Página 1 dos resultados de 102 itens digitais encontrados em 0.067 segundos

## ‣ Wavelet regression with correlated errors on a piecewise Holder class

PORTO, Rogerio F.; MORETTIN, Pedro A.; AUBIN, Elisete C. Q.
Fonte: ELSEVIER SCIENCE BV Publicador: ELSEVIER SCIENCE BV
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
47.999688%
This paper generalizes the methodology of Cat and Brown [Cai, T., Brown, L.D., 1998. Wavelet shrinkage for nonequispaced samples. The Annals of Statistics 26, 1783-1799] for wavelet shrinkage for nonequispaced samples, but in the presence of correlated stationary Gaussian errors. If the true function is a member of a piecewise Holder class, it is shown that, even for long memory errors, the rate of convergence of the procedure is almost-minimax relative to the independent and identically distributed errors case. (c) 2008 Elsevier B.V. All rights reserved.; CNPq; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); FAPESP[03/10105-2]; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

## ‣ Influence diagnostics in heteroscedastic and/or autoregressive nonlinear elliptical models for correlated data

Russo, Cibele M.; Paula, Gilberto A.; Cysneiros, Francisco Jose A.; Aoki, Reiko
Fonte: TAYLOR & FRANCIS LTD; ABINGDON Publicador: TAYLOR & FRANCIS LTD; ABINGDON
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
47.6215%
In this paper, we propose nonlinear elliptical models for correlated data with heteroscedastic and/or autoregressive structures. Our aim is to extend the models proposed by Russo et al. [22] by considering a more sophisticated scale structure to deal with variations in data dispersion and/or a possible autocorrelation among measurements taken throughout the same experimental unit. Moreover, to avoid the possible influence of outlying observations or to take into account the non-normal symmetric tails of the data, we assume elliptical contours for the joint distribution of random effects and errors, which allows us to attribute different weights to the observations. We propose an iterative algorithm to obtain the maximum-likelihood estimates for the parameters and derive the local influence curvatures for some specific perturbation schemes. The motivation for this work comes from a pharmacokinetic indomethacin data set, which was analysed previously by Bocheng and Xuping [1] under normality.; FAPESP; FAPESP; FACEPE; FACEPE; CNPq (Brazil); CNPq, Brazil

## ‣ O uso de ondaletas em modelos FANOVA; Wavelets FANOVA models

Airton Kist
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Relevância na Pesquisa
48.06189%

## ‣ Different Dreams, Same Bed : Collecting, Using, and Interpreting Employment Statistics in Sub-Saharan Africa--The Case of Uganda

Fox, Louise; Pimhidzai, Obert
Fonte: World Bank, Washington, DC Publicador: World Bank, Washington, DC
Português
Relevância na Pesquisa
37.808735%
Employment and earnings statistics are the key link between the size and structure of economic growth and the welfare of households, which is the ultimate goal of development policy, so it is important to monitor employment outcomes consistently. A cursory review of employment data for low-income Sub-Saharan African countries shows both large gaps and improbable variation within countries over time and among countries, suggesting that low quality data are routinely reported by national statistics offices. Unfortunately, policies are formed and projects developed and implemented on the basis of these statistics. Therefore, errors of measurement could be having profound implications on the strategic priorities and policies of a country. This paper explains the improbable results observed by using data from Uganda, where the labor module contains variation both within and across surveys, to show the sensitivity of employment outcomes to survey methodology. It finds that estimates of employment outcomes are unreliable if the questionnaire did not use screening questions...

## ‣ Modeling Compositional Regression with uncorrelated and correlated errors: a Bayesian approach

Shimizu, Taciana K. O.; Louzada, Francisco; Suzuki, Adriano K.; Ehlers, Ricardo S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
57.50794%
Compositional data consist of known compositions vectors whose components are positive and defined in the interval (0,1) representing proportions or fractions of a "whole". The sum of these components must be equal to one. Compositional data is present in different knowledge areas, as in geology, economy, medicine among many others. In this paper, we introduce a Bayesian analysis for compositional regression applying additive log-ratio (ALR) transformation and assuming uncorrelated and correlated errors. The Bayesian inference procedure based on Markov Chain Monte Carlo Methods (MCMC). The methodology is illustrated on an artificial and a real data set of volleyball.

## ‣ Moving Taylor Bayesian Regression for nonparametric multidimensional function estimation with possibly correlated errors

Heitzig, Jobst
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.69107%
We present a nonparametric method for estimating the value and several derivatives of an unknown, sufficiently smooth real-valued function of real-valued arguments from a finite sample of points, where both the function arguments and the corresponding values are known only up to measurement errors having some assumed distribution and correlation structure. The method, Moving Taylor Bayesian Regression (MOTABAR), uses Bayesian updating to find the posterior mean of the coefficients of a Taylor polynomial of the function at a moving position of interest. When measurement errors are neglected, MOTABAR becomes a multivariate interpolation method. It contains several well-known regression and interpolation methods as special or limit cases. We demonstrate the performance of MOTABAR using the reconstruction of the Lorenz attractor from noisy observations as an example.; Comment: 22 pages, 4 figures

## ‣ Optimal designs for random effect models with correlated errors with applications in population pharmacokinetics

Dette, Holger; Pepelyshev, Andrey; Holland-Letz, Tim
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.673174%
We consider the problem of constructing optimal designs for population pharmacokinetics which use random effect models. It is common practice in the design of experiments in such studies to assume uncorrelated errors for each subject. In the present paper a new approach is introduced to determine efficient designs for nonlinear least squares estimation which addresses the problem of correlation between observations corresponding to the same subject. We use asymptotic arguments to derive optimal design densities, and the designs for finite sample sizes are constructed from the quantiles of the corresponding optimal distribution function. It is demonstrated that compared to the optimal exact designs, whose determination is a hard numerical problem, these designs are very efficient. Alternatively, the designs derived from asymptotic theory could be used as starting designs for the numerical computation of exact optimal designs. Several examples of linear and nonlinear models are presented in order to illustrate the methodology. In particular, it is demonstrated that naively chosen equally spaced designs may lead to less accurate estimation.; Comment: Published in at http://dx.doi.org/10.1214/09-AOAS324 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org)

## ‣ Assessing Colour-dependent Occupation Statistics Inferred from Galaxy Group Catalogues

Campbell, Duncan; Bosch, Frank C van den; Hearin, Andrew; Padmanabhan, Nikhil; Berlind, Andreas; Mo, H. J.; Tinker, Jeremy; Yang, Xiaohu
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
38.089312%
We investigate the ability of current implementations of galaxy group finders to recover colour-dependent halo occupation statistics. To test the fidelity of group catalogue inferred statistics, we run three different group finders used in the literature over a mock that includes galaxy colours in a realistic manner. Overall, the resulting mock group catalogues are remarkably similar, and most colour-dependent statistics are recovered with reasonable accuracy. However, it is also clear that certain systematic errors arise as a consequence of correlated errors in group membership determination, central/satellite designation, and halo mass assignment. We introduce a new statistic, the halo transition probability (HTP), which captures the combined impact of all these errors. As a rule of thumb, errors tend to equalize the properties of distinct galaxy populations (i.e. red vs. blue galaxies or centrals vs. satellites), and to result in inferred occupation statistics that are more accurate for red galaxies than for blue galaxies. A statistic that is particularly poorly recovered from the group catalogues is the red fraction of central galaxies as function of halo mass. Group finders do a good job in recovering galactic conformity, but also have a tendency to introduce weak conformity when none is present. We conclude that proper inference of colour-dependent statistics from group catalogues is best achieved using forward modelling (i.e....

## ‣ On the Combination Procedure of Correlated Errors

Erler, Jens
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.53219%
When averages of different experimental determinations of the same quantity are computed, each with statistical and systematic error components, then frequently the statistical and systematic components of the combined error are quoted explicitly. These are important pieces of information since statistical errors scale differently and often more favorably with the sample size than most systematical or theoretical errors. In this communication we describe a transparent procedure by which the statistical and systematic error components of the combination uncertainty can be obtained. We develop a general method and derive a general formula for the case of Gaussian errors with or without correlations. The method can easily be applied to other error distributions, as well. For the case of two measurements, we also define disparity and misalignment angles, and discuss their relation to the combination weight factors.; Comment: 11 pages

## ‣ Regularized estimation in sparse high-dimensional time series models

Basu, Sumanta; Michailidis, George
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
37.808735%
Many scientific and economic problems involve the analysis of high-dimensional time series datasets. However, theoretical studies in high-dimensional statistics to date rely primarily on the assumption of independent and identically distributed (i.i.d.) samples. In this work, we focus on stable Gaussian processes and investigate the theoretical properties of $\ell _1$-regularized estimates in two important statistical problems in the context of high-dimensional time series: (a) stochastic regression with serially correlated errors and (b) transition matrix estimation in vector autoregressive (VAR) models. We derive nonasymptotic upper bounds on the estimation errors of the regularized estimates and establish that consistent estimation under high-dimensional scaling is possible via $\ell_1$-regularization for a large class of stable processes under sparsity constraints. A key technical contribution of the work is to introduce a measure of stability for stationary processes using their spectral properties that provides insight into the effect of dependence on the accuracy of the regularized estimates. With this proposed stability measure, we establish some useful deviation bounds for dependent data, which can be used to study several important regularized estimates in a time series setting.; Comment: Published at http://dx.doi.org/10.1214/15-AOS1315 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org)

## ‣ Computing maximum likelihood estimates in recursive linear models with correlated errors

Drton, Mathias; Eichler, Michael; Richardson, Thomas S.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
47.29115%
In recursive linear models, the multivariate normal joint distribution of all variables exhibits a dependence structure induced by a recursive (or acyclic) system of linear structural equations. These linear models have a long tradition and appear in seemingly unrelated regressions, structural equation modelling, and approaches to causal inference. They are also related to Gaussian graphical models via a classical representation known as a path diagram. Despite the models' long history, a number of problems remain open. In this paper, we address the problem of computing maximum likelihood estimates in the subclass of bow-free' recursive linear models. The term bow-free' refers to the condition that the errors for variables $i$ and $j$ be uncorrelated if variable $i$ occurs in the structural equation for variable $j$. We introduce a new algorithm, termed Residual Iterative Conditional Fitting (RICF), that can be implemented using only least squares computations. In contrast to existing algorithms, RICF has clear convergence properties and finds parameter estimates in closed form whenever possible.; Comment: 22 pages; removed an incorrect identifiability claim

## ‣ Autocovariance estimation in change-point regression with $m$-dependent errors: A difference-based approach

Munk, Axel; Tecuapetla-Gómez, Inder
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
47.89008%
We propose a class of difference-based estimators for the autocovariance in nonparametric regression when the signal is discontinuous (change-point model), possibly highly fluctuating, and the errors form a stationary $m$-dependent Gaussian process. These estimators circumvent the explicit pre-estimation of the unknown regression function, a task that is particularly challenging in change-point regression with correlated errors. We provide finite sample expressions for their mean squared errors when the signal function is piecewise constant. Based on this, we distinguish the signal as the major source of the mean squared errors and derive biased-optimized estimates. These optimal estimates do not depend on the particular (unknown) autocovariance structure, and notably, for positive correlated errors, in addition, they minimize that part of the variance which is influenced by the unknown regression function. Further, we provide some asymptotic analysis of our estimators. We show their $\sqrt{n}$-consistency in the context of a piecewise H\"older signal with non-Gaussian stationary $m$-dependent errors and when the number of change-points tends to infinity. Finally, we combine our biased-optimized autocovariance estimates with a projection-based approach and derive covariance matrix estimates for change-point regression...

## ‣ Correlated samples with fixed and nonnormal latent variables

Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
37.895918%
A general structural equation model is fitted on a panel data set that consists of $I$ correlated samples. The correlated samples could be data from correlated populations or correlated observations from occasions of panel data. We consider cases in which the full pseudo-normal likelihood cannot be used, for example, in highly unbalanced data where the participating individuals do not appear in consecutive years. The model is estimated by a partial likelihood that would be the full and correct likelihood for independent and normal samples. It is proved that the asymptotic standard errors (a.s.e.'s) for the most important parameters and an overall-fit measure are the same as the corresponding ones derived under the standard assumptions of normality and independence for all the observations. These results are very important since they allow us to apply classical statistical methods for inference, which use only first- and second-order moments, to correlated and nonnormal data. Via a simulation study we show that the a.s.e.'s based on the first two moments have negligible bias and provide less variability than the a.s.e.'s computed by an alternative robust estimator that utilizes up to fourth moments. Our methodology and results are applied to real panel data...

## ‣ Estimating Mediation Effects under Correlated Errors with an Application to fMRI

Zhao, Yi; Luo, Xi
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.770713%
Mediation analysis assesses the effect passing through a intermediate variable (mediator) in a causal pathway from the treatment variable to the outcome variable. Structure equation model (SEM) is a popular approach to estimate the mediation effect. However, causal interpretation usually requires strong assumptions, such as ignorability of the mediator, which may not hold in many social and scientific studies. In this paper, we use mediation analysis in an fMRI experiment to assess the effect of randomized binary stimuli passing through a brain pathway of two brain regions. We propose a two-layer SEM framework for mediation analysis that provides valid inference even if correlated additive errors are present. In the first layer, we use a liner SEM to model the subject level fMRI data, where the continuous mediator and outcome variables may contain correlated additive errors. We propose a constrained optimization approach to estimate the model coefficients, analyze its asymptotic properties, and characterize the nonidentifiability issue due to the correlation parameter. To address the identifiability issue, we introduce a linear mixed effects SEM with an innovation to estimate the unknown correlation parameter in the first layer, instead of sensitivity analysis. Using extensive simulated data and a real fMRI dataset...

## ‣ Parameter Estimation from Time-Series Data with Correlated Errors: A Wavelet-Based Method and its Application to Transit Light Curves

Carter, Joshua A.; Winn, Joshua N.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.223076%
We consider the problem of fitting a parametric model to time-series data that are afflicted by correlated noise. The noise is represented by a sum of two stationary Gaussian processes: one that is uncorrelated in time, and another that has a power spectral density varying as $1/f^\gamma$. We present an accurate and fast [O(N)] algorithm for parameter estimation based on computing the likelihood in a wavelet basis. The method is illustrated and tested using simulated time-series photometry of exoplanetary transits, with particular attention to estimating the midtransit time. We compare our method to two other methods that have been used in the literature, the time-averaging method and the residual-permutation method. For noise processes that obey our assumptions, the algorithm presented here gives more accurate results for midtransit times and truer estimates of their uncertainties.; Comment: Accepted in ApJ. Illustrative code may be found at http://www.mit.edu/~carterja/code/ . 17 pages

## ‣ Optimal design for linear models with correlated observations

Dette, Holger; Pepelyshev, Andrey; Zhigljavsky, Anatoly
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
37.718657%
In the common linear regression model the problem of determining optimal designs for least squares estimation is considered in the case where the observations are correlated. A necessary condition for the optimality of a given design is provided, which extends the classical equivalence theory for optimal designs in models with uncorrelated errors to the case of dependent data. If the regression functions are eigenfunctions of an integral operator defined by the covariance kernel, it is shown that the corresponding measure defines a universally optimal design. For several models universally optimal designs can be identified explicitly. In particular, it is proved that the uniform distribution is universally optimal for a class of trigonometric regression models with a broad class of covariance kernels and that the arcsine distribution is universally optimal for the polynomial regression model with correlation structure defined by the logarithmic potential. To the best knowledge of the authors these findings provide the first explicit results on optimal designs for regression models with correlated observations, which are not restricted to the location scale model.; Comment: Published in at http://dx.doi.org/10.1214/12-AOS1079 the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org)

## ‣ How to combine correlated data sets -- A Bayesian hyperparameter matrix method

Ma, Yin-Zhe; Berndsen, Aaron
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
37.872202%
We construct a "hyperparameter matrix" statistical method for performing the joint analyses of multiple correlated astronomical data sets, in which the weights of data sets are determined by their own statistical properties. This method is a generalization of the hyperparameter method constructed by Lahav et al. (2000) and Hobson, Bridle, & Lahav (2002) which was designed to combine independent data sets. The advantage of our method is to treat correlations between multiple data sets and gives appropriate relevant weights of multiple data sets with mutual correlations. We define a new "element-wise" product, which greatly simplifies the likelihood function with hyperparameter matrix. We rigorously prove the simplified formula of the joint likelihood and show that it recovers the original hyperparameter method in the limit of no covariance between data sets. We then illustrate the method by applying it to a demonstrative toy model of fitting a straight line to two sets of data. We show that the hyperparameter matrix method can detect unaccounted systematic errors or underestimated errors in the data sets. Additionally, the ratio of Bayes' factors provides a distinct indicator of the necessity of including hyperparameters. Our example shows that the likelihood we construct for joint analyses of correlated data sets can be widely applied to many astrophysical systems.; Comment: 13 pages...

## ‣ Observation of correlated-photon statistics using a single detector

Kim, Yoon-Ho; Grice, Warren P.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
47.47378%
We report experimental observations of correlated-photon statistics in the single-photon detection rate. The usual quantum interference in a two-photon polarization interferometer always accompanies a dip in the single detector counting rate, regardless of whether a dip or peak is seen in the coincidence rate. This effect is explained by taking into account all possible photon number states that reach the detector, rather than considering just the state post-selected by the coincidence measurement. We also report an interferometeric scheme in which the interference peak or dip in coincidence corresponds directly to a peak or dip in the single-photon detection rate.; Comment: 4 pages, two-column (minor errors corrected.)

## ‣ An extension to GUM methodology: degrees-of-freedom calculations for correlated multidimensional estimates

Tipo: Artigo de Revista Científica
Relevância na Pesquisa
48.03918%
The Guide to the Expression of Uncertainty in Measurement advocates the use of an 'effective number of degrees of freedom' for the calculation of an interval of measurement uncertainty. However, it does not describe how this number is to be calculated when (i) the measurand is a vector quantity or (ii) when the errors in the estimates of the quantities defining the measurand (the 'input quantities') are not incurred independently. An appropriate analysis for a vector-valued measurand has been described (Metrologia 39 (2002) 361-9), and a method for a one-dimensional measurand with dependent errors has also been given (Metrologia 44 (2007) 340-9). This paper builds on those analyses to present a method for the situation where the problem is multidimensional and involves correlated errors. The result is an explicit general procedure that reduces to simpler procedures where appropriate. The example studied is from the field of radio-frequency metrology, where measured quantities are often complex-valued and can be regarded as vectors of two elements.; Comment: 30 pages with 2 embedded figures

## ‣ Evaluation of the occurrence and type of antiretroviral and opportunistic infection medication errors within the inpatient setting

Tipo: info:eu-repo/semantics/article; journal article; info:eu-repo/semantics/publishedVersion Formato: text/html; application/pdf