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The least squares estimator is probably the most frequently used estimation method in regression analysis. Unfortunately, it is also quite sensitive to data contamination and model misspecification. Although there are several robust estimators designed for parametric regression models that can be used in place of least squares, these robust estimators cannot be easily applied to models containing binary and categorical explanatory variables. Therefore, I design a robust estimator that can be used for any linear regression model no matter what kind of explanatory variables the model contains. Additionally, I propose an adaptive procedure that maximizes the efficiency of the proposed estimator for a given data set while preserving its robustness.
In the last ten years, there has been increasing interest and activity in the general area of partially linear regression smoothing in statistics. Many methods and techniques have been proposed and studied. This monograph hopes to bring an up-to-date presentation of the state of the art of partially linear regression techniques. The emphasis is on methodologies rather than on the theory, with a particular focus on applications of partially linear regression techniques to various statistical problems. These problems include least squares regression, asymptotically efficient estimation, bootstrap resampling, censored data analysis, linear measurement error models, nonlinear measurement models, nonlinear and nonparametric time series models.
Offering an in-depth treatment of robust and resistant regression, this volume takes an applied approach and offers readers empirical examples to illustrate key concepts.
In many scientific studies the goal is to determine the effect of a particular feature or variable on a given outcome in order to help understand, identify, and quantify the driving factors behind a particular phenomena. This type of analysis is commonly referred to as variable importance analysis. Parametric methods used to estimate these effects are prone to bias. This bias is often the result of incorrect model specification and improper inference for the parameter of interest. Alternative machine learning techniques, such as Random Forest, often result in abstract measures of importance whose inference depends on a computationally intensive bootstrap analysis. In this thesis, robust estimators for variable importance based on targeted maximum likelihood methodology are presented and developed for three types of outcomes (1) univariate continuous, (2) multivariate continuous, and (3) binary outcome. These estimators are specifically designed to target the effect of a variable of interest on an outcome while adjusting for confounders when the variable of interest is of general form (i.e. continuous or discrete). When the outcome is continuous (1,2), the effect is on an additive scale. When the outcome is binary (3), the effect is on a multiplicative scale, and the importance measure is a relative risk. The estimators are developed under a flexible semiparametric model, in which only components related to the variable of interest must be fully specified, and effect modification can be easily incorporated. Based on targeted maximum likelihood theory, the presented estimators are double robust and locally efficient, and correct inference for the parameter of interest is available using the corresponding influence curve. In this thesis, the three estimators relating to the three outcomes are derived from targeted maximum likelihood methodology and implemented by adapting standard statistical regression software. These estimators are applied in both simulation and application. In a simulated biomarker discovery analysis, the robustness of the estimator for a univariate continuous outcome is compared to other common methods of variable importance under increasing correlation among the covariates. In a repeated measures setting, the double robust property of the estimator for a multivariate continuous outcome is demonstrated in simulation, and the estimator is applied in a transcription factor analysis to determine the activity level of transcription factors during the cell cycle in yeast. For a binary outcome, the estimator for the relative risk is applied to estimate the effect of HIV genetic susceptibility scores on viral response. Effect modification is also explored and model selection methodology is introduced.
The statistical and mathematical principles of smoothing with a focus on applicable techniques are presented in this book. It naturally splits into two parts: The first part is intended for undergraduate students majoring in mathematics, statistics, econometrics or biometrics whereas the second part is intended to be used by master and PhD students or researchers. The material is easy to accomplish since the e-book character of the text gives a maximum of flexibility in learning (and teaching) intensity.