| 1. |
- Liang, Yuli, 1985-, et al.
(författare)
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On estimation in multilevel models with block circular symmetric covariance structure
- 2012
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Ingår i: Acta et Commentationes Universitatis Tartuensis de Mathematica. - Tartu : University of Tartu Press. - 1406-2283 .- 2228-4699. ; 16:1, s. 83-96
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Tidskriftsartikel (refereegranskat)abstract
- In this article we consider a multilevel model with block circular symmetric covariance structure. Maximum likelihood estimation of the parameters of this model is discussed. We show that explicit maximum likelihood estimators of variance components exist under certain restrictions on the parameter space.
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| 2. |
- Aad, G., et al.
(författare)
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- 2012
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swepub:Mat__t (refereegranskat)
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| 3. |
- Ahmed, S. Ejaz, et al.
(författare)
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Estimation of Several Intraclass Correlation Coefficients
- 2015
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Ingår i: Communications in statistics. Simulation and computation. - : Informa UK Limited. - 0361-0918 .- 1532-4141. ; 44:9, s. 2315-2328
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Tidskriftsartikel (refereegranskat)abstract
- An intraclass correlation coefficient observed in several populations is estimated. The basis is a variance-stabilizing transformation. It is shown that the intraclass correlation coefficient from any elliptical distribution should be transformed in the same way. Four estimators are compared. An estimator where the components in a vector consisting of the transformed intraclass correlation coefficients are estimated separately, an estimator based on a weighted average of these components, a pretest estimator where the equality of the components is tested and then the outcome of the test is used in the estimation procedure, and a James-Stein estimator which shrinks toward the mean.
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| 4. |
- Hao, Chengcheng, 1986-
(författare)
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Explicit Influence Analysis in Crossover Models
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation develops influence diagnostics for crossover models. Mixed linear models and generalised mixed linear models are utilised to investigate continuous and count data from crossover studies, respectively.For both types of models, changes in the maximum likelihood estimates of parameters, particularly in the estimated treatment effect, due to minor perturbations of the observed data, are assessed. The novelty of this dissertation lies in the analytical derivation of influence diagnostics using decompositions of the perturbed mixed models. Consequently, the suggested influence diagnostics, referred to as the delta-beta and variance-ratio influences, provide new findings about how the constructed residuals affect the estimation in terms of different parameters of interest.The delta-beta and variance-ratio influence in three different crossover models are studied in Chapters 5-6, respectively. Chapter 5 analyses the influence of subjects in a two-period continuous crossover model. Possible problems with observation-level perturbations in crossover models are discussed. Chapter 6 extends the approach to higher-order crossover models. Furthermore, not only the individual delta-beta and variance-ratio influences of a subject are derived, but also the joint influences of two subjects from different sequences. Chapters 5-6 show that the delta-beta and variance-ratio influences of a particular parameter are decided by the special linear combination of the constructed residuals. In Chapter 7, explicit delta-beta influence on the estimated treatment effect in the two-period count crossover model is derived. The influence is related to the Pearson residuals of the subject. Graphical tools are developed to visualise information of influence concerning crossover models for both continuous and count data. Illustrative examples are provided in each chapter.
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| 5. |
- Hao, Chengcheng, 1986-, et al.
(författare)
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Explicit Influence Analysis in Two-Treatment Balanced Crossover Models
- 2015
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Ingår i: Mathematical Methods of Statistics. - New York, NY, United States : Allerton Press, Inc.. - 1066-5307 .- 1934-8045. ; 24:1, s. 16-36
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers how to detect influential observations in crossover models with random individual effects. Two influence measures, the delta-beta influence and variance-ratio influence, are utilized as tools to evaluate the influence of the model on the estimates of mean and variance parameters with respect to case-weighted perturbations, which are introduced to the model for studying the ‘influence’ of cases. The paper provides explicit expressions of the delta-beta and variance-ratio influences for the general two-treatment balanced crossover models when the proposed decompositions for the perturbed models hold. The influence measures for each parameter turn out to be closed-form functions of orthogonal projections of specific residuals in the unperturbed model.
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| 6. |
- Hao, Chengcheng, et al.
(författare)
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Influence analysis in two-treatment cross-over designs with special reference to the ABBA|BAAB design
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This work is to develop methodology to detect influential observations in linear mixed model for multiple-period two-treatment cross-over designs. Existence of explicit maximum likelihood estimates (MLEs) of variance parameters as well as of mean parameters in the mixed model with treatment, residual, period and sequence effects is proven. Special reference is taken to the four-period ABBA|BAAB design. Case-weighted perturbations are performed. The influence quantities on each parameter estimate and their dispersion matrix are presented as closed-form functions of residuals in the unperturbed model.
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| 7. |
- Hao, Chengcheng, 1986-, et al.
(författare)
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Influence diagnostics for count data under AB-BA crossover trials
- 2017
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Ingår i: Statistical Methods in Medical Research. - : SAGE Publications. - 0962-2802 .- 1477-0334. ; 26:6, s. 2938-2950
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Tidskriftsartikel (refereegranskat)abstract
- This paper aims to develop diagnostic measures to assess the influence of data perturbations on estimates in AB-BA crossover studies with a Poisson distributed response. Generalised mixed linear models with normally distributed random effects are utilised. We show that in this special case, the model can be decomposed into two independent sub-models which allow to derive closed-form expressions to evaluate the changes in the maximum likelihood estimates under several perturbation schemes. The performance of the new influence measures is illustrated by simulation studies and the analysis of a real dataset.
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| 8. |
- Hao, Chengcheng, 1986-
(författare)
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Local Influence Analysis and Cross-over Studies
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With a special reference to cross-over design models with random individual effects, the purpose of this dissertation is to develop new methodology to detect influential observations in the context of mixed linear models with explicit maximum likelihood estimators (MLEs).Case-weighted perturbation schemes within and between subjects in mixed models are constructed. It is emphasised that perturbations should be performed under the restriction that explicit MLEs can be obtained in the perturbed model. Two influence functions, the delta-beta influence and variance-ratio influence, are tools to evaluate the influence on the estimates of mean parameters and variance parameters, respectively, with respect to the used perturbations.The proposed approach, named the delta-beta-based local influence approach, derives the expressions of the delta-beta and variance-ratio influences for two specific cross-over designs. In both the AB|BA design (2 X 2 cross-over design) and the ABBA|BAAB design, the applied influence functions turn out to have closed-form expressions of residuals from the unperturbed models. Some graphical tools are also presented.
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| 9. |
- Hao, Chengcheng, et al.
(författare)
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Local influence analysis in 2 X 2 cross-over designs
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this work is to develop new methodology to detect influential observations in cross-over design models with random individual effects. Various case-weighted perturbations are performed. We derive the exact solution of influence of the perturbations on each parameter estimate and their dispersion matrix. Closed-form maximum likelihood estimates (MLEs) of variance parameters as well as fixed effect parameters in the cross-over design models are utilised. The work exhibits the possibility to produce closed-form expressions of the influence using the residuals in mixed models. A discussion on restrictions of the case-weighted perturbation schemes is given. Some graphical tools are also presented.
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| 10. |
- Hao, Chengcheng, et al.
(författare)
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Local Influence Analysis in AB–BA Crossover Designs
- 2014
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Ingår i: Scandinavian Journal of Statistics. - : Wiley. - 0303-6898 .- 1467-9469. ; 41:4, s. 1153-1166
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this article is to develop methodology for detecting influential observations in crossover models with random individual effects. Various case-weighted perturbations are performed. We obtain the influence of the perturbations on each parameter estimator and on their dispersion matrices. The obtained results exhibit the possibility to obtain closed-form expressions of the influence using the residuals in mixed linear models. Some graphical tools are also presented.
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