The Analysis of interval censoring and double censoring via Markov chain Monte Carlo methods

L'Anàlisi de la supervivència s'utilitza en diferents camps per analitzar el temps transcorregut entre dos esdeveniments. El que distingeix l'anàlisi de la supervivència d'altres àrees de l'estadística és que les dades normalment estan censurades. La censura en un interval apareix quan l'esdeveniment final d'interès no és directament observable i només se sap que el temps de fallada està en un interval concret. Un esquema de censura més complex encara apareix quan tant el temps inicial com el temps final estan censurats en un interval. Aquesta situació s'anomena doble censura. En aquest article donem una descripció formal d'un mètode bayesà paramètric per a l'anàlisi de dades censurades en un interval i dades doblement censurades així com unes indicacions clares de la seva utilització o pràctica. La metodologia proposada s'ilustra amb dades d'una cohort de pacients hemofílics que es varen infectar amb el virus VIH a principis dels anys 1980's.Survival analysis is used in different fields to analyse the elapsed time between two events. What distinguishes survival analysis from other areas in statistics is that data are usually censored. Interval censoring arises when the occurrence of the final event of interest cannot be exactly observed and the failure time is only known to lie in an interval. A more complex censoring scheme is found when both initial and final times are interval–censored. This situation is referred as double censoring. In this paper we provide a formal description of a parametric Bayesian method for the analysis of interval–censored and doubly–censored data and clear guide lines for its practical use. The proposed methodology is illustrated with data from a cohort of haemophilia patients who were infected with HIV in the early1980’s.El análisis de la supervivencia se utiliza en diferentes campos para analizar el tiempo transcurrido entre dos sucesos. Lo que distingue el análisis de la supervivencia de otras áreas de la estadística es que los datos normalmente están censurados. La censura en un intervalo aparece cuando el suceso final de interés no es directamente observable y sólo se sabe que el tiempo de fallo está en un intervalo concreto. Un esquema de censura más complejo todavía aparece cuando tanto el tiempo inicial como el tiempo final están censurados en un intervalo. Esta situación se denomina doble censura. En este artículo damos una descripción formal de un método bayesiano paramétrico para el análisis de datos censurados en un intervalo y datos doblemente censurados, así como unas indicaciones claras de su utilización práctica. La metodología propuesta se ilustra con datos de una cohorte de pacientes hemofílicos que se infectaron con el virus VIH a principios de los años 1980

Electrochemical Investigation of Corrosion in the Space Shuttle Launch Environment

Corrosion studies began at NASA/Kennedy Space Center in 1966 during the Gemini/Apollo Programs with the evaluation of long-term protective coatings for the atmospheric protection of carbon steel. An outdoor exposure facility on the beach near the launch pad was established for this purpose at that time. The site has provided over 35 years of technical information on the evaluation of the long-term corrosion performance of many materials and coatings as well as on maintenance procedures. Results from these evaluations have helped NASA find new materials and processes that increase the safety and reliability of our flight hardware, launch structures, and ground support equipment. The launch environment at the Kennedy Space Center (KSC) is extremely corrosive due to the combination of ocean salt spray, heat, humidity, and sunlight. With the introduction of the Space Shuttle in 1981, the already highly corrosive conditions at the launch pad were rendered even more severe by the acidic exhaust from the solid rocket boosters. It has been estimated that 70 tons of hydrochloric acid (HC1) are produced during a launch. The Corrosion Laboratory at NASA/KSC was established in 1985 to conduct electrochemical studies of corrosion on materials and coatings under conditions similar to those encountered at the launch pads. I will present highlights of some of these investigations

Incertesa i coneixement

Incertesa i coneixement són dos conceptes oposats, antagònics, però que molt sovint es veuen obligats a conviure. La seva gran rivalitat serà una constant que els acompanyarà en tot moment,cadascun amb l’objectiu d’apoderar-se de l’altre. L’estadística serà el pont entre aquest dos vells enemics

On the driver of relativistic effects strength in Seyfert galaxies

Spectroscopy of X-ray emission lines emitted in accretion discs around supermassive black holes is one of the most powerful probes of the accretion flow physics and geometry, while also providing in principle observational constraints on the black hole spin.[...] We aim at determining the ultimate physical driver of the strength of this relativistic reprocessing feature. We first extend the hard X-ray flux-limited sample of Seyfert galaxies studied so far (FERO, de la Calle Perez et al. 2010) to obscured objects up to a column density N_H=6x10^23 atoms/cm/cm. We verify that none of the line properties depends on the AGN optical classification, as expected from the Seyfert unification scenarios. There is also no correlation between the accretion disc inclination, as derived from formal fits of the line profiles, and the optical type or host galaxy aspect angle, suggesting that the innermost regions of the accretion disc and the host galaxy plane are not aligned. [...]. Data are not sensitive enough to the detailed ionisation state of the line-emitting disc. However, the lack of dependency of the line EW on either the luminosity or the rest-frame centroid energy rules out that disc ionisation plays an important role on the EW dynamical range in Seyferts. The dynamical range of the relativistically broadened K-alpha iron line EW in nearby Seyferts appears to be mainly determined by the properties of the innermost accretion flow. We discuss several mechanisms (disc ionisation, disc truncation, aberration due to a mildly relativistic outflowing corona) which can explain this. [...] Observational data are still not in contradiction with scenarios invoking different mechanisms for the spectral complexity around the iron line, most notably the "partial covering" absorption scenario. (abridged).Comment: Accepted for publication on Astronomy & Astrophysics. 14 pages, 9 figure

The analysis of interval-censored survival data. From a Nonparametric perspective to a nonparametric Bayesian approach

This work concerns some problems in the area of survival analysis that arise in real clinical or epidemiological studies. In particular, we approach the problem of estimating the survival function based on interval-censored data or doubly-censored data. We will start defining these concepts and presenting a brief review of different methodologies to deal with this kind of censoring patterns.Survival analysis is the term used to describe the analysis of data that correspond to the time from a well defined origin time until the occurrence of some particular event of interest. This event need not necessarily be death, but could, for example, be the response to a treatment, remission from a disease, or the occurrence of a sympto