça serien les notícies falses, en què els usuaris difonen i comparteixen informació falsa, inexacta o enganyosa. La informació falsa no es limita a la informació verificable, sinó que també inclou informació que s'utilitza amb fins nocius. A més, un dels desafiaments als quals s'enfronten els investigadors és la gran quantitat d'usuaris en les plataformes de xarxes socials, on detectar els difusors d'informació falsa no és tasca fàcil. Els treballs previs que s'han proposat per limitar o estudiar el tema de la detecció d'informació falsa s'han centrat en comprendre el llenguatge de la informació falsa des d'una perspectiva lingüística. En el cas d'informació verificable, aquests enfocaments s'han proposat en un entorn monolingüe. A més, gairebé no s'ha investigat la detecció de les fonts o els difusors d'informació falsa a les xarxes socials. En aquesta tesi estudiem la informació falsa des de diverses perspectives. En primer lloc, atès que els treballs anteriors es van centrar en l'estudi de la informació falsa en un entorn monolingüe, en aquesta tesi estudiem la informació falsa en un entorn multilingüe. Proposem diferents enfocaments multilingües i els comparem amb un conjunt de baselines monolingües. A més, proporcionem estudis sistemàtics per als resultats de l'avaluació dels nostres enfocaments per a una millor comprensió. En segon lloc, hem notat que el paper de la informació afectiva no s'ha investigat en profunditat. Per tant, la segona part del nostre treball de recerca estudia el paper de la informació afectiva en la informació falsa i mostra com els autors de contingut fals l'empren per manipular el lector. Aquí, investiguem diversos tipus d'informació falsa per comprendre la correlació entre la informació afectiva i cada tipus (Propaganda, Trucs / Enganys, Clickbait i Sàtira). Finalment, però no menys important, en un intent de limitar la seva propagació, també abordem el problema dels difusors d'informació falsa a les xarxes socials. En aquesta direcció de la investigació, ens enfoquem en explotar diverses característiques basades en text extretes dels missatges de perfils en línia de tals difusors. Estudiem diferents conjunts de característiques que poden tenir el potencial d'ajudar a discriminar entre difusors d'informació falsa i verificadors de fets. / [EN] In the recent years, the development of social media and online news agencies has brought several challenges and threats to the Web. These threats have taken the attention of the Natural Language Processing (NLP) research community as they are polluting the online social media platforms. One of the examples of these threats is false information, in which false, inaccurate, or deceptive information is spread and shared by online users. False information is not limited to verifiable information, but it also involves information that is used for harmful purposes. Also, one of the challenges that researchers have to face is the massive number of users in social media platforms, where detecting false information spreaders is not an easy job. Previous work that has been proposed for limiting or studying the issue of detecting false information has focused on understanding the language of false information from a linguistic perspective. In the case of verifiable information, approaches have been proposed in a monolingual setting. Moreover, detecting the sources or the spreaders of false information in social media has not been investigated much. In this thesis we study false information from several aspects. First, since previous work focused on studying false information in a monolingual setting, in this thesis we study false information in a cross-lingual one. We propose different cross-lingual approaches and we compare them to a set of monolingual baselines. Also, we provide systematic studies for the evaluation results of our approaches for better understanding. Second, we noticed that the role of affective information was not investigated in depth. Therefore, the second part of our research work studies the role of the affective information in false information and shows how the authors of false content use it to manipulate the reader. Here, we investigate several types of false information to understand the correlation between affective information and each type (Propaganda, Hoax, Clickbait, Rumor, and Satire). Last but not least, in an attempt to limit its spread, we also address the problem of detecting false information spreaders in social media. In this research direction, we focus on exploiting several text-based features extracted from the online profile messages of those spreaders. We study different feature sets that can have the potential to help to identify false information spreaders from fact checkers. / Ghanem, BHH. (2020). On the Detection of False Information: From Rumors to Fake News [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/158570 / Compendio
394

Referrals from Primary Eye Care: An Investigation into their quality, levels of false positives and psychological effect on patients.

Davey, Christopher J. January 2011 (has links)
Previous research into the accuracy of referrals for glaucoma has shown that a large number of referrals to the Hospital Eye Service are false positive. Research in areas of healthcare other than ophthalmology has shown that psychological distress can be caused by false positive referrals. The present study aimed to evaluate the quality of referrals to the HES for all ocular pathologies, and also to quantify the proportion of these referrals that were false positive. Any commonality between false positive referrals was investigated. The psychological effect of being referred to the HES was also evaluated using the Hospital Anxiety and Depression Scale (HADS) and State-Trait Anxiety Inventory (STAI). Both scales were validated in this population with Rasch analysis before use. A final aim was to develop an improvement to the present referral pathway in order to reduce numbers of false positive referrals. The accuracy of referrals to the HES appears to improve as clinicians become more experienced, and greater numbers of false positive referrals are generated by female clinicians. Optometrists refer patients with a wide range of ocular diseases and in most cases include both fundus observations and visual acuity measurements in their referrals. GPs mainly refer patients with anterior segment disorders, particularly lid lesions, based on direct observation and symptoms. Illegibility and missing clinical information reduce the quality of many optometric referrals. Patients referred to the HES experience raised levels of anxiety as measured by the STAI and raised levels of depression as measured by the HADS-Depression subscale. As a method of assessing psychological distress, the questionnaires HADS-T (all items), STAI-S (State subscale) and STAI-T (Trait subscale) show good discrimination between patients when administered to a population of new ophthalmic outpatients, despite all having a floor effect. Subsequently a referral refinement service was developed which reduced numbers of unnecessary referrals and reduced costs for the NHS.
395

Visualising network security attacks with multiple 3D visualisation and false alert classification

Musa, Shahrulniza January 2008 (has links)
Increasing numbers of alerts produced by network intrusion detection systems (NIDS) have burdened the job of security analysts especially in identifying and responding to them. The tasks of exploring and analysing large quantities of communication network security data are also difficult. This thesis studied the application of visualisation in combination with alerts classifier to make the exploring and understanding of network security alerts data faster and easier. The prototype software, NSAViz, has been developed to visualise and to provide an intuitive presentation of the network security alerts data using interactive 3D visuals with an integration of a false alert classifier. The needs analysis of this prototype was based on the suggested needs of network security analyst's tasks as seen in the literatures. The prototype software incorporates various projections of the alert data in 3D displays. The overview was plotted in a 3D plot named as "time series 3D AlertGraph" which was an extension of the 2D histographs into 3D. The 3D AlertGraph was effectively summarised the alerts data and gave the overview of the network security status. Filtering, drill-down and playback of the alerts at variable speed were incorporated to strengthen the analysis. Real-time visual observation was also included. To identify true alerts from all alerts represents the main task of the network security analyst. This prototype software was integrated with a false alert classifier using a classification tree based on C4.5 classification algorithm to classify the alerts into true and false. Users can add new samples and edit the existing classifier training sample. The classifier performance was measured using k-fold cross-validation technique. The results showed the classifier was able to remove noise in the visualisation, thus making the pattern of the true alerts to emerge. It also highlighted the true alerts in the visualisation. Finally, a user evaluation was conducted to find the usability problems in the tool and to measure its effectiveness. The feed backs showed the tools had successfully helped the task of the security analyst and increased the security awareness in their supervised network. From this research, the task of exploring and analysing a large amount of network security data becomes easier and the true attacks can be identified using the prototype visualisation tools. Visualisation techniques and false alert classification are helpful in exploring and analysing network security data.
396

The Influence of Population on Wrongful Convictions

Whittington, Kari 12 1900 (has links)
With criminal cases continuing to be exonerated across the United States, research must be done on the subject to advance current practices to reduce its occurrence in the future. This study combines county population data with the National Registry of Exonerations to analyze the contributing factors to wrongful convictions and the possible effect of population on their frequency. The objective of this study was to identify specific policy changes based on the five contributing factors to wrongful convictions that could be applied to population specific areas. The results yielded multiple patterns that are discussed thoroughly. These findings allowed the introduction of policy changes and proposals for future research.
397

Derrida et Bergson : dialogue médiat sur la question de l'immédiat

Fradet, Pierre-Alexandre 08 1900 (has links)
Si le rapport entre Derrida et Bergson n’a pas fait l’objet de nombreuses études, les commentaires existants témoignent à peu près tous d’une vision commune : entre les deux philosophes, les divergences peuvent être atténuées, voire dissoutes, par la considération de convergences plus fondamentales. Les pages qui suivent seront l’occasion pour nous de faire contrepoids à cette vulgate interprétative. Sans nier l’existence de points de contact entre Derrida et Bergson, nous voudrions en effet montrer qu’un important désaccord subsiste entre eux au sujet de la possibilité de l’intuition. Alors que Derrida met en cause les doctrines intuitionnistes, Bergson érige l’intuition en méthode philosophique. Le présent mémoire prendra pour fil conducteur les motifs de cette discorde. Réduit à sa plus simple expression, l’objectif que nous y poursuivrons sera de montrer que les pensées bergsonienne et derridienne, lorsque mises en dialogue, révèlent un désaccord partiel qui permet de réfléchir de façon féconde sur la possibilité de l’intuition. Pour être plus exact, nous caresserons ici une triple ambition : i/ cerner étroitement l’objet du litige entre Derrida et Bergson, trop peu souligné par les commentateurs, et dont nous montrons qu’il s’articule à une entente partielle ; ii/ tirer au clair les diverses raisons qui amènent l’un à s’en prendre à l’intuition, l’autre à embrasser la méthode intuitive ; iii/ établir que certains arguments de Bergson, bien qu’ils connaissent un regain d’intérêt depuis quelques années, paraissent lacunaires lorsqu’on les confronte à différentes objections. / Although studies of the relation between Derrida and Bergson are few and far between, they nearly all share a common vision: that of attenuating – or even altogether eliminating – the divisions between the two philosophers’ thought, by considering their more fundamental convergences. The following pages will allow us to counterbalance this common interpretation. Without denying the points that Derrida and Bergson do have in common, we will show an important divergence in opinion between the two on the idea that intuition is possible and founded. While Derrida lays doubt on intuitionist doctrine, Bergson establishes intuition as a philosophical method. This thesis examines the motives behind this divergence. Put simply, a comparison of Derridian and Bergsonian thought reveals a partial disagreement that enables fruitful reflection about whether or not intuition is possible. More precisely, we pursue three objectives here: i/ to clearly identify the scope of the disagreement between Derrida and Bergson, often overlooked by previous commentaries, showing that it includes a partial agreement; ii/ to clarify the diverse reasons leading Derrida to deny the very existence of intuition while Bergson embraces intuition as a philosophical method; and iii/ to show that certain Bergsonian arguments, although enjoying a resurge in interest in recent years, appear unable to stand up to several different objections.
398

Désintégration du faux vide médiée par des kinks en 1+1 dimensions

Ung, Yvan 07 1900 (has links)
Dans ce mémoire, on étudie la désintégration d’un faux vide, c’est-à-dire un vide qui est un minimum relatif d’un potentiel scalaire par effet tunnel. Des défauts topologiques en 1+1 dimension, appelés kinks, apparaissent lorsque le potentiel possède un minimum qui brise spontanément une symétrie discrète. En 3+1 dimensions, ces kinks deviennent des murs de domaine. Ils apparaissent par exemple dans les matériaux magnétiques en matière condensée. Un modèle à deux champs scalaires couplés sera étudié ainsi que les solutions aux équations du mouvement qui en découlent. Ce faisant, on analysera comment l’existence et l’énergie des solutions statiques dépend des paramètres du modèle. Un balayage numérique de l’espace des paramètres révèle que les solutions stables se trouvent entre les zones de dissociation, des régions dans l’espace des paramètres où les solutions stables n’existent plus. Le comportement des solutions instables dans les zones de dissociation peut être très différent selon la zone de dissociation dans laquelle une solution se trouve. Le potentiel consiste, dans un premier temps, en un polynôme d’ordre six, auquel on y rajoute, dans un deuxième temps, un polynôme quartique multiplié par un terme de couplage, et est choisi tel que les extrémités du kink soient à des faux vides distincts. Le taux de désintégration a été estimé par une approximation semi-classique pour montrer l’impact des défauts topologiques sur la stabilité du faux vide. Le projet consiste à déterminer les conditions qui permettent aux kinks de catalyser la désintégration du faux vide. Il appert qu’on a trouvé une expression pour déterminer la densité critique de kinks et qu’on comprend ce qui se passe avec la plupart des termes. / In this thesis, we study the tunneling decay of the false vacuum, that is, a vacuum that is a relative minimum of a scalar potential. Topological defects in 1+1 dimension, called kinks, appear when the potential possesses a minimum that spontaneously breaks a discrete symmetry. In 3+1 dimensions, these kinks become domain walls. For instance, they appear in magnetic materials in condensed matter. A model with two coupled scalar fields will be studied, as well as the solutions to the equations of motion that arise from it. We will then analyze how the energy of the static solutions depend on the parameters of the model. A numerical survey of parameter space reveals that the stable solutions are located between dissociation zones, areas in parameter space where stable solutions no longer exist. The behavior of the unstable solutions in the dissociation zones can be very different depending on which dissociation zone a solution is found near the dissociation zone. The potential first consists in a sixth-order polynomial, to which is added a quartic polynomial multiplied by a coupling term, and is chosen such that the extremities of the kink are at distinct false vacua. The decay rate has been estimated by a semiclassical approximation to show the impact of topological defects on the stability of the false vacuum. The project consists in determining the conditions that allow the kinks to catalyze false vacuum decay. It appears that we found an expression for the critical kink density and that we understand what happens with most terms.
399

L'effet des croyances sur la production des faux souvenirs : études des témoignages de phénomènes aérospatiaux non identifiés / The impact of beliefs on the production of false memories : study evidence of unidentified aerospace phenomena

Bouvet, Romain 27 June 2014 (has links)
Cette thèse porte sur les effets des croyances dans la production des faux souvenirs, phénomène qui présente une importance toute particulière dans le contexte judiciaire. Nous avons trouvé avec le CNES (Centre National d’Études Spatiales) un terrain d’étude particulièrement fonctionnel pour comprendre les mécanismes de la production des faux souvenirs. En effet, le CNES a créé un service, le GEIPAN (Groupe d’Études et d’Informations sur les Phénomènes Aérospatiaux Non Identifiés), chargé de recueillir les témoignages à propos de PAN (Phénomènes Aérospatiaux Non identifiés) sur le territoire français. Le GEIPAN reçoit un volume conséquent de témoignages (en moyenne deux par jour) qui présentent bien souvent une grande homogénéité. Cette collaboration avec le CNES a permis de réaliser des études de terrain qui, articulées avec des études expérimentales de laboratoire, ont permis de mettre en évidence les conditions de production des faux souvenirs. Nous observons que les faux souvenirs sont dus à l’impact des croyances (ici à propos des OVNI) dès lors qu’un délai a permis une confusion entre différentes sources du souvenir. Dans une dernière partie de la thèse, nous montrons que l’adaptation d’une technique d’entretien judiciaire (l’entretien cognitif sur internet) à la problématique de ces témoignages permet de résoudre en partie la production des faux souvenirs. Nous évoquons dans la discussion les pistes d’utilisation des travaux de cette thèse dans le contexte des enquêtes judiciaires. / This doctoral dissertation focuses on the impact of beliefs on the development of false memories. Such memories have been reported to be important as far as the judicial system is concerned. The CNES (Centre National d’ Études Spatiales) has been an ideal study site for a depth understanding of the development of false memories. Indeed, the CNES has developed the GEPAN (Groupe d’Études et d’Informations sur les Phénomènes Aérospatiaux Non Identifiés), a service aiming to collect French testimonies regarding PAN (Phénomènes Aérospatiaux Non identifiés). The GEIPAN receive plenty of testimonies (2 testimonies per day). This collaboration with the GEIPAN allowed to conduct both field researches and experimental studies aiming to highlight processes leading to false memories. False memories were related to beliefs about UFOs as long as a period led to a confusion between the several sources memories. In a last part of this doctoral dissertation, the adaptation of judicial interview (cognitive interview on the Internet) for these testimonies will be presented as well as its utility to for solving a part of false memories. These findings and their utility in criminal investigation will be discussed in the discussion section.
400

La caractérisation des exoplanètes en transit par vélocimétrie radiale

Santerne, Alexandre 26 October 2012 (has links)
La recherche et caractérisation de planètes extrasolaires en transit (i.e., qui passent devant leur étoile, vue depuis la Terre) est un domaine important de la planétologie car ces planètes permettent de contraindre les processus de formation, d'évolution et de migration des systèmes planétaires. Les missions spatiales CoRoT et Kepler ont permis, ces dernières années, de découvrir plusieurs milliers de candidats-planètes en transit. Cependant, ces candidats-planètes doivent être confirmés afin d'exclure tout scénario de faux-positifs pouvant imiter un transit d'une exo-planète. Pour cela, l'une des méthodes possible consiste à mener des observations complémentaires de vitesse radiale permettant de mesurer la masse et les paramètres orbitaux de l'objet qui transite et ainsi de pouvoir déterminer la nature des candidats-planètes. Au cours de ma thèse, je me suis attaché à résoudre la nature des candidats-planètes en transit issues des missions spatiales CoRoT et Kepler en menant des observations avec les spectrographes SOPHIE et HARPS, ce qui m'a permis d'identifier plusieurs nouvelles planètes extrasolaires en transit. J'ai également pu mesurer le taux de faux-positif de la mission Kepler, égal à 35% pour les candidats planètes-géantes à courte période orbitale, contredisant les précédentes estimations, beaucoup plus optimistes. J'ai également participé au développement d'un nouveau logiciel, "PASTIS", qui permet de valider statistiquement des planètes extrasolaires de faible masse, trop petites pour être caractérisées grâce aux spectrographes actuels. Ce logiciel permettra, à terme, de valider des dizaines de planètes de faible masse issues des missions CoRoT et Kepler. / The search and characterization of transiting extrasolar planets (i.e. that pass in front of their host star, as seen from the Earth) is an important domain of planetology since these planets constrain the formation, evolution and migration process of planetary systems. The CoRoT (CNES) and Kepler (NASA) space missions permit, these last years, to discover several thousand of transiting-planet candidates. However, these planet candidates need to be confirmed in order to exclude all false positive scenario that can mimic a planetary transit. For that, one of the method consist on performing radial velocity follow-up observations to measure the transiting object's mass and orbital parameters and thus, to determine the nature of planet candidates.During my PhD thesis, I tried to resolve the nature of transiting planet candidates from the CoRoT and Kepler space missions. For that, I performed follow-up observations with the SOPHIE (OHP) and HARPS (ESO) spectrographs that were used to discover several new transiting extrasolar planets. I also measured the Kepler false-positive rate, equal to 35% for giant close-in exoplanet candidates, contradicting previous estimations, much more optimistic.I also participate to the development of a new software, called "PASTIS", which objective is to validate statistically low-mass transiting exoplanets out of reach for current spectrographs. This new tool will, in a near future, validate tens of low-mass planets from the CoRoT and Kepler space missions.

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