Página 1 dos resultados de 1453 itens digitais encontrados em 0.019 segundos

- AMER DAIRY SCIENCE ASSOC-ADSA
- ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD
- Biblioteca Digitais de Teses e Dissertações da USP
- Universidade de Évora
- Copyright © 2007 by the Genetics Society of America
- BioMed Central
- Blackwell Science Inc
- Oxford University Press
- SAGE Publications
- Universidade Carlos III de Madrid
- The Royal Society
- Universidade Cornell
- Mais Publicadores...

## Parametric correlation functions to model the structure of permanent environmental (co)variances in milk yield random regression models

Fonte: AMER DAIRY SCIENCE ASSOC-ADSA
Publicador: AMER DAIRY SCIENCE ASSOC-ADSA

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.04%

#covariance function#genetic parameter#parametric correlation structure#GENETIC-PARAMETERS#COVARIANCE STRUCTURE#LONGITUDINAL DATA#LACTATION#CATTLE#COWS#Agriculture, Dairy & Animal Science#Food Science & Technology

The objective of the present study was to estimate milk yield genetic parameters applying random regression models and parametric correlation functions combined with a variance function to model animal permanent environmental effects. A total of 152,145 test-day milk yields from 7,317 first lactations of Holstein cows belonging to herds located in the southeastern region of Brazil were analyzed. Test-day milk yields were divided into 44 weekly classes of days in milk. Contemporary groups were defined by herd-test-day comprising a total of 2,539 classes. The model included direct additive genetic, permanent environmental, and residual random effects. The following fixed effects were considered: contemporary group, age of cow at calving (linear and quadratic regressions), and the population average lactation curve modeled by fourth-order orthogonal Legendre polynomial. Additive genetic effects were modeled by random regression on orthogonal Legendre polynomials of days in milk, whereas permanent environmental effects were estimated using a stationary or nonstationary parametric correlation function combined with a variance function of different orders. The structure of residual variances was modeled using a step function containing 6 variance classes. The genetic parameter estimates obtained with the model using a stationary correlation function associated with a variance function to model permanent environmental effects were similar to those obtained with models employing orthogonal Legendre polynomials for the same effect. A model using a sixth-order polynomial for additive effects and a stationary parametric correlation function associated with a seventh-order variance function to model permanent environmental effects would be sufficient for data fitting.; State of Sao Paulo Research Foundation (Fapesp); National Council of Technological and Scientific Development (CNPq)

Link permanente para citações:

## Covariance structure in the skull of Catarrhini: a case of pattern stasis and magnitude evolution

Fonte: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD
Publicador: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

56.24%

#Quantitative genetics#P-matrix#G-matrix#Covariance structure#Skull morphology#Old World monkeys#Apes#QUANTITATIVE GENETIC-ANALYSIS#NEW-WORLD MONKEYS#G-MATRIX#PHENOTYPIC CORRELATIONS

The study of the genetic variance/covariance matrix (G-matrix) is a recent and fruitful approach in evolutionary biology, providing a window of investigating for the evolution of complex characters. Although G-matrix studies were originally conducted for microevolutionary timescales, they could be extrapolated to macroevolution as long as the G-matrix remains relatively constant, or proportional, along the period of interest. A promising approach to investigating the constancy of G-matrices is to compare their phenotypic counterparts (P-matrices) in a large group of related species; if significant similarity is found among several taxa, it is very likely that the underlying G-matrices are also equivalent. Here we study the similarity of covariance and correlation structure in a broad sample of Old World monkeys and apes (Catarrhini). We made phylogenetically structured comparisons of correlation and covariance matrices derived from 39 skull traits, ranging from between species to the superfamily level. We also compared the overall magnitude of integration between skull traits (r(2)) for all Catarrhim genera. Our results show that P-matrices were not strictly constant among catarrhines, but the amount of divergence observed among taxa was generally low. There was significant and positive correlation between the amount of divergence in correlation and covariance patterns among the 30 genera and their phylogenetic distances derived from a recently proposed phylogenetic hypothesis. Our data demonstrate that the P-matrices remained relatively similar along the evolutionary history of catarrhines...

Link permanente para citações:

## Análise de modelos lineares mistos com um fator longitudinal quantitativo e um qualitativo ordinal; Analysis of linear mixed models with one quantitative and one ordinal qualitative longitudinal factor

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Tese de Doutorado
Formato: application/pdf

Publicado em 08/08/2014
PT

Relevância na Pesquisa

46.11%

#Covariance structure#Dois fatores longitudinais#Estrutura de Covariância#Linear mixed-effects model#Massa seca de raiz#Modelo linear misto#Root dry mass#Two longitudinal factors

Os experimentos agronômicos que envolvem somente um fator longitudinal são bastante comuns. No entanto, existem casos em que as observações são tomadas considerando dois ou mais desses fatores, como nos casos em que são feitas medidas de uma variável resposta em profundidades diferentes ao longo do tempo, por exemplo. Admite-se que essas observações, tomadas de modo sistemático em cada unidade experimental, sejam correlacionadas e as variâncias nos diferentes níveis do fator longitudinal sejam heterogêneas. Com o uso de modelos mistos, essa correlação entre medidas repetidas e a heterogeneidade de variâncias podem ser modeladas convenientemente. Para que esses modelos sejam ajustados a um conjunto de dados envolvendo presença de dois fatores longitudinais, existe a necessidade de se adaptarem algumas estruturas de variâncias e covariâncias que são comuns em experimentos com somente um fator longitudinal. O objetivo do presente trabalho é utilizar a classe dos modelos lineares mistos para estudar a massa seca de raiz no solo de uma plantação de cana-de-açúcar. O experimento foi casualizado em blocos e as parcelas receberam quatro doses de nitrogênio. Foram feitas medidas repetidas ao longo de dois fatores longitudinais...

Link permanente para citações:

## Small Sample Bias of Alternative Estimation Methods for Moment Condition Models: Monte Carlo Evidence for Covariance Structures and Instrumental Variables

Fonte: Universidade de Évora
Publicador: Universidade de Évora

Tipo: Trabalho em Andamento

ENG

Relevância na Pesquisa

56.07%

#GMM#Continuous Updating#Empirical Likelihood#Exponential Tilting#Analytical and Bootstrap Bias-Adjusted Estimators#Covariance Structure Models#Instrumental Variables#Monte Carlo Simulation

It is now widely recognized that the most commonly used efficient two-step GMM estimator may have large bias in small samples. This problem has motivated the search for alternative estimators with better finite sample properties. Two classes of alternatives are considered in this paper. The first includes estimators which are asymptotically first-order equivalent to the GMM estimator, namely the continuous-updating, exponential tilting, and empirical likelihood estimators. Analytical and bootstrap bias-adjusted GMM estimators form the second class of alternatives. Two extensive Monte Carlo simulation studies are conducted in this paper for covariance structure and instrumental variable models. We conclude that all alternative estimators offer much reduced bias as compared to the GMM estimator, particularly the empirical likelihood and some of the bias-corrected GMM estimators analyzed.

Link permanente para citações:

## A simulation study of the effects of assignment of prior identity-by-descent probabilities to unselected sib pairs, in covariance-structure modeling of a quantitative-trait locus.

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /01/1999
EN

Relevância na Pesquisa

46.11%

Sib pair-selection strategies, designed to identify the most informative sib pairs in order to detect a quantitative-trait locus (QTL), give rise to a missing-data problem in genetic covariance-structure modeling of QTL effects. After selection, phenotypic data are available for all sibs, but marker data-and, consequently, the identity-by-descent (IBD) probabilities-are available only in selected sib pairs. One possible solution to this missing-data problem is to assign prior IBD probabilities (i.e., expected values) to the unselected sib pairs. The effect of this assignment in genetic covariance-structure modeling is investigated in the present paper. Two maximum-likelihood approaches to estimation are considered, the pi-hat approach and the IBD-mixture approach. In the simulations, sample size, selection criteria, QTL-increaser allele frequency, and gene action are manipulated. The results indicate that the assignment of prior IBD probabilities results in serious estimation bias in the pi-hat approach. Bias is also present in the IBD-mixture approach, although here the bias is generally much smaller. The null distribution of the log-likelihood ratio (i.e., in absence of any QTL effect) does not follow the expected null distribution in the pi-hat approach after selection. In the IBD-mixture approach...

Link permanente para citações:

## A Covariance Structure Model for the Admixture of Binary Genetic Variation

Fonte: Copyright © 2007 by the Genetics Society of America
Publicador: Copyright © 2007 by the Genetics Society of America

Tipo: Artigo de Revista Científica

Publicado em /08/2007
EN

Relevância na Pesquisa

46.01%

I derive a covariance structure model for pairwise linkage disequilibrium (LD) between binary markers in a recently admixed population and use a generalized least-squares method to fit the model to two different data sets. Both linked and unlinked marker pairs are incorporated in the model. Under the model, a pairwise LD matrix is decomposed into two component matrices, one containing LD attributable to admixture, and another containing, in an aggregate form, LD specific to the populations forming the mixture. I use population genetics theory to show that the latter matrix has block-diagonal structure. For the data sets considered here, I show that the number of source populations can be determined by statistical inference on the canonical correlations of the sample LD matrix.

Link permanente para citações:

## Use of the score test as a goodness-of-fit measure of the covariance structure in genetic analysis of longitudinal data

Fonte: BioMed Central
Publicador: BioMed Central

Tipo: Artigo de Revista Científica

Publicado em 15/03/2003
EN

Relevância na Pesquisa

46.11%

Model selection is an essential issue in longitudinal data analysis since many different models have been proposed to fit the covariance structure. The likelihood criterion is commonly used and allows to compare the fit of alternative models. Its value does not reflect, however, the potential improvement that can still be reached in fitting the data unless a reference model with the actual covariance structure is available. The score test approach does not require the knowledge of a reference model, and the score statistic has a meaningful interpretation in itself as a goodness-of-fit measure. The aim of this paper was to show how the score statistic may be separated into the genetic and environmental parts, which is difficult with the likelihood criterion, and how it can be used to check parametric assumptions made on variance and correlation parameters. Selection of models for genetic analysis was applied to a dairy cattle example for milk production.

Link permanente para citações:

## Nonparametric Modeling of Longitudinal Covariance Structure in Functional Mapping of Quantitative Trait Loci

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /12/2009
EN

Relevância na Pesquisa

46.17%

Estimation of the covariance structure of longitudinal processes is a fundamental prerequisite for the practical deployment of functional mapping designed to study the genetic regulation and network of quantitative variation in dynamic complex traits. We present a nonparametric approach for estimating the covariance structure of a quantitative trait measured repeatedly at a series of time points. Specifically, we adopt Huang et al.’s (2006a) approach of invoking the modified Cholesky decomposition and converting the problem into modeling a sequence of regressions of responses. A regularized covariance estimator is obtained using a normal penalized likelihood with an L2 penalty. This approach, embedded within a mixture likelihood framework, leads to enhanced accuracy, precision and flexibility of functional mapping while preserving its biological relevance. Simulation studies are performed to reveal the statistical properties and advantages of the proposed method. A real example from a mouse genome project is analyzed to illustrate the utilization of the methodology. The new method will provide a useful tool for genome-wide scanning for the existence and distribution of quantitative trait loci underlying a dynamic trait important to agriculture...

Link permanente para citações:

## Developmental plasticity in covariance structure of the skull: effects of prenatal stress

Fonte: Blackwell Science Inc
Publicador: Blackwell Science Inc

Tipo: Artigo de Revista Científica

EN

Relevância na Pesquisa

46.28%

Environmental perturbations of many kinds influence growth and development. Little is known, however, about the influence of environmental factors on the patterns of phenotypic integration observed in complex morphological traits. We analyze the changes in phenotypic variance–covariance structure of the rat skull throughout the early postnatal ontogeny (from birth to weaning) and evaluate the effect of intrauterine growth retardation (IUGR) on this structure. Using 2D coordinates taken from lateral radiographs obtained every 4 days, from birth to 21 days old, we show that the pattern of covariance is temporally dynamic from birth to 21 days. The environmental perturbation provoked during pregnancy altered the skull growth, and reduced the mean size of the IUGR group. These environmental effects persisted throughout lactancy, when the mothers of both groups received a standard diet. More strikingly, the effect grew larger beyond this point. Altering environmental conditions did not affect all traits equally, as revealed by the low correlations between covariance matrices of treatments at the same age. Finally, we found that the IUGR treatment increased morphological integration as measured by the scaled variance of eigenvalues. This increase coincided and is likely related to an increase in morphological variance in this group. This result is expected if somatic growth is a major determinant of covariance structure of the skull. In summary...

Link permanente para citações:

## Constrained Maximum Likelihood Estimation for Two-level Mean and Covariance Structure Models2

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em 22/03/2011
EN

Relevância na Pesquisa

45.97%

Maximum likelihood is commonly used for estimation of model parameters in analysis of two-level structural equation models. Constraints on model parameters could be encountered in some situations such as equal factor loadings for different factors. Linear constraints are the most common ones and they are relatively easy to handle in maximum likelihood analysis. Nonlinear constraints could be encountered in complicated applications. In this paper we develop an EM-type algorithm for estimating model parameters with both linear and nonlinear constraints. The empirical performance of the algorithm is demonstrated by a Monte Carlo study. Application of the algorithm for linear constraints is illustrated by setting up a two-level mean and covariance structure model for a real two-level data set and running an EQS program.

Link permanente para citações:

## Modelling marginal covariance structures in linear mixed models

Fonte: IWSM
Publicador: IWSM

Tipo: info:eu-repo/semantics/conferenceObject; all_ul_research; ul_published_reviewed

ENG

Relevância na Pesquisa

46.17%

peer-reviewed; Pourahmadi (1999) provided a convenient reparameterisation of the marginal
covariance matrix arising in longitudinal studies. We exploit his work to
model the dependence of this covariance structure on baseline covariates,
time and their interaction. The rationale for this approach is the realisation
that in linear mixed models (LMMs) the assumption of a homogeneous covariance
structure with respect to the covariate space is a testable model
choice. Accordingly, we provide methods for testing this assumption and
re-analyse Kenward’s (1987) cattle data set using our new model.

Link permanente para citações:

## Modelling of covariance structure in constrained marginal models for longitudinal data

Fonte: IWSM
Publicador: IWSM

Tipo: info:eu-repo/semantics/conferenceObject; all_ul_research; ul_published_reviewed

ENG

Relevância na Pesquisa

45.98%

#longitudinal data#inequality constraints#marginal models#covariance modelling#Cholesky decomposition

peer-reviewed; A data-driven method for modelling intra-subject covariance matrix is introduced to constrained marginal models with longitudinal data. A constrained iteratively re-weighted least squares algorithm is presented consequently.
Asymptotic properties of the constrained ML estimates, including strong consistency, approximate representation and asymptotic distribution, are given. Real
data analysis and simulations are conducted to compare our new approach with
classical menu-selection-based modelling technique.

Link permanente para citações:

## Modelling covariance structure in bivariate marginal models for longitudinal data

Fonte: Oxford University Press
Publicador: Oxford University Press

Tipo: info:eu-repo/semantics/article; all_ul_research; ul_published_reviewed

ENG

Relevância na Pesquisa

46.11%

#longitudinal data#bivariate marginal models#covariance modelling#block triangular factorization#matrix logarithm#modelling log-innovation matrices

peer-reviewed; It can be more challenging and demanding to efficiently model the covariance matrices for
multivariate longitudinal data than for univariate case because of the correlations between
responses arising from multiple variables and repeated measurements over time. In addition to the more complicated covariance structures, the positive-definiteness constraint
is still the major obstacle in modelling covariance matrices as in univariate case. In this
paper, we develop a data-based method to model the covariance structures. Using this
method, the constrained and hard-to-model parameters of ∑i are traded in for uncon-
strained and interpretable parameters. Estimates of these parameters, together with the
parameters in the mean, are obtained by maximum likelihood approach, and the large-
sample asymptotic properties are derived when the observations are normally distributed.
A simulation is carried out to illustrate the asymptotics. Application to a set of bivariate
visual data shows that our method performs very well even when modelling bivariate
nonstationary dependence structures.

Link permanente para citações:

## The impact of covariance misspecification in group-based trajectory models for longitudinal data with non-stationary covariance structure

Fonte: SAGE Publications
Publicador: SAGE Publications

Tipo: Artigo de Revista Científica

Publicado em //2015
EN

Relevância na Pesquisa

46.2%

#Covariance#model misspecification#mixture models#longitudinal data#group-based trajectory modelling

One purpose of a longitudinal study is to gain a better understanding of how an outcome of interest changes among a given population over time. In what follows, a trajectory will be taken to mean the series of measurements of the outcome variable for an individual. Group-based trajectory modelling methods seek to identify subgroups of trajectories within a population, such that trajectories that are grouped together are more similar to each other than to trajectories in distinct groups. Group-based trajectory models generally assume a certain structure in the covariances between measurements, for example conditional independence, homogeneous variance between groups or stationary variance over time. Violations of these assumptions could be expected to result in poor model performance. We used simulation to investigate the effect of covariance misspecification on misclassification of trajectories in commonly used models under a range of scenarios. To do this we defined a measure of performance relative to the ideal Bayesian correct classification rate. We found that the more complex models generally performed better over a range of scenarios. In particular, incorrectly specified covariance matrices could significantly bias the results but using models with a correct but more complicated than necessary covariance matrix incurred little cost; Christopher E Davies...

Link permanente para citações:

## Some remarks on estimating a covariance structure model from a sample correlation matrix

Fonte: Universidade Carlos III de Madrid
Publicador: Universidade Carlos III de Madrid

Tipo: Trabalho em Andamento
Formato: application/octet-stream; application/octet-stream; application/pdf

Publicado em /09/2000
ENG

Relevância na Pesquisa

46.4%

A popular model in structural equation modeling involves a multivariate normal density with a structured covariance matrix that has been categorized according to a set of thresholds. In this setup one may estimate the covariance structure parameters from the sample tetrachoricl polychoric correlations but only if the covariance structure is scale invariant. Doing so when the covariance structure is not scale invariant results in estimating a more restricted covariance structure than the one intended. When the covariance structure is not scale invariant, then the model parameters must be estimated jointly from the sample thresholds and tetrachoricl polychoric correlations. In general, when fitting a covariance structure from a sample correlation matrix one should consider the population correlation structure under the null hypothesis. This is obtained by pre and post-multiplying the covariance structure by a diagonal matrix consisting of the inverse of the square root of the diagonal of the covariance structure under consideration. We provide computer algebra code for assessing whether a covariance structure is scale invariant and for assessing the identification of threshold and correlation structures.

Link permanente para citações:

## Advances in covariance modelling

Fonte: IWSM
Publicador: IWSM

Tipo: info:eu-repo/semantics/conferenceObject; all_ul_research; ul_published_reviewed

ENG

Relevância na Pesquisa

46.2%

peer-reviewed; Conventionally, in longitudinal studies, the mean structure has been thought
to be more important than the covariance structure between the repeated
measures on the same individual. Often, it has been argued that, with re-
spect to the mean, the covariance was merely a `nuisance parameter' and,
consequently, was not of `scientific interest'. Today, however, one can see
that from a formal statistical standpoint, the inferential problem is entirely
symmetric in both parameters. In recent years there has been a steady
stream of new results and we pause to review some key advances in the expanding field of covariance modelling, In particular, developments since the
seminal work by Pourahmadi (1999, 2000) are traced. While the main focus
is on longitudinal data with continuous responses, emerging approaches to
joint mean-covariance modelling in the GEE, and GLMM arenas are also
considered briefly.

Link permanente para citações:

## The beak of the other finch: coevolution of genetic covariance structure and developmental modularity during adaptive evolution

Fonte: The Royal Society
Publicador: The Royal Society

Tipo: Artigo de Revista Científica

Publicado em 12/04/2010
EN

Relevância na Pesquisa

46.11%

The link between adaptation and evolutionary change remains the most central and least understood evolutionary problem. Rapid evolution and diversification of avian beaks is a textbook example of such a link, yet the mechanisms that enable beak's precise adaptation and extensive adaptability are poorly understood. Often observed rapid evolutionary change in beaks is particularly puzzling in light of the neo-Darwinian model that necessitates coordinated changes in developmentally distinct precursors and correspondence between functional and genetic modularity, which should preclude evolutionary diversification. I show that during first 19 generations after colonization of a novel environment, house finches (Carpodacus mexicanus) express an array of distinct, but adaptively equivalent beak morphologies—a result of compensatory developmental interactions between beak length and width in accommodating microevolutionary change in beak depth. Directional selection was largely confined to the elimination of extremes formed by these developmental interactions, while long-term stabilizing selection along a single axis—beak depth—was mirrored in the structure of beak's additive genetic covariance. These results emphasize three principal points. First...

Link permanente para citações:

## Tests for Large Dimensional Covariance Structure Based on Rao's Score Test

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

46.17%

This paper proposes a new test for covariance matrices structure based on the
correction to Rao's score test in large dimensional framework. By generalizing
the CLT for the linear spectral statistics of large dimensional sample
covariance matrices, the test can be applicable for large dimensional
non-Gaussian variables in a wider range without the restriction of the 4th
moment. Moreover, the amending Rao's score test is also powerful even for the
ultra high dimensionality as $p \gg n$, which breaks the inherent idea that the
corrected tests by RMT can be only used when $p

Link permanente para citações:

## Limiting Laws of Coherence of Random Matrices with Applications to Testing Covariance Structure and Construction of Compressed Sensing Matrices

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/02/2011

Relevância na Pesquisa

46.07%

Testing covariance structure is of significant interest in many areas of
statistical analysis and construction of compressed sensing matrices is an
important problem in signal processing. Motivated by these applications, we
study in this paper the limiting laws of the coherence of an $n\times p$ random
matrix in the high-dimensional setting where $p$ can be much larger than $n$.
Both the law of large numbers and the limiting distribution are derived. We
then consider testing the bandedness of the covariance matrix of a high
dimensional Gaussian distribution which includes testing for independence as a
special case. The limiting laws of the coherence of the data matrix play a
critical role in the construction of the test. We also apply the asymptotic
results to the construction of compressed sensing matrices.

Link permanente para citações:

## Gaussian Multiresolution Models: Exploiting Sparse Markov and Covariance Structure

Fonte: IEEE
Publicador: IEEE

Tipo: Article; PeerReviewed
Formato: application/pdf

Publicado em /03/2010

Relevância na Pesquisa

46.17%

In this paper, we consider the problem of learning Gaussian multiresolution (MR) models in which data are only available at the finest scale, and the coarser, hidden variables serve to capture long-distance dependencies. Tree-structured MR models have limited modeling capabilities, as variables at one scale are forced to be uncorrelated with each other conditioned on other scales. We propose a new class of Gaussian MR models in which variables at each scale have sparse conditional covariance structure conditioned on other scales. Our goal is to learn a tree-structured graphical model connecting variables across scales (which translates into sparsity in inverse covariance), while at the same time learning sparse structure for the conditional covariance (not its inverse) within each scale conditioned on other scales. This model leads to an efficient, new inference algorithm that is similar to multipole methods in computational physics. We demonstrate the modeling and inference advantages of our approach over methods that use MR tree models and single-scale approximation methods that do not use hidden variables.

Link permanente para citações: