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Generalized Estimating Equations For Clustered Survival Data , Xiaohong Zhang Jan 2006

Generalized Estimating Equations For Clustered Survival Data , Xiaohong Zhang

Retrospective Theses and Dissertations

Although time to event data is traditionally analyzed assuming independent responses, it is common to encounter correlated time to event data in the form of repeated measurements on subjects or clusters of subjects formed by genetic or social relationships. The objective of this research is to develop estimation procedures for clustered survival data that improve efficiency in estimating regression coefficients in Cox proportional hazards model without imposing overwhelming computational burdens;A commonly used method for clustered survival data obtains parameter estimates from the partial likelihood score equations based on a model that incorrectly assumes independent observations. This independent working model ...


Applications Of Nonparametric Regression In Survey Statistics , Xiaoxi Li Jan 2006

Applications Of Nonparametric Regression In Survey Statistics , Xiaoxi Li

Retrospective Theses and Dissertations

Systematic sampling is a frequently used sampling method in natural resource surveys, because of its ease of implementation and its design efficiency. An important drawback of systematic sampling, however, is that no direct estimator of the design variance is available. We propose an estimator of the model-based expectation of the design variance, under a nonparametric model for the population. The nonparametric model is sufficiently flexible that it can be expected to hold at least approximately for many practical situations. We prove that the nonparametric variance estimator is both a consistent estimator for the model-based expectation of the design variance and ...


Estimation For Two-Phase Longitudinal Surveys With Application To The National Resources Inventory , Jason Colin Legg Jan 2006

Estimation For Two-Phase Longitudinal Surveys With Application To The National Resources Inventory , Jason Colin Legg

Retrospective Theses and Dissertations

Many longitudinal surveys can be represented as two-phase samples, where the entire set of observed units can be considered to be a first-phase sample and the second-phase samples are panels that are observed at times determined by an observation scheme. Common longitudinal schemes such as pure panel, rotating panels, and supplemented panels fit this two-phase sample description. We propose a cell-mean model with a fixed number of time points and panels where the model links the vector of second-phase panel means to the vector of time point population means. Covariance matrix estimation techniques that rely on fitting theoretical autocovariance functions ...


Estimation Of Accelerated Failure Time Models With Random Effects , Yaqin Wang Jan 2006

Estimation Of Accelerated Failure Time Models With Random Effects , Yaqin Wang

Retrospective Theses and Dissertations

Correlated survival data with possible censoring are frequently encountered in survival analysis. This includes multi center studies where subjects are clustered by clinical or other environmental factors that influence expected survival time, studies where times to several different events are monitored on each subject, and studies using groups of genetically related subjects. To analyze such data, we propose accelerated failure time (AFT) models based on lognormal frailties. AFT models provide a linear relationship between the log of the failure time and covariates that affect the expected time to failure by contracting or expanding the time scale. These models account for ...


New Statistical Methods In Bioinformatics: For The Analysis Of Quantitative Trait Loci (Qtl), Microarrays, And Eqtls , Rhonda Decook Jan 2006

New Statistical Methods In Bioinformatics: For The Analysis Of Quantitative Trait Loci (Qtl), Microarrays, And Eqtls , Rhonda Decook

Retrospective Theses and Dissertations

This thesis focuses on new statistical methods in the area of bioinformatics which uses computers and statistics to solve biological problems. The first study discusses a method for detecting a quantitative trait locus (QTL) when the trait of interest has a zero-inflated Poisson (ZIP) distribution. Though existing methods based on normality may be reasonably applied to some ZIP distributions, the characteristics of other ZIP distributions make such an application inappropriate. We compare our method to an existing non-parametric approach, and we illustrate our method using QTL data collected on two ecotypes of the Arabidopsis thaliana plant where the trait of ...


Weedy Setaria Species-Group Seed Heteroblasty Blueprints Seedling Recruitment , Kari Ann Jovaag Jan 2006

Weedy Setaria Species-Group Seed Heteroblasty Blueprints Seedling Recruitment , Kari Ann Jovaag

Retrospective Theses and Dissertations

The relationship between weedy Setaria seed dormancy and subsequent behaviors in the soil culminating in seedling recruitment is elucidated. Weedy Setaria seed dormancy capacity heterogeneity (heteroblasty) at the time of dispersal was characterized for 45 locally adapted Setaria populations, as influenced by parental genotype (species, time of embryogenesis) and environment (year, location). Taken together, the 45 responses represented Setaria's "seed dormancy phenotype space". The fate of heteroblastic seed entering the soil post-abscission was studied in four of the populations. When dispersed, heteroblastic Setaria seeds introduced into the soil form long-lived pools with varying cycles of dormancy, germination and death ...


New Methods For Designing And Analyzing Microarray Experiments For The Detection Of Differential Expression, Justin C. Recknor Jan 2006

New Methods For Designing And Analyzing Microarray Experiments For The Detection Of Differential Expression, Justin C. Recknor

Retrospective Theses and Dissertations

This thesis is divided into three sections all pertaining to microarray experimental design and analysis. Microarrays are a tool used in biological research which enables scientists to measure the relative level of expression many genes within an organism at the same time. Microarrays have also opened new research areas in statistics which are currently being investigated concerning different aspects of data normalization, experimental design, and analysis;The first chapter entails a comparison of two commonly used experimental designs in two-dye microarray experiments. Both designs are applicable only to experiments containing treatments with two levels. One design is shown to be ...


Poisson Process Models For A Combination Of Points And Counts In Space , Han Wu Jan 2006

Poisson Process Models For A Combination Of Points And Counts In Space , Han Wu

Retrospective Theses and Dissertations

A spatial point process is a stochastic model determining the locations of events in some region A ⊂ realsd. Events may be nests in a breeding colony of birds, tress in a forest, or cities in a country. One goal of spatial statistics is to model the underlying process and thus interpret a complicated point process through some parameter estimates based on the known locations of events from some spatial point processes;Techniques have been developed for estimating the parameters of spatial point process, given data at either the aggregate or point levels. However, it remains unclear how to model aggregate ...


Statistics On Stiefel Manifolds , Gabriel Camano-Garcia Jan 2006

Statistics On Stiefel Manifolds , Gabriel Camano-Garcia

Retrospective Theses and Dissertations

When the orientation of an object lies in a space of non-zero curvature usual distributions of probability cannot be used to describe its directions. One of such spaces is the Stiefel manifold. We focus on a probability distribution defined on that space, the matrix Langevin distribution. Classical and Bayesian methods of estimation of the parameter of the distribution are discussed. As the dimension of the Stiefel manifold increases, the more complicated the estimation process becomes given the complexity of the functions to be evaluated. A method is given that efficiently parameterizes the elements of the singular value decomposition of the ...


Statistical Inference For Particle Systems From Sieving Studies , Norma Leyva-Estrada Jan 2006

Statistical Inference For Particle Systems From Sieving Studies , Norma Leyva-Estrada

Retrospective Theses and Dissertations

This dissertation considers several aspects of inference from particle sieving data. Such data comprise interval-censored particle sizes, and weight fractions of particles in each size interval;Under a model of random sampling of particles up to a target total weight, a sample of particles can be described using renewal theory, and the asymptotic distribution of the empirical weight fraction vector is multivariate normal. The model assumptions are that the particle size distribution being sampled has a standard probability density and that the first two moments of the conditional distribution of weight given size can be described with a power law ...


Extensions Of Small Area Models With Applications To The National Resources Inventory , Pushpal Mukhopadhyay Jan 2006

Extensions Of Small Area Models With Applications To The National Resources Inventory , Pushpal Mukhopadhyay

Retrospective Theses and Dissertations

The National Resources Inventory (NRI) is a longitudinal survey of non federal lands in the US. The objectives of the survey are to produce estimates for variables related to land use, land cover and soil erosion at the national and sub-national level. Three extensions of existing small area models are proposed for estimating soil erosion for counties;A transformed Fay-Herriot model is developed to estimate wind erosion for the counties in Iowa. A soil erodibility index is available from administrative records for each county and is used as the predictor. The response variable is the soil loss as recorded in ...


Statistical Analysis Of Safety And Health Issues , Qi Jiang Jan 2006

Statistical Analysis Of Safety And Health Issues , Qi Jiang

Retrospective Theses and Dissertations

Statistical analysis is an essential tool in safety and health research. This thesis is composed of statistical analyses for three different types of safety and health projects: power-take-off entanglements, farm stress, and heart problems and diabetes, in additional the related statistical issues are discussed;In the power-take-off entanglements project, an experimental design was used to study the risk of entanglement in power-take-off driveline. The response variable was a binary variable indicating whether a dangerous "entanglement" occurred or not. The experimental factors were angle of introduction, length of specimen, and type (stiffness) of material. The main conclusion was that the entanglement ...


New Aspects Of Statistical Methods For Missing Data Problems, With Applications In Bioinformatics And Genetics , Dong Wang Jan 2006

New Aspects Of Statistical Methods For Missing Data Problems, With Applications In Bioinformatics And Genetics , Dong Wang

Retrospective Theses and Dissertations

As missing data problems become more commonplace in biological research and other areas, a method with relaxed assumptions while flexible enough to accommodate a wide range of situations is highly desired. We propose a nonparametric imputation method for data with missing values. The inference on the parameter defined by general estimating equations is performed using an empirical likelihood method. It is shown that the nonparametric imputation method together with empirical likelihood can reduce bias and improve efficiency of the estimate relative to inference using only complete cases of the dataset. The confidence regions obtained by empirical likelihood demonstrate good coverage ...


Advanced Statistical Methods For Analysis Of Nde Data , Yurong Wang Jan 2006

Advanced Statistical Methods For Analysis Of Nde Data , Yurong Wang

Retrospective Theses and Dissertations

Nondestructive Evaluation (NDE) is a quality-ensuring technique widely used in modern industry. For example, ultrasonic inspection is a routine NDE method to detect flaws/defects in rotating components of jet engines. However, there are random factors that can affect the performance and reliability of NDE systems. Probability of detection (POD) is an important metric for quantifying NDE capability and reliability. The most commonly used POD assessment method is known as the a versus a method. However, the standard a versus a method can not be directly applied to some new modern NDE applications. The objective of this research is to ...


Toxicokinetic-Based Survival Model For Analysis Of Toxicity Data With Changing Dose , Xia Xu Jan 2006

Toxicokinetic-Based Survival Model For Analysis Of Toxicity Data With Changing Dose , Xia Xu

Retrospective Theses and Dissertations

In toxicology studies, some data sets are dose-time-response data with changing doses over time and death/alive as response. We developed a toxicokinetic-based survival model that relates survivorship and bioaccumulation. The model assumes toxicant uptake and elimination follow a single compartment model and the hazard rate is proportional to the internal concentration. Several diagnostics methods were described for this model to check assumptions of proportional hazard and the functional form of covariates. Maximum likelihood and least square estimation were compared by comparing the estimating equations and a simulation study. The results showed that maximum likelihood is the most unbiased and ...


The Enhancement Of A Block-Oriented Modeling Method To Improve Inference And Prediction , Gabrielle Lynn Larson Jan 2006

The Enhancement Of A Block-Oriented Modeling Method To Improve Inference And Prediction , Gabrielle Lynn Larson

Retrospective Theses and Dissertations

In process identification (i.e., dynamic model development) information on the precision and reliability of a parameter estimate is conveyed by a confidence interval. The best confidence interval is the one with the shortest width for a given level of confidence. Confidence interval widths widen as the standard error increases or as the number of estimated parameters increases. This thesis focuses on minimizing the width of confidence intervals by reducing the number of estimated dynamic parameters in the context of block-oriented modeling. This objective is accomplished by the development of a procedure that finds parameter equivalencies within (intra) and between ...