A different approach to multiple correspondence analysis (MCA) than that of specific MCA

Dans l'analyse des correspondances multiples, la contribution de chaque variable nominale à l’inertie est différente selon le nombre des modalités, ou des catégories de cette variable. Habituellement, pour les variables ayant beaucoup de modalités (ou catégories) il y a des modalités peu fréquentes (les « classes faibles ») qui contribuent à l'inertie de la variable correspondante de façon disproportionnée. Souvent, ces modalités contribuent fortement à la détermination des premiers axes factoriels ce qui a pour conséquence de ne pas correctement représenter le problème étudié. L'analyse spécifique des correspondances multiples traite le problème des modalités peu fréquentes en les supprimant. C'est-à-dire, qu’elle les ignore purement et simplement dans le calcul des distances entre les individus [Le Roux B., 1999 ; Le Roux B., Rouanet H., 2004].Dans cet article, nous traitons ce problème d'une façon différente. Nous maintenons les modalités faibles dans l'analyse. Ce que réalise notre analyse, c’est de remplacer la métrique du khi-deux, par une nouvelle métrique qui prend également en compte le nombre de modalités de chaque variable : c’est aussi d’attribuer un effet raisonnable aux modalités faibles et d’équilibrer toutes les variables nominales. En outre, nous tenons compte, uniformément, des modalités faibles, qu’elles dérivent de beaucoup ou de peu de variables, bien que la plupart des cas « dangereux » sont ceux des variables qui ont beaucoup de modalités. Seules les variables à deux modalités ne sont pas concernéesIn multiple correspondence analysis, each nominal variable affects the analysis with a different amount of inertia, depending on the number of its modalities or categories. Usually in variables with many modalities – categories created infrequent (weak classes) modalities which contribute disproportionally to the inertia of the corresponding variable. Often these modalities contribute heavily to the determination of the first factorial axes and as a result this cannot clearly represent the investigated problem. Specific multiple correspondence analysis deals with the problem of infrequent (weak) modalities by removing them. That is, it simply ignores them in the calculation of distances between individuals [Le Roux B., 1999; Le Roux B., Rouanet H., 2004].In this paper we deal with this problem in a different manner. We keep the weak modalities in the analysis. Replacing the khi2 metric by a new metric which also takes into account the number of modalities of each variable, a reasonable effect of the weak modalities and a balancing of all the nominal variables is achieved in the analysis.We also encounter uniformly the weak modalities, whether they derive from many or few variables, even though the most “dangerous” case is the one variables where have many modalities. Only variables of two modalities are not affected

The stochastic gravitational-wave background in the absence of horizons

Gravitational-wave astronomy has the potential to explore one of the deepest and most puzzling aspects of Einstein's theory: the existence of black holes. A plethora of ultracompact, horizonless objects have been proposed to arise in models inspired by quantum gravity. These objects may solve Hawking's information-loss paradox and the singularity problem associated with black holes, while mimicking almost all of their classical properties. They are, however, generically unstable on relatively short timescales. Here, we show that this 'ergoregion instability' leads to a strong stochastic background of gravitational waves, at a level detectable by current and future gravitational-wave detectors. The absence of such background in the first observation run of Advanced LIGO already imposes the most stringent limits to date on black-hole alternatives, showing that certain models of 'quantum-dressed' stellar black holes can be at most a small percentage of the total population. The future LISA mission will allow for similar constraints on supermassive black-hole mimickers

Superradiant instability of black resonators and geons

Abstract: Black resonators and geons in global AdS are rapidly rotating, low-energy solutions with a helical Killing field. We study the linear mode stability of equal angular momenta, five-dimensional black resonators and geons under scalar, electromagnetic, and gravitational perturbations. We find that black resonators are unstable to the superradiant instability, in agreement with previously known results. Perhaps surprisingly, many geons appear linearly stable, despite having an ergoregion. This apparent stability implies that geons are important long-lived, low-energy states in the dual gauge theory. However, we do find that geons are unstable within a certain range of parameter space. We comment on the nature of this instability and to its possible endpoints. We also report on new non-spinning oscillating geons, which we construct within a cohomogeneity two ansatz. Given the existing arguments that suggest our linear stability results may be extended nonlinearly, our findings indicate that most geons are generic and long-lived solutions

Impact of hepatitis B virus infection on the progression of AIDS and mortality in HIV-infected individuals: A cohort study and meta-analysis

Background. The effect of hepatitis B virus (HBV) infection on the natural history of human immunodeficiency virus (HIV) disease remains uncertain. Therefore, a retrospective cohort study was conducted to examine the influence of HIV-HBV coinfection on AIDS development and overall mortality. Moreover, our results were added to those of previous studies in a literature-based meta-analysis. Methods. Serum samples obtained from HIV-seropositive patients from 1984 through 2003 were retrospectively tested for hepatitis B surface antigen. Multivariable analyses were performed using Poisson and logistic regression models. For meta-analytic purposes, eligible articles were identified and relevant data were abstracted. Pooled estimates of effect were calculated applying fixed and random effects models. Results. The prevalence of chronic HBV infection (documented hepatitis B surface antigen seropositivity for >6 months) among 1729 HIV-positive patients was ∼6%. The multivariable analyses in our primary study revealed no significant impact of concomitant HIV-HBV infection on progression to AIDS and all-cause mortality. However, a meta-analysis performed on data from 12,382 patients enrolled in 11 studies revealed a significant effect of HIV- HBV coinfection on overall mortality (pooled effect estimate, 1.36; 95% confidence interval, 1.12-1.64). The increased rate of death among coinfected individuals was observed in the meta-analyses of studies conducted both before (pooled effect estimate, 1.60; 95% confidence interval, 1.07-2.39) and after (pooled effect estimate, 1.28; 95% confidence interval, 1.03-1.60) commencement of highly active antiretroviral therapy. Conclusions. HIV-HBV coinfection seems to affect all-cause mortality, and strategies to reduce liver damage in patients coinfected with HIV and HBV are justified. © 2009 by the Infectious Diseases Society of America. All rights reserved

HIV/HBV co-infection and rate of antiretroviral treatment change after highly active antiretroviral treatment initiation in a cohort of HIV-infected patients in Greece

The current study investigated the impact of human immunodeficiency virus (HIV)/hepatitis B virus (HBV) co-infection on the rate of change of antiretroviral drugs after the initiation of highly active antiretroviral treatment (HAART). The data on 1425 HIV-positive patients with recorded serology for hepatitis B surface antigen (HBsAg) were retrospectively analysed. The estimated rate of treatment change was slightly higher in the HBsAg-positive group (0.57 per year) compared with the HBsAg-negative group (0.50 per year). Although this difference was insignificant in multivariable modelling, the confidence intervals of the estimates barely included unity. Antiretroviral drug family, calendar period, prior exposure to antiretrovirals and the diagnosis of acquired immunodeficiency syndrome were independently associated with the number of drug alterations. A slight impact of co-infection on the frequency of treatment change after the beginning of HAART cannot be excluded. However, the paucity of studies on this issue necessitates the conduct of further research