345 research outputs found
High-breakdown estimation of multivariate mean and covariance with missing observations
No full textIn this paper, we consider the problem of outliers in incomplete multivariate data, when the aim is to estimate a measure of mean and covariance as it is the case for example in factor analysis. In such a situation the ER algorithm of Little and Smith (1987) which combines the EM algorithm for missing data and a robust estimation step based on an Mestimator could be used. However, the ER algorithm as originally proposed can fail to be robust in some cases especially in high dimensions. We propose here two alternatives to avoid the problem. One is to combine a small modification of the ER algorithm with a socalled high breakdown estimator as starting point for the iterative procedure and the other is to base the estimation step of the ER algorithm on a high breakdown estimator. Among the high breakdown estimators which are actually built to keep their robustness properties even if the number of variables is relatively large, we consider here the minimum covariance determinant (MCD) estimator and the t-biweight S-estimator. Simulated and real data are used to compare and illustrate the different procedures
Distributional Analysis: a Robust Approach
No full textDistributional dominance criteria are commonly applied to draw welfare inferences about comparisons, but conclusions drawn from empirical implementations of dominance criteria may be influenced by data contamination. We show the conditions under which this may occur and propose empirical methods to work round the proble using both non-parametric and parametric approaches
Resistant Modelling of Income Distributions and Inequality Measures
No full textWe review the use and the interpretation of some robustness concepts and techniques in some economic applications. We focus on estimation techniques in income distribution analysis and we discuss the reliability of inequality measures
Microscale study of frictional properties of graphene in ultra high vacuum
No full textWe report on the frictional properties of epitaxial graphene on SiC in ultra high vacuum. Measurements have been performed using a microtribometer in the load regime of 0.5 to 1 mN. We observed that a ruby sphere sliding against graphene results in very low friction coefficients ranging from 0.02 to 0.05. The friction and also the stability of the graphene layer is higher than that under similar conditions in ambient conditions. The friction shows a load dependence. Finally it was found that graphene masks the frictional anisotropy which was observed on the SiC surface
Fast Robust Model Selection in Large Datasets
No full textLarge datasets are more and more common in many research fields. In particular, in the linear regression context, it is often the case that a huge number of potential covariates are available to explain a response variable, and the first step of a reasonable statistical analysis is to reduce the number of covariates. This can be done in a forward selection procedure that includes the selection of the variable to enter, the decision to retain it or stop the selection and estimation of the augmented model. Least squares plus t-tests can be fast, but the outcome of a forward selection might be suboptimal when there are outliers. In this paper, we propose a complete algorithm for fast robust model selection, including considerations for huge sample sizes. Since simply replacing the classical statistical criteria by robust ones is not computationally possible, we develop simplified robust estimators, selection criteria and testing procedures for linear regression. The robust estimator is a one-step weighted M-estimator that can be biased if the covariates are not orthogonal. We show that the bias can be made smaller by [...
Probleme des Alkoholmissbrauchs junger Soldaten im Vergleich zu gleichaltrigen Zivilpersonen T. 1: Kasuistische Vergleichsuntersuchung
No full textUuStB Koeln(38)-950106571 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Probleme des Alkoholmissbrauchs junger Soldaten im Vergleich zu gleichaltrigen Zivilpersonen T. 2: Repraesentative Vergleichsuntersuchung
No full textUuStB Koeln(38)-950106572 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Predicting LyC emission of galaxies using their physical and Ly? emission properties
No full textAims. The primary difficulty in understanding the sources and processes that powered cosmic reionization is that it is not possible to directly probe the ionizing Lyman-continuum (LyC) radiation at that epoch as those photons have been absorbed by the intervening neutral hydrogen. It is therefore imperative to build a model to accurately predict LyC emission using other properties of galaxies in the reionization era.
Methods. In recent years, studies have shown that the LyC emission from galaxies may be correlated to their Lyman-alpha (LyAlpha) emission. In this paper we study this correlation by analyzing thousands of simulated galaxies at high redshift in the SPHINX cosmological simulation. We post-process these galaxies with the LyAlpha radiative transfer code RASCAS and analyze the LyAlpha - LyC connection.
Results. We find that the LyAlpha and LyC luminosities are strongly correlated with each other, although with dispersion. There is a positive correlation between the escape fractions of LyAlpha and LyC radiations in the brightest Lyman-alpha emitters (LAEs; escaping LyAlpha luminosity L^LyAlpha esc > 1041 erg s1), similar to that reported by recent observational studies. However, when we also include fainter LAEs, the correlation disappears, which suggests that the observed relation may be driven by selection effects. We also find that the brighter LAEs are dominant contributors to reionization, with L^LyAlpha_esc > 1040 erg s1 galaxies accounting for >90% of the total amount of LyC radiation escaping into the intergalactic medium in the simulation. Finally, we build predictive models using multivariate linear regression, where we use the physical and LyAlpha properties of simulated reionization era galaxies to predict their LyC emission.
We build a set of models using different sets of galaxy properties as input parameters and predict their intrinsic and escaping LyC luminosity with a high degree of accuracy (the adjusted R2 of these predictions in our fiducial model are 0.89 and 0.85, respectively, where R2 is a measure of how much of the response variance is explained by the model). We find that the most important galaxy properties for predicting the escaping LyC luminosity of a galaxy are its LLyAlpha
esc , gas mass, gas metallicity, and star formation rate.
Conclusions. These results and the predictive models can be useful for predicting the LyC emission from galaxies using their physical and LyAlpha properties and can thus help us identify the sources of reionization
Thermal conductance of interfaces between titanium nitride and group IV semiconductors at high temperatures
No full textThis article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Samreen Khan, Xinping Shi, Joseph Feser, Richard Wilson; Thermal conductance of interfaces between titanium nitride and group IV semiconductors at high temperatures. Appl. Phys. Lett. 22 July 2024; 125 (4): 041601. https://doi.org/10.1063/5.0220124 and may be found at https://doi.org/10.1063/5.0220124.
© 2024 Author(s). Published under an exclusive license by AIP Publishing.
This article will be embargoed until 07/22/2025.Measuring the temperature dependence of material properties is a standard method for better understanding the microscopic origins for that property. Surprisingly, only a few experimental studies of thermal boundary conductance at high temperatures exist. This lack of high temperature data makes it difficult to evaluate competing theories for how inelastic processes contribute to thermal conductance. To address this, we report time domain thermoreflectance measurements of the thermal boundary conductance for TiN on diamond, silicon-carbide, silicon, and germanium between 120 and 1000 K. In all systems, the interface conductance increases monotonically without stagnating at higher temperatures. For TiN/SiC interfaces, ranges from 330 to 1000 MW/m2-K, with a room temperature conductance of 750 MW/m2-K. The interface conductance for TiN/diamond ranges from 140 to 950 MW/m2-K. Notably, for all four interfacial systems, the conductance continues to increase with temperature even after all phonon modes in the vibrationally soft material are thermally excited. This observation suggests that inelastic processes are significant contributors to the thermal conductance in all four interfacial systems, regardless of whether the materials forming the interface are vibrationally similar or dissimilar. Our study fills a notable gap in the literature for how interfacial conductance evolves at high temperatures and tests burgeoning theories for the role of inelastic processes in interfacial thermal transport.This work was supported as part of ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0021230
Robust MM-Estimation and Inference in Mixed Linear Models
No full textMixed linear models are used to analyse data in many settings. These models generally rely on the normality assumption and are often fitted by means of the maximum likelihood estimator (MLE) or the restricted maximum likelihood estimator (REML). However, the sensitivity of these estimation techniques and related tests to this underlying assumption has been identified as a weakness that can even lead to wrong interpretations. Recently Copt and Victoria-Feser(2005) proposed a high breakdown estimator, namely an S-estimator, for general mixed linear models. It has the advantage of being easy to compute - even for highly structured variance matrices - and allow the computation of a robust score test. However this proposal cannot be used to define a likelihood ratio type test which is certainly the most direct route to robustify an F-test. As the latter is usually a key tool to test hypothesis in mixed linear models, we propose two new robust estimators that allow the desired extension. They also lead to resistant Wald-type tests useful for testing contrasts and covariate efects. We study their properties theoretically and by means of simulations. An analysis of a real data set illustrates the advantage of the new approach in the presence of outlying observations.
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